Miguel Angel Gonzalez Balanta

P B Alves De Oliveira

H Albalawi

Yara Galvão Gobato

Helder Vinícius

Avanço Galeti

A D Rodrigues

Mohamed Henini

Samy Almosni

Cédric Robert

Andrea Balocchi

M A G Balanta

P B A De Oliveira

H V A Galeti

Y Léger

H Carrère

M Bahri

G Patriarche

X Marie

C Cornet

Effects of nitrogen incorporation and thermal annealing on the optical and spin properties of GaPN dilute nitride alloys

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

III-V dilute nitride alloys have recently attracted a great of attention for applications in optoelectronics particularly for solar cells [1-5]. Among these alloys, GaPN is an interesting material for pseudomorphic integration of photonics with silicon [START_REF] Kharel | Band structure and absorption properties of (Ga, In)/(P, As, N) symmetric and asymmetric quantum wells and super-lattice structures: Towards lattice-matched III-V/Si tandem[END_REF][START_REF] Almosni | Evaluation of InGaPN and GaAsPN materials lattice-matched to Si for multi-junction solar cells[END_REF]. Furthermore, recently it was found that GaNPbased nanowires are a promising material for innovative device applications due to their beneficial properties related to their architecture [START_REF] Steidl | Impact of N Incorporation on VLS Growth of GaP(N) Nanowires Utilizing UDMH[END_REF][START_REF] Steidl | MOVPE growth of GaP/GaPN core-shell nanowires: N incorporation, morphology and crystal structure[END_REF][10][START_REF] Filippov | Strongly polarized quantum-dot-like light emitters embedded in GaAs/GaNAs core/shell nanowires[END_REF]. Gallium phosphide (GaP) has the smallest lattice mismatch with silicon (0.37% at room temperature) amongst all binary III-V compound semiconductors. In addition, the residual strain due to this lattice mismatch can be compensated by the incorporation of a few per cent of nitrogen (N) in GaP [START_REF] Lazarenko | Photoluminescence of heterostructures with GaP 1 x N x and GaP 1xy N x As y layers grown on GaP and Si substrates by molecular-beam epitaxy[END_REF][START_REF] Shokhovets | Optical constants and origin of the absorption edge of GaPN lattice-matched to Si[END_REF]. It is therefore important to investigate the effect of incorporation of N in GaPN in order to obtain high crystal quality and lattice matched heteroepitaxial GaPN thin layers on silicon substrates for applications in optoelectronics and photovoltaics. In fact, the doping of GaP semiconductor, which is an indirect bandgap semiconductor, with (N) isoelectronic impurity has been widely used in the past to improve its radiative efficiency, and thus, making it suitable for optoelectronic

Experimental Details

In this work, the samples were grown by Solid-Source Molecular Beam Epitaxy (SSMBE) using the apparatus described in ref. [START_REF] Quinci | Defects limitation in epitaxial GaP on bistepped Si surface using UHVCVD-MBE growth cluster[END_REF]. Phosphorus and atomic N were generated with a valved cracker cell and a valved RF plasma N 2 cell operating at a 400 W power and with a 0.5 sccm high purity N 2 flow rate, respectively. Most of the samples have nominally the same structure, i.e. after the oxide desorption performed at 650 °C, a 100 nm-thick GaP buffer layer is grown at 580°C, followed by a 100 nm-thick GaPN epilayer grown at 480°C. Finally, a 10 nm GaP cap layer was deposited, in order to avoid surface contribution to the optical properties of GaPN. Specific samples (with similar growth conditions) were grown for detailed annealing studies by TRPL and transmission electron microscopy, where the GaPN active layers are 50nm-ACCEPTED MANUSCRIPT -CLEAN COPY thick and the GaP cap layers are 100 nm, in order to study structurally the N exodiffusion during annealing. The N composition in the GaPN layer was controlled mainly by the growth rate, V/III beam equivalent pressure ratio and N plasma cell valve opening, as described in details in ref. [START_REF] Quinci | Defects limitation in epitaxial GaP on bistepped Si surface using UHVCVD-MBE growth cluster[END_REF]. The GaP substrates were carefully selected to avoid N incorporation inhomogeneities due to roughening [START_REF] Cornet | Preferential incorporation of substitutional nitrogen near the atomic step edges in diluted nitride alloys[END_REF]. The N content was deduced from standard θ-2θ high resolution X-ray diffraction (HRXRD) for the different samples, assuming a linear dependency of the GaPN lattice parameter with the N composition, as already shown in ref. [START_REF] Almosni | Evaluation of InGaPN and GaAsPN materials lattice-matched to Si for multi-junction solar cells[END_REF] and 22 for similar thicknesses. In the following, the N contents measured for each sample are given with the corresponding experimental data. The GaPN layer thickness is limited to 100 nm in order to avoid plastic relaxation. Rapid Thermal Annealing (RTA) was performed at 800°C for 5 minutes in a N 2 and H 2 mixture atmosphere for some of the samples studied. PL measurements were performed at 10K using a Janis closed-cycle refrigerator cryostat and Andor 0.5m spectrometer coupled with Si CCD. Polarization resolved PL measurements under magnetic field were performed at 2K in a 15T Oxford Magnet with the magnetic field parallel to the growth direction (Faraday configuration). A linearly-polarized blue line (488 nm) from a continuous-wave Ar+ laser was used as optical excitation. Therefore, the photogenerated carriers in the GaPN layers should not present any preferential spin polarization. Right (  ) and left (  ) circularly-polarized PL signals were selected with appropriate optical components (achromatic quarter wave plate and Glan-Thompson Calcite Polarizer). Raman scattering measurements at room temperature were performed using 488nm laser line and recorded with a triple grating spectrometer coupled with CCD detector. Experimental parameters were set to furnish a spectral resolution ~ 1.5cm -1 . PLE measurements were carried out in a cold finger cryostat at low temperature (~10 K). A 50 W tungsten halogen lamp combined with a 1-m single grating monochromator served as tunable light source for both PL and PLE measurements. The emission from the layers was dispersed by a 0.75-m double monochromator and detected with a liquid-N cooled S1-photomultiplier. TRPL measurements were performed using a pulse-picker to decrease the Titanium-Sapphire laser repetition frequency to 4 MHz which is placed before a non-linear crystal to double the fundamental Ti:Sa emission energy to 3.1 eV ACCEPTED MANUSCRIPT -CLEAN COPY (400 nm) in order to excite GaP (E GAP =2.27 eV). The laser light was then focussed to a 100 m spot diameter with a time-averaged power around 0.5 mW. The PL signal is detected by using a Hamamatsu S1 streak camera. The PL decay times are then fitted using a double exponential decay function.

STEM-HAADF (High-angle annular dark-field scanning transmission electron microscopy) measurements were performed using a Cs corrected TEM/STEM Jeol2200Fs. The microscope was operated at 200 KV. The beam convergence angle was 30 mrad. For the HAADF detector, the collection angles used were 62.3 mrad and 170 mrad for the internal and external angles, respectively. It is well known that N forms perturbed host conduction states which shift to lower energy when increasing the N content. In Fig. 1(b), bandgap energies calculated with tight-binding (TB) simulations are superimposed with experimental data, showing an excellent agreement. A 50 meV systematic shift was added to TB bandgaps that are performed nominally at 0K. However, the incorporation of N in GaP also introduces important disorder effects which affect considerably the optical properties. These effects will be investigated later in this paper by different techniques such as Raman Spectroscopy, power and temperature dependent PL, TRPL and PLE. Figure 2 shows typical PL spectra for different laser power at low temperature (10K). For all samples, different broad bands were observed at lower energies and sharp PL peaks at higher energies ( > 2eV) depending on laser power. These broad bands at lower energy are usually associated with exciton localization effects. The observed sharp emissions are due to NNi (nitrogen-nitrogen interstitial) centers. Actually, these sharp peaks are usually observed in the doping regime at low ACCEPTED MANUSCRIPT -CLEAN COPY incorporation of N in GaP and in non-intentionally doped GaP due to residual doping. In GaP crystal, all these states are localized below the conduction band.

Results and Discussion

Photoluminescence general properties

In our case, the observed sharp lines are associated with the emission in the GaP substrate. In addition, the observed asymmetry of the broad band is usually attributed with disorder effects such as random fluctuations in alloy composition which could result in a smearing of band edges and in an extended density of states into the band gap. This asymmetry of the PL spectrum is well known for dilute nitrides. Due to the fact that the high energy tail is enhanced by thermal filling of the bands, the asymmetric shape is more pronounced at lower temperatures than at room temperature and higher laser power. Particularly, it was observed a blue shift of the PL band with increasing laser power and a reduction of band tail at lower energy. Furthermore, it was also found (not shown here) that the integrated PL intensity has a nearly linear variation as a function of excitation density. These features are usually explained by a gradual filling of energy states resulting from localized centers particularly due to alloy fluctuations in the low energy band tail. The radiative recombinations associated with the band tail are expected to be the dominant mechanism which would be responsible for this laser power dependence of PL spectrum. We have investigated the temperature dependence of GaPN layers with different N % content as shown in figure 3. Figure 3 (a) shows the temperature dependence of PL peak energy. An anomalous behavior of PL peak position versus T was observed. Particularly, the PL peak energy cannot be fitted by the Varshni equation. This behavior, which is similar to the reported results on other dilute nitride such as GaAsN and InGaAsN, evidences important effects of localization of excitons by disorder for all investigated GaPN layers. In general, it is well known that for dilute nitrides the temperature dependence of PL peak show an S-shape behavior [33-38]. However, only for the 2.7% N content it was possible to clearly evidenced this behavior, this result suggests a high carrier localization on this sample, we reinforces this fact with the next results . In order to improve the optical quality of our GaPN samples, we have performed thermal annealing treatment. Figure 3b shows a typical temperature dependence of PL peak energy for annealed samples using laser excitation at 488nm and 30mW.

A typical S-shape dependence of PL peak versus temperature was observed after annealing. Figure 4 illustrates a typical PL spectrum for 1.4% N at 300K ACCEPTED MANUSCRIPT -CLEAN COPY before and after thermal annealing at 800°C and 5mins. An important improvement of PL intensity for all samples was ascertained. Particularly for the 1.4% N content this enhancement of PL intensity is about 3 times after thermal annealing. In addition, an important blue shift of PL spectrum after thermal annealing was determined. This result is in agreement with previous reports [START_REF] Rubel | Quantitative description of disorder parameters in (GaIn)(NAs) quantum wells from the temperature-dependent photoluminescence spectroscopy[END_REF][START_REF] Francoeur | Luminescence of As-Grown And Thermally Annealed GaAsN/GaAs[END_REF]. Usually, at lower temperatures the PL spectrum is dominated by recombination of localized excitons. Above 200 K, the PL peak usually presents a blue shift and an increase of PL linewidth. At this condition, the localized carriers have enough thermal energy and starts to escape to the quasidelocalized states. As mentioned before this effect was typically observed in dilute nitrides and is usually called the S-shape behavior.

Structural properties

Before discussing in details the impact of annealing and N composition on advanced optical properties, structural investigations have been performed by Raman spectroscopy and transmission electron microscopy to find out the main physical processes involved during annealing. Fig. 5( Surprisingly, GaPN layer has higher HAADF intensity compared to GaP ones. It worth noting that the HAADF image mode is also called Z-contrast imaging. It is proportional to the square of the atomic number. Thus, the heavier the elements the higher the contrast, that should lead to a larger HAADF intensity for the GaP. This effect has been reported recently in GaAs/GaAsN layers [START_REF] Grieb | Quantitative chemical evaluation of dilute GaNAs using ADF STEM: avoiding surface strain induced artifacts[END_REF][START_REF] Grieb | Determination of Nitrogen Concentration in Dilute GaNAs ACCEPTED MANUSCRIPT -CLEAN COPY by STEM HAADF Z-Contrast Imaging and improved STEM-HAADF strain state analysis[END_REF]. It is here attributed to the static atomic displacement effect [START_REF] Grillo | Influence of the static atomic displacement on atomic resolution Z-contrast imaging[END_REF]. The corresponding contrast profiles are also given along the growth direction (Fig. 6(c)). From this figure, we can conclude that as-grown and annealed GaPN/GaP interfaces are sharp and well-defined and that structural inhomogeneity is not observed inside the GaPN epilayers, as confirmed also by X-ray diffraction experiments, see ref.

22. From the important findings that no modification of the GaPN is observed during the annealing, keeping the same interface quality and HAADF contrast, we evidence that exodiffusion of N out of the GaPN layers did not occur during the annealing process. This tends to indicate that structural non-radiative defects that are impacted by annealing are not related to N local inhomogeneities, but more probably to Ga-vacancies induced during the growth-process, which also explains why the impact of annealing cannot be clearly seen from X-ray diffraction and Raman experiments.

PLE results

Figure 7(a) shows the PL (open circles and solid square) and PLE (solid line) spectra at 10K for the as-grown and annealed samples for N= 1.4%. We have observed that the PLE spectra is dominated by a broad band centered at ~2.3 ACCEPTED MANUSCRIPT -CLEAN COPY eV. This broad band has been reported previously [START_REF] Thomas | Isoelectronic Traps Due to Nitrogen in Gallium Phosphide[END_REF][START_REF] Izadifard | Evaluation of optical quality and defect properties of GaNxP1-x alloys lattice matched to Si[END_REF] for N concentration up to 0.56% and was associated with a miniband created by the coupling between N-levels and the conduction band. For low N% concentration a PLE peak associated with an exciton bound to an isolated N atom is usually observed and labelled A-line. By increasing the N concentration this line results in a broader band. A shoulder at ~2.2 eV was also observed and attributed to the isoelectronic traps formed by NN pairs (NN1). The PLE spectra were also measured for several detection energies (not shown here) for both samples and showed a similar result indicating that the emission and absorption bands are due to the same layer. In addition, a large energy separation between the PL and PLE peaks was observed. Particularly, a Stokes Shift of ~190 meV was seen for N=1.4% as-grown sample. Since the PLE spectra are related to the density of states, the high density of the continuous states may dominate, whereas the radiative transitions occur for the occupied states, at lower energy (localized states). This effect explains the large energy separation between absorption and emission observed in Fig. 7(a), which is typical for GaPN layers.

After thermal annealing, this energy separation was drastically reduced to ~130 meV, suggesting that thermal annealing reduces the localized states, possibly due to defects and potential fluctuations, resulting in a PL blue shift and a decreasing of the PL broadening. However, it was observed that the peak position of the PLE band was not affected by the thermal annealing. On the other hand, the PLE band width was reduced which could also be related to the reduction of the localized states in the sample. Considering that the N states become a mini-band in the gap, it is reasonable to consider a parabolic band around the edge, i.e. the density of states can be described as for the minimum energy edge, where A 0 is a constant and E 0 is the minimum band edge. Similarly, for maximum edge the density of states can be given by . where A F is a constant and E t is a threshold band edge. We can extract the absorption edge (E 0 and E t ) for each PLE spectra. Considering that the PLE band shown in Fig. 7(a) is related to the lower branch of the impurity mini-band, we can estimate band width by (E t -E 0 ). The values of E 0 , E t , and (E t -E 0 ) are plotted in figure 7(c) as function of N concentration for annealed samples. The value of E t is independent of the N composition, while, as ACCEPTED MANUSCRIPT -CLEAN COPY expected, E 0 decreases and (E t -E 0 ) increases due to the increase of the coupling as N increases. We also plot the energy separation between E 0 and PL peak (Stokes shift) in Fig. 7(b). The Stokes shift increases from N content from 1.4% to 2.1% and then remains constant. The localization effects seem to have a saturation for large values of N%.

Time Resolved Photoluminescence

Typical TRPL streak images for a N= 1% sample before and after annealing are shown in Figure 8 (a) and (b), respectively, at T=10K. In general, it was observed that the decay time of GaP(N) is usually of several nanoseconds probably due to the indirect nature of the recombination. The temporal and spectral evolutions presented in the figures do not change drastically from one sample to the other, the overall shape is always similar. But looking more carefully at the two presented streak images, one can see that annealing slightly impacts the time-resolved PL properties in terms of a narrowing of the spectrum and modification of the radiative lifetime. From streak images, PL decay times are fitted by using a double exponential decay function, τ 1 and τ 2 being the associated lifetimes. The origin of these two decay times could be ascribed to: (1) delocalised state emissions associated with shorter decay times and (2) emissions associated with localised density of states (longer decay times). Both of them being impacted by the presence of non-radiative centers.

Figure 8 (c) and (d) present the PL decay times measured for N= 1% sample before and after annealing. In this case, beyond the spectral narrowing clearly observed in the annealed sample, the decay time evolution does not change significantly, and a small decrease of the τ 2 value at 10K is even measured, as compared to the as-grown samples. It is then concluded that in this range of compositions, the annealing mainly plays an important role on the homogeneity of the samples, but does not impact significantly on non-radiative recombination contributions. It was also observed that the longer decay times are more sensitive to thermal annealing. Particularly, a characteristic S-shape as a function of the temperature was observed after annealing, similar to the behaviour usually observed for the temperature variation of the PL peak in ACCEPTED MANUSCRIPT -CLEAN COPY dilute nitrides. This result points to the existence, after annealing, of shallower and fewer N-induced defects which can be ionised at relatively low temperature: an increase of the decay time is observed starting from a temperature as low as 40 K, in parallel to a symmetric increase of the band gap (not shown). The same analysis is now performed on samples having N= 2%. Decay times for asgrown and annealed samples are reported in fig. 9 (a) and (b), respectively. In this case, both decay times are always significantly larger after annealing, which evidences an overall improved optical quality of the material. Whereas for the as-grown sample the decay times monotonously decrease with the temperature, for the annealed sample it is clear that temperature-activated defects dominate only above 150K. These improved optical qualities are confirmed by the observation of a sizeable larger activation energy of the decrease of the PL integrated intensity as a function of temperature (Fig. 9(c)) and a marked overall narrower linewidth (Fig. 9(d)). Therefore, the benefits of post-thermal annealing are significantly more important in samples with high N contents. All these data are consistent with an improved optical quality of the annealed GaPN layer, a key result for the realisation of buffer layers for the monolithic integration of optoelectronic devices on silicon [47-50].

Magneto-PL

We have performed a detailed study of magneto-optical properties of GaPN layers. It is important to mention that no results of magnetoluminescence resolved in polarization measurements in GaPN systems have been reported previously. Figure 10 shows typical PL spectra as function of magnetic field at 2K and 30mW of laser excitation for the sample with N= 2.1%.

It was observed that the diamagnetic shift, which is obtained from the difference between the energy peak under magnetic field and zero magnetic field, is almost zero which evidences important effects of exciton localization in GaPN layers. Similar results were observed in all as-grown and post-thermal annealed GaPN samples with different N concentrations. These results evidence important effect of exciton localization [START_REF] Balanta | Polarization resolved photoluminescence in GaAs1-xBix/GaAs quantum wells[END_REF][START_REF] Balanta | Optical and spin properties of localized and free excitons in GaBiAs /GaAs multiple quantum wells[END_REF] in GaPN layers probably due to composition variation which cannot be changed after thermal annealing ACCEPTED MANUSCRIPT -CLEAN COPY treatment. We have also investigated the polarization resolved PL for as-grown and annealed layers. Figure 11 show typical σ+ and σpolarization resolved PL spectra for N= 2.1% samples (as-grown and annealed) at 2K and under 15 T. It was observed that only for N= 2.7% sample the polarization increases linearly for all the range of the magnetic field applied. Nevertheless, this behavior was also observed in the other samples at low fields, ~B = 6 T and tends to saturate at higher magnetic fields, reaching values up to ~ 40%. A similar behavior was observed in InGaAs quantum well structures where the magnetic field dependence of the polarization was attributed to the fact that it is proportional to the spin polarization of holes once both electron spin levels below E F are equally occupied. At low magnetic fields the polarization of holes is determined by g h μ B B/(k B T) and then saturates at high B because of the similar occupation of spin levels for holes [START_REF] Ya | Magneto-photoluminescence of InAs/InGaAs/InAlAs Quantum Well Structures[END_REF][START_REF] Meier | Optical Orientation[END_REF]. Here, g h is the hole g-factor and  B is the Bohr magneton. The fact that we found a linear behavior only in the sample with higher content of N, indicates a higher efficiency of the annealing eliminated more significantly the scattering centers for the relaxation of the spin of the holes in this sample. A significant value of odd circular polarization degree for GaPN layers was obtained. In addition, an increase of the degree of polarization was observed after thermal annealing which could be explained by important changes of spin relaxation and recombination times after thermal annealing due to the reduction of defects [START_REF] Lombez | Spin Dynamics in Dilute Nitride Semiconductors At Room Temperature[END_REF][START_REF] Zhao | Electron Spin Control in Dilute Nitride Semiconductors[END_REF]. This effect is illustrated in figure 12 which shows the magnetic field dependence of degree of polarization for the as-grown and annealed samples. An interesting effect is observed in these GaPN layers.

We remark that the difference between the circular polarization degree of the as-grown samples and annealing samples increases as the N concentration increases. This result is consistent with the fact that the effect of the annealing must be more efficient in reducing the defects which cause spin relaxation in samples with higher N concentration. In the diluted limit of the impurity concentration, the annealing is less efficient in decreasing those scattering centers. Moreover, the higher polarization degree is obtained for the sample with N= 1% for both as-grown and annealed samples. This fact reinforces the previous results related with the increase of the polarization degree Postthermal annealing which indicates that for this N concentration, less scattering ACCEPTED MANUSCRIPT -CLEAN COPY centers for spin relaxation are present in the sample. Finally, we have determined the g exc factor for GaPN layers. The inset of the figure 11 (a) presents the normalized PL spectra for  and  emission in order to obtain the spin splitting for sample with N= 2.1% at 15T. From this value we have extracted a g exc factor of ~ 2 which is similar for all the samples studied. Despite a relatively high circular polarization of the light emitted in relation to other systems with localized effects, the small spin splitting is related with the fact that the relaxation of the spin via scattering from defects and impurities in this system, is accompanied by a decrease of the Zeeman energy, which requires coupling of the spin system with the lattice [57].

Conclusion

We have studied the effects of N incorporation and thermal annealing on the optical, structural and spin properties of GaPN layers. We have evidenced important effects of localization of carriers by disorder. In addition, it was also shown that the annealing process successfully improves the optical qualities of the epilayers in terms of PL intensity, PL decay time, activation energy, linewidth and polarization degree. Nevertheless, no modification of the GaPN structural properties was observed after thermal annealing. In addition, We presented for the first time a study of polarization resolved PL in these systems.

The magneto-PL results revealed that the diamagnetic shift is almost zero which also evidences important effects of exciton localization in GaPN layers. A relatively high circular polarization of the light emitted was observed for all samples at high magnetic fields which are higher than the values observed in other systems with localized effects. Finally, our results evidenced an improvement of the optical properties after thermal annealing of the samples which is essential for potential manufacture of GaPN based semiconductors devices.

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  applications. It has been shown that the incorporation of N in GaP leads to an extremely large band-gap bowing and N-induced transformation from an indirect to a pseudo-direct band gap which consequently increases the emission intensity of the semiconductor. It was also previously shown that the incorporation of small content of N into P-rich GaPAs leads to the formation of intermediate band and the change of the nature of the fundamental band gap from indirect to direct [14]. The observed N-induced modification of the electronic band structure was also explained by the Band Anti Crossing (BAC) model. The changes of band structure is usually described by this model that calculates the changes of band structure induced by isovalent N atom in a III-V host matrix . According to this model, the interaction of the localized N level with the states of the GaPAs or GaP host leads to the formation of E-and E+ bands in GaNP or GaPN alloy, respectively [14]. In addition, the observation of the isolated intermediate band offers a potential of using P-rich GaNPAs or GaP alloys for intermediate band solar cells [15,16]. However, GaPN semiconductors have many non-radiative recombination channels, mainly ACCEPTED MANUSCRIPT -CLEAN COPY associated with the formation of defects, such as vacancies, interstitials , and impurity atoms that have a tendency to increase with the increasing of the N content. [17-20]. Particularly, the optical quality can be improved if the density of defects are reduced which can be realized by optimizing the growth conditions or by performing post-growth annealing [21,22]. In the present work, we explored this effect by investigating as-grown and thermal annealed GaPN samples containing different N contents between 0.5 and 3%. The effect of thermal annealing was not previously investigated for GaPN layers. In addition, we present for the first time magneto-optical results for GaPN layers under high magnetic fields for as-grown and annealed samples. Structural properties were studied by high resolution X-ray diffraction and Raman scattering. Optical properties were analysed by photoluminescence (PL), photoluminescence excitation (PLE) and time resolved photoluminescence (TRPL). In general, our results have also revealed the observation of the isolated intermediate band at low N concentration and important effects of exciton localization and improvement of the optical properties after thermal annealing of the GaPN layers which are essential for potential manufacture of GaPN based semiconductors devices. In addition, a g exc factor of ~ 2 was observed which is similar for all the samples studied.

Figure 1 (

 1 Figure 1 (a) shows the typical PL spectra obtained for different N concentrations at 300K. In general, it was observed a red-shift of the PL energy peak with the increase of N content, as shown in Fig. 1 (b). This expected behavior is usually associated with the giant band gap bowing effect in dilute nitride alloys [25-32].

  a), presents the Raman spectra at 300K for GaPN layers with different N concentrations. The most intense peak at around 402cm -1 is related to GaP LO-phonons. At 365 cm -1 is the TO-phonons peak, which is forbidden by Raman selection rules in backscattering geometry, but in this case they are activated by disorder.Spectral features with wave-numbers lower than 300cm -1 are associated with the density of states (DOS) of acoustic phonons. It is noticed that the LOphonon peaks (inset of Fig.5 (a)) present the same width for N contents up to 2.1 %, however, a large broadening is observed for N= 2.7%. Generally, peak broadening occurs as a consequence of a breakdown in the selection rule for the momentum conservation. In the presence of crystalline disorder, phonons become localized. The decrease in the uncertainty of the phonon localizations corresponds to an increasing in the uncertainty of their momenta, allowing phonons far from the Brillouin Zone center to contribute to the scattering process. Since GaP LO phonons present negative dispersion relation [41], the resulting spectral line will be broadened towards lower wavenumbers, indicating ACCEPTED MANUSCRIPT -CLEAN COPY that the N incorporation has induced strong structural disorder in the layer with higher N proportion. Raman spectra of the samples after annealing were also measured. In fig5 (b), the Raman data for the sample with N= 2.1%, are displayed as an example. No measurable change were found in the spectra taken before and after the annealing process, indicating that the thermal treatment did not produce significant structural modifications. STEM and XRD measurements were performed on similar samples (N= 2% but with larger GaP capping layer and thinner GaPN layer) in order to investigate possible structural changes of these samples after annealing. Two cross-sectional STEM-HAADF images are shown in Fig.6before (a) and after (b) the annealing process.

Figure 1 :

 1 Figure 1: (a) Typical PL specta at 300K for GaPN samples with various N compositions (b) Energy of the maximum room temperature PL intensity as a function of N content at room temperature. The red solid line is the calculated bandgap using tight-binding models from ref. [29]

Figure 2 :Figure 3 .

 23 Figure 2: Typical PL spectra at low temperature (T=10K) for different laser powers. (a) N=2% and (b) N= 2.7% Figure 3. PL peak energy versus T (solid circles) and Vashni fitting (solid lines) at 30mW. (a) as-grown sample, (b) after annealing.

Figure 4 :

 4 Figure4: PL spectra at 300K before and after thermal annealing for a laser wavelength of 488nm and power of 100mW.

Figure 5 :

 5 Figure 5: (a) Raman spectra at 300K for samples with different N% incorporation using a laser of 488nm (2.54 eV). The inset shows the details of the LO-mode peak. (b) Raman spectra of the as-grown and annealed samples with N= 2.1%,.

Figure 6 :

 6 Figure 6: STEM-HAADF images for a GaP 0.98 N 0.02 sample (a) as-grown and (b) annealed. A comparison of HAADF intensity profiles normalized to the one of GaP is given in (c).

Figure 7 .

 7 Figure 7. (a) Typical PL and PLE for N= 1.4% at 10K for as-grown and annealed samples. (b) Stokes shift (c) edge and threshold energy (left) and band width dependence with the nitrogen concentration for annealed samples (right).

Figure 8 .Figure 9 :

 89 Figure 8. Typical Streak camera images for N= 1% at 10K for (a) as-grown and (b)annealed samples. The temperature dependence of the measured PL decay times of the 1%N containing GaP sample as grown (c) and after annealing (d) Figure 9: Decay times measured in the GaP 0.98 N 0.02 sample as a function of temperature for (a) as-grown sample and (b) annealed sample. Associated Boltzmann plots are drawn in (c), with corresponding extracted activation energies. PL linewidths as function of temperature are given in (d).

Figure 10 :

 10 Figure 10: Magnetic dependence of the PL spectra for the N= 2.1 % sample.The inset shows the normalized spectra for 0 T and 15 T. Measurements taken at 2K with a power excitation of 30mW.

Figure 11 .

 11 Figure 11. Polarization resolved spectra obtained at 2K, 15T at and power excitation of 25 mW for both (a) as-grown and (b) annealed GaPN samples with N= 2.1%. The inset shows the normalized spectra for  and  emission.

Figure 12 .

 12 Figure 12. Magnetic field dependence of polarization degree at 2K before and after thermal annealing for GaPN layers for different N% contents

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Chen SL, Chen WM, Ishikawa F, Buyanova IA. Suppression of nonradiative surface recombination by N incorporation in GaAs/GaNAs core/shell nanowires. Sci Rep 5:11653 (2015).

Acknowledgements

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We acknowledge the Brazilianfinancial agencies CNPq, CAPES, FAPESP and FAPEMIG. Y.G.G. and H.V.A.G. acknowledge FAPESP grants 18/01808-5,16/10668-7 and 14/50513-7. M. A. G. B. and H.V.A.G acknowledge Capes grants 88881.068192/2014-01. M. A. G. B. acknowledge the financial support from PNPD -Capes-PPG-UFSCar. M. A. G. B. acknowledges FAPEMIG grants CEX-APQ-00753-18. M.H. and H.A.acknowledge the support by a grant from the deanship of scientific research, Princess Nourah bint Abdulrahman University, Riyadh, KSA.Finally the authors are grateful to Grupo de PropriedadesOticas (GPO) at Universidade Estadual de Campinas (UNICAMP) for PLE andmagneto-photoluminescence measurements and to Professor F. Iikawa for fruitful discussions. Highlights Comprehensive characterization of the structural and optical properties of GaPN. Investigation of the synthesis and physical properties of dilute nitrides. Study of alloys which have great potential for the development of high efficiency solar cells ACCEPTED MANUSCRIPT -CLEAN COPY


Clément Gilbert
email: clement.gilbert@egce.cnrs-gif.fr

Jean Peccoud

Richard Cordaux

Transposable elements and the evolution of insects

Keywords: Transposable elements, adaptation, antiviral immunity, aging, evolution, horizontal transfer

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Introduction

Transposable elements (TEs) are pieces of DNA able to move from one genomic locus to another, often duplicating themselves in the process [START_REF] Bourque | Ten things you should know about transposable elements[END_REF]. The large diversity of TE types is classified in two classes: retrotransposons (Class-I TEs), which move through a replicative process involving reverse transcription of the TE mRNA, and DNA transposons (Class-II TEs), which mobilize through replicative or non-replicative transposition of DNA copies [START_REF] Craig | Mobile DNA III[END_REF]144). TEs are found in the genomes of virtually all organisms, often in very large amounts. The structure of TEs, their mobility and repeated nature have profoundly shaped the evolution of their hosts [START_REF] Cordaux | The impact of retrotransposons on human genome evolution[END_REF][START_REF] Feschotte | DNA transposons and the evolution of eukaryotic genomes[END_REF][START_REF] Hua-Van | The struggle for life of the genome's selfish architects[END_REF]123).

TEs were discovered in the 1940s by Barbara McClintock, who showed that some genetic elements could move in the maize genome. This activity would induce chromosome breaks and sometimes modify the expression of neighboring genes with visible changes in maize kernel color [START_REF] Mcclintock | The origin and behavior of mutable loci in maize[END_REF]. These initial studies have been mostly ignored for over three decades until they were finally rewarded by the Nobel prize of Physiology or Medicine (in 1983). TEs then became increasingly considered as important drivers of evolution, notably owing to major discoveries made in insects, mainly in Drosophila flies [START_REF] Biémont | A Brief History of the Status of Transposable Elements: From Junk DNA to Major Players in Evolution: Figure 1[END_REF]. Examples include the hybrid dysgenesis phenomenon, whereby F1 hybrids between males harboring P elements (class-II TEs) and females lacking them showed several defects such as gonadal atrophy and sterility (17,[START_REF] Kelleher | Reexamining the P -Element Invasion of Drosophila melanogaster Through the Lens of piRNA Silencing[END_REF][START_REF] Kidwell | Cytoplasm-chromosome interactions in Drosophila melanogaster[END_REF]. The molecular underpinnings of hybrid dysgenesis were only deciphered in 2008 through the discovery of the PIWI-associated RNAs (piRNAs)-mediated control of TEs [START_REF] Brennecke | Discrete small RNA-generating loci as master regulators of transposon activity in Drosophila[END_REF]. In the meantime, molecular cloning and extensive studies of the P element led to the development of this TE as one of the most widely used germline transformation tools in Drosophila [START_REF] Majumdar | P Transposable Elements in Drosophila and other Eukaryotic Organisms[END_REF]125). Other TE-based transformation systems were developed for non-Drosophila species, including the one using Piggybac, a TE from the cabbage looper moth (Trichoplusia ni) that was discovered because of its ability to jump into a baculovirus [START_REF] Fraser | Acquisition of Host Cell DNA Sequences by Baculoviruses: Relationship Between Host DNA Insertions and FP Mutants of Autographa californica and Galleria mellonella Nuclear Polyhedrosis Viruses[END_REF][START_REF] Handler | The lepidopteran transposon vector, piggyBac, mediates germ-line transformation in the Mediterranean fruit fly[END_REF].

Surveys of the distribution of P elements in wild and laboratory strains of various Drosophila species also revealed that horizontal transfer (HT) of genetic material (here TEs) was not restricted to bacteria but that it could occur in eukaryotes [START_REF] Daniels | Evidence for horizontal transmission of the P transposable element between Drosophila species[END_REF][START_REF] Gilbert | Horizontal acquisition of transposable elements and viral sequences: patterns and consequences[END_REF]. Other landmark TE discoveries made in Drosophila include the so-called P neogenes derived from stationary P element copies, which led to the realization that TE protein domains can be co-opted to fulfill cellular functions [START_REF] Miller | Molecular domestication--more than a sporadic episode in evolution[END_REF]102,108).

Perhaps even more surprising, the finding that the classical telomeres had been replaced in Drosophila flies with HetA/TART/TAHRE retrotransposons, which only transpose at chromosome ends, revealed that bona fide TEs could be beneficial to their host [START_REF] Biessmann | HeT-A, a transposable element specifically involved in "healing" broken chromosome ends in Drosophila melanogaster[END_REF]101). Finally, seminal studies mapped the genetic basis of insecticide resistance in the tobacco budworm (Heliothis virescens) and D. melanogaster to TE-driven mutations, which contributed to set TEs as powerful facilitators of genome evolution and host adaptation [START_REF] Daborn | A single p450 allele associated with insecticide resistance in Drosophila[END_REF][START_REF] Gahan | Identification of a gene associated with Bt resistance in Heliothis virescens[END_REF][START_REF] Oliver | Transposable elements and viruses as factors in adaptation and evolution: an expansion and strengthening of the TE-Thrust hypothesis[END_REF]116).

These case-studies along with more recent multi-species, genome-wide surveys allow painting a general picture of the composition, evolution and potential roles of TEs in insects, which is the topic of this review. We first expose emerging macroevolutionary trends that underlie insect TE genomic landscapes. We then discuss how the latest major discoveries on the impact of TEs in insect adaptation, antiviral immunity and aging are continuously changing the way we view these genomic symbionts.

-Evolution of TEs within and across insect orders -Diversity and abundance of insect TEs

The first comprehensive characterizations of TE abundance, diversity, age and genome distribution in an insect were made possible thanks to analyses of the D. melanogaster genome [START_REF] Bartolomé | On the abundance and distribution of transposable elements in the genome of Drosophila melanogaster[END_REF][START_REF] Bergman | Recurrent insertion and duplication generate networks of transposable element sequences in the Drosophila melanogaster genome[END_REF][START_REF] Kaminker | The transposable elements of the Drosophila melanogaster euchromatin: a genomics perspective[END_REF][START_REF] Kapitonov | Molecular paleontology of transposable elements in the Drosophila melanogaster genome[END_REF]. These studies illustrated how estimates of TE abundance and diversity heavily depend on the completeness and quality of assembled genomes as well as on the methods used to annotate TEs. TE content ranged from 2% of the D. melanogaster genome in early studies of euchromatic regions relying only on similarity-based approaches, to about 15 % when heterochromatic regions were included and combined de novo annotation pipelines were developed. The D. melanogaster TE landscape is dominated by LTR-retrotransposons, which make up 10% of the genome, with non-LTR retrotransposons and DNA transposons altogether occupying ~5% of the genome. The TE distribution along D. melanogaster chromosomes is non-random, with a much lower density in euchromatic regions where TEs generally occur as single full-length copies, than in pericentromeric heterochromatin where TEs are arranged in dense islands of nested and often rearranged copies.

Bergman et al. [START_REF] Bergman | Recurrent insertion and duplication generate networks of transposable element sequences in the Drosophila melanogaster genome[END_REF] proposed that expression of such TE islands would produce transcripts made of chimeric TEs which could be involved in suppressing the activity of multiple TE families. The hypothesis was later confirmed with the discovery of dozens of pericentromeric TE-rich regions called piRNA clusters acting as TE traps, the expression of which initiates piRNA-mediated TE suppression [START_REF] Brennecke | Discrete small RNA-generating loci as master regulators of transposon activity in Drosophila[END_REF]150). Early studies based on between-copy genetic distances within TE families found that most euchromatic TE copies resulted from very recent transposition events [START_REF] Bowen | Drosophila euchromatic LTR retrotransposons are much younger than the host species in which they reside[END_REF][START_REF] Kapitonov | Molecular paleontology of transposable elements in the Drosophila melanogaster genome[END_REF]. Their recent origin is in agreement with subsequent studies showing that >80% of polymorphic TE copies segregate at low (<0.2) frequency in D. melanogaster populations [START_REF] Kofler | Tempo and Mode of Transposable Element Activity in Drosophila[END_REF].

The analysis of other genomes quickly revealed that characteristics of fly TEs were not necessarily representative of those found in other insects and that TE landscapes were highly dynamic in this group [START_REF] Maumus | Impact of transposable elements on insect genomes and biology[END_REF]. For example, although the overall content of classified TEs in the mosquito Anopheles gambiae (about 13%) was close to that of D. melanogaster, LTR retrotransposons occupy about the same amount of genomic space as non-LTR retrotransposons and DNA transposons in this species [START_REF] Holt | The genome sequence of the malaria mosquito Anopheles gambiae[END_REF][START_REF] Neafsey | Mosquito genomics. Highly evolvable malaria vectors: the genomes of 16 Anopheles mosquitoes[END_REF]. Genome sequencing of another mosquito (Aedes aegypti), a Coleoptera (Tribolium castaneum) and a Lepidoptera (Bombyx mori) further revealed that TE content varies substantially in insects (from 6% in T. castaneum to 48% in A. aegypti) and that both DNA transposons and non-LTR retrotransposons can be the dominant TEs in other species [START_REF] Nene | Genome sequence of Aedes aegypti, a major arbovirus vector[END_REF]100,139). The decrease in sequencing costs was then accompanied by a rapid augmentation of the number of genome papers with detailed sections on TE annotation (e.g. [START_REF] Chang | Islands of retroelements are major components of Drosophila centromeres[END_REF][START_REF] Chung | Cis -Regulatory Elements in the Accord Retrotransposon Result in Tissue-Specific Expression of the Drosophila melanogaster Insecticide Resistance Gene Cyp6g1[END_REF][START_REF] Cosby | Host-transposon interactions: conflict, cooperation, and cooption[END_REF][START_REF] Gadau | The genomic impact of 100 million years of social evolution in seven ant species[END_REF][START_REF] Gavotte | Costs and benefits of Wolbachia infection in immature Aedes albopictus depend upon sex and competition level[END_REF]. One study reported that the antarctic midge (Belgica antarctica) had both the smallest genome and the lowest TE content among arthropods [START_REF] Kelley | Compact genome of the Antarctic midge is likely an adaptation to an extreme environment[END_REF]. It is now common to see papers reporting comparative analyses of multiple insect genomes with efforts dedicated to surveying the evolution of TE contents across genomes (e.g. [START_REF] Aminetzach | Pesticide Resistance via Transposition-Mediated Adaptive Gene Truncation in Drosophila[END_REF][START_REF] Bast | No Accumulation of Transposable Elements in Asexual Arthropods[END_REF][START_REF] Brennecke | Discrete small RNA-generating loci as master regulators of transposon activity in Drosophila[END_REF][START_REF] Chung | Cis -Regulatory Elements in the Accord Retrotransposon Result in Tissue-Specific Expression of the Drosophila melanogaster Insecticide Resistance Gene Cyp6g1[END_REF][START_REF] De Cecco | Genomes of replicatively senescent cells undergo global epigenetic changes leading to gene silencing and activation of transposable elements[END_REF][START_REF] Fabrick | Insertion of an intact CR1 retrotransposon in a cadherin gene linked with Bt resistance in the pink bollworm, Pectinophora gossypiella: CR1 retrotransposon and Bt resistance[END_REF][START_REF] Gilbert | Horizontal acquisition of transposable elements and viral sequences: patterns and consequences[END_REF], as well as studies specifically dedicated to comparisons of TE contents at scales ranging from insect genus to whole arthropods [START_REF] Han | iMITEdb: the genome-wide landscape of miniature inverted-repeat transposable elements in insects[END_REF]105,110,114,148). We ourselves performed an automated TE annotation on 195 publicly available insect genomes (103) (Figure 1).

A recent study also reported automated de novo TE-annotation and comparison of 73 arthropod genomes (including 62 insects) (105). An interesting outcome of this analysis, as well as ours (Figure 1), is that the nature of a large fraction of the repetitive content of most insect genomes (11% in D. simulans up to 93% in Ephemera danica; mean 56%, (105)) cannot be assessed using automated procedures. Thus, interpretations on variation in TE content in insects must be taken with caution.

Bearing this limitation in mind, broad-scale TE landscape analyses reveal a high diversity of TEs in insects, with most known TE superfamilies present in most insect species (105, 148). They also unveil some degree of homogeneity in TE composition among insect groups spanning diverse taxonomic levels (105). For example, in six tsetse fly species (Glossina spp), the genome proportion occupied by DNA transposons (12.8-14.7%), LTR (0.4-0.8%) and non-LTR (5.8-2.9%) retrotransposons has remained stable over 25 million years [START_REF] Attardo | Comparative genomic analysis of six Glossina genomes, vectors of African trypanosomes[END_REF]. In three Hemipteran species having diverged >200 million years ago (Cimex lectularius, Halyomorpha halys and Oncopeltus fasciatus), genome proportions of TE classes differ between species but non-LTR retrotransposons (7-14%) are always in higher amounts than DNA transposons (1.3-4%) and LTR retrotransposons (0.6-1.4%). The situation is similar in four lepidopteran species which diverged >100 million years ago (Helicoverpa armigera, Bombyx mori, Danaus plexippus, Heliconius melpomene), in which non-LTR retrotransposons (4.2-20.4%) are also always in higher amount than DNA transposons (0.9-3.1%) and LTR retrotransposons (0.3-0.7%).

While homogeneity in TE composition exists in some cases, pronounced differences have been found in other cases. In hemipterans for example, SINEs and DNA transposons of the pea aphid (Acyrtosiphon pisum) occupy the largest genome fraction (7.1% and 8.6%, respectively) while they are either present in low amount (mean=1.2% and 2.3%, respectively) and/or occupy the lowest genome fraction in the other six surveyed species (105). In coleopterans, DNA transposons occupy a much larger genome fraction than retrotransposons in T. castaneum and Anoplophora glabripennis while they are in minority compared to retrotransposons in Agrilus plannipenis and Leptinotarsa decemlineata (105). Large differences in TE landscapes also exist at the genus level. Among 16

Heliconius butterfly species, TE composition may be similar at the level of TE class (DNA transposons versus retrotransposons), but may differ drastically within each TE Class (110). For example, LTR retrotransposons are almost absent in H. doris and H. burneyi but they make up 2 to 6% of the genome of the other 14 species. Similarly, eight Heliconius species have a genome proportion of SINEs <1% while SINEs make up 2 to 6% of the genome of the remaining eight species.

-2 The influence of vertical and horizontal transmission on insect TE landscapes

The TE content of a genome is an equilibrium between the rate of TE acquisition, the replication dynamics of TEs within the genome and the rate of TE loss by degradation/deletion. TEs are acquired from ancestors (vertical inheritance) and potentially from other organisms (HT). TE replication dynamics reflect rates of TE insertions (TE activity), which may be suppressed by host factors, as well as the fate of these insertions under the influence of natural selection and genetic drift [START_REF] Kofler | Dynamics of Transposable Element Invasions with piRNA Clusters[END_REF][START_REF] Rouzic | Genome ecosystem and transposable elements species[END_REF]115,135). While the relative importance of the factors and processes shaping TE landscapes is not well understood, insects are one of the groups in which they have been the most intensely studied so far (see also (129) for a study in nematodes).

Multiple large-scale studies indicate a positive correlation between phylogenetic relatedness and similarity of TE landscapes in insects (see above and refs. (105, 148)), although this correlation is not formally tested. Variation in genome size also shows a strong phylogenetic signal and is positively correlated to TE content both at the scale of arthropods (102, 146) and within insect orders: in Drosophila species [START_REF] Fonseca | The mobilome of Drosophila incompta, a flower-breeding species: comparison of transposable element landscapes among generalist and specialist flies[END_REF]122), wood-white butterflies (130) and fireflies [START_REF] Lower | Genome Size in North American Fireflies: Substantial Variation Likely Driven by Neutral Processes[END_REF]. This observation is somewhat expected given that TEs, like any other genome component, are inherited from parents to descendants. Yet in several pairs of closely related species showing relatively similar TE landscapes, such as D. melanogaster and D. simulans, most TE copies have originated from transposition events that are much more recent than the species split [START_REF] Bowen | Drosophila euchromatic LTR retrotransposons are much younger than the host species in which they reside[END_REF]105). Their recent origin is further supported by the fact that TE insertions in D. melanogaster are generally polymorphic within the species [START_REF] Kofler | Tempo and Mode of Transposable Element Activity in Drosophila[END_REF]. As a result, relatively few TE copies may be inherited over millions of years in a context of ongoing TE activity, and species would be expected to diverge quickly with respect to their TE content.

On the other hand, a systematic survey of HT of TEs (HTT) between D. melanogaster, D. simulans and D. yakuba, which diverged 11 million years ago, revealed that at least one third of the TE families in these three species have been acquired through HTT, with an overall HTT rate of 0.035 events per TE family per million years [START_REF] Bartolome | Widespread evidence for horizontal transfer of transposable elements across Drosophila genomes[END_REF]. It was also found that the number of HTT was higher between closely related species than between distantly related ones, with twice as many events between D. melanogaster and D. simulans as between any of these species and D. yakuba [START_REF] Bartolome | Widespread evidence for horizontal transfer of transposable elements across Drosophila genomes[END_REF]. A similar study extended to 195 insect genomes inferred no fewer than 2248 HTT events between species having diverged >40 million years ago (103). In addition to showing that HTT is not infrequent and has had a strong impact on genome evolution in insects, with on average 2% of insect genome content deriving from horizontally transferred TEs (and up to 25%), this study confirmed the role of phylogenetic relatedness in favoring HTT. Preferential HTT between related species may thus explain the phylogenetic signal in insect TE landscape variation in conjunction to inheritance from shared ancestors.

In addition to phylogeny, geography was found to partly explain the distribution of HTT along the insect tree. More HTT was detected between species originating from the same biogeographic realm than between species from different ones (103). The observation that species more likely to be in (direct or indirect) contact are also more likely to exchange TEs supports a role for ecological relationships in shaping HTT. This is in line with a series of studies reporting HTT between animal hosts and their parasites [START_REF] Gilbert | A role for host-parasite interactions in the horizontal transfer of transposons across phyla[END_REF][START_REF] Kuraku | Horizontal transfers of Tc1 elements between teleost fishes and their vertebrate parasites, lampreys[END_REF]128,137,141), but not with a study that did not detect HTT between Drosophila species and two of their parasitoid wasps [START_REF] Ortiz | An evaluation of the ecological relationship between Drosophila species and their parasitoid wasps as an opportunity for horizontal transposon transfer[END_REF]. Given the relatively high rate of HTT in insects (103), it is likely that future studies implementing appropriate taxon sampling will further delineate the types of ecological relationships, including host-parasite relationships, that affect HTT patterns in insects (135). Interestingly, if was recently found that lepidopterans were significantly more prone to HTT than other insect orders (112). The higher susceptibility of lepidopterans to HTT may be explained by a number of different factors, one of which being exposure to large double-stranded DNA viruses belonging to the Baculoviridae family. Most known baculoviruses indeed infect lepidopterans and a large number of TEs are able to jump from infected moth species to the genome of several baculoviruses, suggesting a role for these viruses as vectors of HTT [START_REF] Gilbert | Continuous Influx of Genetic Material from Host to Virus Populations[END_REF].

-3 Are insect TE landscapes shaped by ecological and life history traits?

The replication dynamics of TEs may depend on certain life history traits of their hosts, as these traits may condition the impact of TE insertions on fitness and how these insertions evolve in host populations. One candidate trait is reproductive mode, since reduced recombination rates in asexual taxa may lead to the accumulation of TE insertions, as seen in non-recombinant sex chromosomes [START_REF] Chalopin | Transposable elements and early evolution of sex chromosomes in fish[END_REF]. However, asexual arthropods (including asexual wasps) do not have more TEs than their sexual counterparts [START_REF] Bast | No Accumulation of Transposable Elements in Asexual Arthropods[END_REF]77,119). By contrast, social complexity is negatively correlated with TE abundance and TE diversity in bees, which may be explained by increased recombination rates in highly social insects and/or the pronounced hygienic behavior in these insects, leading to a lower exposure to parasites that may act as vectors of HTT [START_REF] Kapheim | Social evolution. Genomic signatures of evolutionary transitions from solitary to group living[END_REF]. Yet, in social ants displaying hygienic behavior, TE content and diversity are similar to non-solitary insects [START_REF] Gadau | The genomic impact of 100 million years of social evolution in seven ant species[END_REF]. Furthermore, in termites, the socially more complex Macrotermes natalensis has a larger genome (1.31 Gb) and higher TE content (about 30%) than Zootermopsis nevadensis (562 Mb and about 16%, respectively) [START_REF] Korb | A genomic comparison of two termites with different social complexity[END_REF]. Thus, whether or not TE landscapes are shaped by levels of sociality in bees, the influence of this factor may not hold for other social insects.

The ecological niche has been proposed as a potential factor shaping insect TE landscapes. For example, the low TE content of the antarctic midge B. antartica (0.12%) is thought to result from the paucity of contacts this extreme specialist has with other organisms in the species-poor polar environment, combined to strong selective pressures due to pronounced physiological and ecological constraints [START_REF] Kelley | Compact genome of the Antarctic midge is likely an adaptation to an extreme environment[END_REF]. In apparent agreement with a link between ecological niche and TE content, another extreme specialist, the fig wasp Ceratosolen solmsi also has one of the lowest TE contents of all insects (6.4%) (149). Yet, when comparing TE landscapes between Drosophila species in a phylogenetic context, no significant difference was found between specialists and generalists [START_REF] Fonseca | The mobilome of Drosophila incompta, a flower-breeding species: comparison of transposable element landscapes among generalist and specialist flies[END_REF].

The level of ecological specialization of Drosophila species, which is not as extreme as in B. antartica and C. solmsi, may not be high enough to impact TE content. Thus, reevaluating TE content in a controlled phylogenetic framework including related specialist and generalist species is required to properly test the influence of ecological niche on this variable. More generally, how host physiology and ecology govern TE replication dynamics deserves further studies. Such works may help to assess how the similarity in TE contents among related species, despite sustained TE activity, reflects some form of stabilizing/parallel evolution under similar constraints.

-The fate and impact of TEs in insect genomes

The development of next-generation sequencing technologies and analytical tools have enabled studies of the population genomics of insect TEs, many of which have focussed on D. melanogaster [START_REF] Barrón | Population Genomics of Transposable Elements in Drosophila[END_REF][START_REF] Bourgeois | On the Population Dynamics of Junk: A Review on the Population Genomics of Transposable Elements[END_REF]. These studies have shown that a large fraction of the TEs inserted in the D. melanogaster reference genome are not fixed in the species (106). In addition, thousands of non-reference TE insertions segregate in D. melanogaster populations (25, [START_REF] Kofler | Sequencing of Pooled DNA Samples (Pool-Seq) Uncovers Complex Dynamics of Transposable Element Insertions in Drosophila melanogaster[END_REF][START_REF] Lerat | Population-specific dynamics and selection patterns of transposable element insertions in European natural populations[END_REF][START_REF] Linheiro | Whole Genome Resequencing Reveals Natural Target Site Preferences of Transposable Elements in Drosophila melanogaster[END_REF]109).

The majority of polymorphic TE insertions are present at lower frequencies than expected for neutral variants, suggesting that purifying selection (deleterious effects) plays a major role in TE population dynamics in D. melanogaster [START_REF] Blumenstiel | An age-of-allele test of neutrality for transposable element insertions[END_REF]. A similar pattern of TE dynamics has been found in D. simulans [START_REF] Kofler | Tempo and Mode of Transposable Element Activity in Drosophila[END_REF]. The deleterious effect of TEs can in part be explained by insertional mutagenesis upon integration in the genome. Consistently, more than half of spontaneous mutant phenotypes in D. melanogaster are caused by TE insertions disrupting gene function [START_REF] Eickbush | Fruit flies and humans respond differently to retrotransposons[END_REF] and very few TE insertions are found in protein-coding genes in natural populations [START_REF] Kofler | Sequencing of Pooled DNA Samples (Pool-Seq) Uncovers Complex Dynamics of Transposable Element Insertions in Drosophila melanogaster[END_REF]106). At the post-insertional level, an important cause for TE deleteriousness is ectopic recombination between non allelic, homologous TE insertions, which can generate deleterious genomic rearrangements [START_REF] Kofler | Sequencing of Pooled DNA Samples (Pool-Seq) Uncovers Complex Dynamics of Transposable Element Insertions in Drosophila melanogaster[END_REF]106). It will be interesting to assess to what extent TE insertions are deleterious for insects other than Drosophila.

-1 Insect adaptation

Even though most TE insertions are deleterious or neutral, some insertions are expected to be beneficial to their carriers. This is the case of several TE-mediated mutations underlying resistance to insecticides in the pink bollworm Pectinophora gossypiella, a major pest of cotton, and in D. melanogaster (116). In P. gossypiella, several independent TE insertions disrupting the PgCad1 gene confer resistance to the widely used Bacillus thuringiensis Cry1Ac toxin [START_REF] Fabrick | Insertion of an intact CR1 retrotransposon in a cadherin gene linked with Bt resistance in the pink bollworm, Pectinophora gossypiella: CR1 retrotransposon and Bt resistance[END_REF]138). In D. melanogaster, cis-regulatory sequences of an Accord LTR retrotransposon inserted in the 5' regulatory region of the Cyp6g1 gene are responsible for Cyp6g1 upregulation and increased resistance to DDT [START_REF] Chung | Cis -Regulatory Elements in the Accord Retrotransposon Result in Tissue-Specific Expression of the Drosophila melanogaster Insecticide Resistance Gene Cyp6g1[END_REF][START_REF] Daborn | A single p450 allele associated with insecticide resistance in Drosophila[END_REF].

Furthermore, an adaptive allelic series in Cyp6g1 involving successive mutations (including P and HMS-Beagle TE insertions) was later found to be associated with increasing DDT resistance levels (120). Similarly, resistance to organophosphate insecticide is caused by a Doc1420 LINE retrotransposon that truncates the CHKov1 gene and produces altered transcripts [START_REF] Aminetzach | Pesticide Resistance via Transposition-Mediated Adaptive Gene Truncation in Drosophila[END_REF]. Interestingly, the new variant may have initially been selected as a defense against viral infections, thereby preadapting flies to insectides [START_REF] Magwire | Successive Increases in the Resistance of Drosophila to Viral Infection through a Transposon Insertion Followed by a Duplication[END_REF]. Yet another mechanism underlies TE-mediated increased resistance to cyclodiene dieldrin: Rdl gene duplication through ectopic recombination between Roo LTR retrotransposons (113). DNA transposons can also be involved in xenobiotic resistance, as examplified by the Pogo TE affecting polyadenylation signal choice in the CG11699 gene and conferring resistance to carbofuran and benzaldehyde [START_REF] Mateo | A Transposable Element Insertion Confers Xenobiotic Resistance in Drosophila[END_REF]. These examples serve to highlight the diversity of TE families and molecular mechanisms through which TEs can be adaptive to their hosts, which is all the more spectacular with respect to insecticide resistance in D. melanogaster, when considering that this species is not a pest and has not been targeted by insecticides.

The contribution of TEs to D. melanogaster adaptation is not restricted to insecticide resistance, as genome-wide studies of TE insertion polymorphisms have identified dozens of candidate adaptive TE insertions [START_REF] González | Genome-Wide Patterns of Adaptation to Temperate Environments Associated with Transposable Elements in Drosophila[END_REF][START_REF] González | High Rate of Recent Transposable Element-Induced Adaptation in Drosophila melanogaster[END_REF][START_REF] Kofler | Sequencing of Pooled DNA Samples (Pool-Seq) Uncovers Complex Dynamics of Transposable Element Insertions in Drosophila melanogaster[END_REF][START_REF] Lerat | Population-specific dynamics and selection patterns of transposable element insertions in European natural populations[END_REF]111). Population frequency distributions of TE insertions suggest that some may play a role in adaptation to temperate climates (D. melanogaster has originated in the afrotropics), often by induction of regulatory changes in nearby genes [START_REF] González | Genome-Wide Patterns of Adaptation to Temperate Environments Associated with Transposable Elements in Drosophila[END_REF][START_REF] González | High Rate of Recent Transposable Element-Induced Adaptation in Drosophila melanogaster[END_REF]. Functional gene categories such as stress response, behavior and development appear to be particularly prone to TEmediated adaptation (111). In some cases, ecologically-relevant fitness benefits have been clearly connected to specific TE insertions, such as a Bari1 DNA transposon mediating oxidative stress response [START_REF] Gonzalez | A Recent Adaptive Transposable Element Insertion Near Highly Conserved Developmental Loci in Drosophila melanogaster[END_REF][START_REF] Guio | The transposable element Bari-Jheh mediates oxidative stress response in Drosophila[END_REF], P, Jockey and Roo elements involved in thermotolerance [START_REF] Lerman | Naturally occurring transposable elements disrupt hsp70 promoter function in Drosophila melanogaster[END_REF][START_REF] Merenciano | Multiple Independent Retroelement Insertions in the Promoter of a Stress Response Gene Have Variable Molecular and Functional Effects in Drosophila[END_REF] and an invader4 retrotransposon inducing faster development time (133). Interestingly, negative fitness effects of such TE insertions have sometimes been identified, which may represent costs of selection and may explain why the TE insertions have not reached fixation [START_REF] Gonzalez | A Recent Adaptive Transposable Element Insertion Near Highly Conserved Developmental Loci in Drosophila melanogaster[END_REF]133).

Adaptive TE insertions have also been found in various insects other than D. melanogaster. For example, in the peppered moth Biston betularia, industrial melanism is caused by a carb-TE DNA transposon in the first intron of the cortex gene which affects gene expression [START_REF] Hof Ae Van't, Campagne | The industrial melanism mutation in British peppered moths is a transposable element[END_REF]. This mutation underlies the replacement of a pale form by a black form driven by the interplay between bird predation and coal pollution during the Industrial Revolution in the UK. Wing color polymorphism is also associated with a TE insertion in the Colias crocea butterfly, confering developmental and reproductive advantages (147). In Chinese populations of the migratory locust (Locusta migratoria), an Lm1 SINE insertion in the third exon of the Hsp90 gene produces an alternative splicing form associated with faster development and higher developmental synchrony [START_REF] Chen | Transposable Element-Mediated Balancing Selection at Hsp90 Underlies Embryo Developmental Variation[END_REF]. Remarkably, only heterozygotes are present in natural populations, suggesting there is an heterozygote advantage that is maintained by balancing selection. It has also been proposed that TEs may be powerful agents of rapid adaptation in invasive species (126), an hypothesis that has received empirical support from several insects such as the ant Cardiocondyla obscurior (121) and the Asian tiger mosquito Aedes albopictus [START_REF] Goubert | High-throughput sequencing of transposable element insertions suggests adaptive evolution of the invasive Asian tiger mosquito towards temperate environments[END_REF].

While the aforementioned examples of adaptive TEs consider individual copies, the repetitive nature of TEs and existence of multiple homologous sequences scattered in genomes offer the perspective for TEs to collectively impact cellular processes. For example, G2/Jockey-3 retrotransposons appear to contribute to the structure and function of centromeres in D. melanogaster and D. simulans [START_REF] Chang | Islands of retroelements are major components of Drosophila centromeres[END_REF].

TEs may also rewire regulatory networks by supplying cis regulatory elements at chromosomal scale, as shown with the recruitment of Helitron elements to regulate dosage compensation in D. miranda sex chromosomes [START_REF] Ellison | Dosage Compensation via Transposable Element Mediated Rewiring of a Regulatory Network[END_REF][START_REF] Ellison | Non-allelic gene conversion enables rapid evolutionary change at multiple regulatory sites encoded by transposable elements[END_REF]. There is however no formal demonstration that TEs have actually improved these cellular processes from their ancestral state. The involvement of TEs in these processes could represent neutral or even slightly deleterious mutational changes.

-Antiviral immunity

In addition to their role in adaptation of insect populations, recent studies are setting TEs as an important component of insect antiviral immunity. In insects, viruses are mainly restricted by RNA interference (RNAi) (134). This pathway relies on the recognition and cleavage of intracellular long viral double-stranded (ds) RNAs by the endoribonuclease Dicer2, yielding 21-nucleotide small interfering RNAs (siRNAs). One strand of siRNAs is then used by the endoribonuclease Argonaute 2 to target and cleave complementary long single-stranded viral RNAs, resulting in viral restriction. While this pathway confers immunity to infected insect cells, it has long remained unclear whether such immunity could spread systemically. In plants and nematodes, systemic RNAi spread relies on amplification of dsRNA molecules by RNA-dependent RNA-polymerases (RdRp) [START_REF] Himber | Transitivity-dependent andindependent cell-to-cell movement of RNA silencing[END_REF]124), which are absent in insects [START_REF] Lewis | Pan-arthropod analysis reveals somatic piRNAs as an ancestral defence against transposable elements[END_REF]. Yet, Goic et al. [START_REF] Goic | Virus-derived DNA drives mosquito vector tolerance to arboviral infection[END_REF][START_REF] Goic | RNA-mediated interference and reverse transcription control the persistence of RNA viruses in the insect model Drosophila[END_REF] showed that antiviral RNAi can be amplified through a hitherto unknown mechanism in Drosophila flies and Aedes mosquitoes, which relies on the formation of viral DNA (vDNA). Remarkably, vDNAs are produced through reverse transcription of viral RNAs by insect retrotransposon proteins, instead of an RdRp. This process primarily takes place in hemocytes, in which viral dsRNAs released by infected cells accumulate (107, 131). vDNAs are then transcribed and they trigger the formation of secondary viral siRNAs which are loaded into exosomelike vesicles. They are then secreted in the extracellular environment and spread over the entire insect body, conferring systemic antiviral immunity (131). vDNAs are generated as both circular and linear molecules, some of which may end up being integrated into the host genome (132). Isolation and sequencing of episomal circular DNA from infected mosquito cells and Drosophila flies revealed that many circular vDNAs (cvDNAs) correspond to virus-LTR retrotransposon chimeras. Strikingly, the amount of circular DNA containing virus-LTR retrotransposon chimeras produced by mosquito cells is typically much higher than that containing chimeras of non-virus host sequence and retrotransposons (132). Furthermore, the viral portion of virus-LTR retrotransposon chimeras is enriched for defective viral genomes (107). Together, these observations suggest that viral defective RNAs are recognized and preferentially reverse-transcribed over non-viral RNAs by LTR retrotransposon-encoded reverse transcriptases, through a process involving copy-choice recombination and seemingly depending on Dicer2 (107, 131, 132). Importantly, inhibition of reverse transcriptase activity in flies and mosquitoes leads to accelerated death of infected individuals [START_REF] Goic | Virus-derived DNA drives mosquito vector tolerance to arboviral infection[END_REF][START_REF] Goic | RNA-mediated interference and reverse transcription control the persistence of RNA viruses in the insect model Drosophila[END_REF]. Thus, in the context of viral infections, the presence of LTR retrotransposons capable of producing active reverse transcriptases is crucial for fly and mosquito survival. In principle, as long as sources of reverse transcriptase activity are available throughout the evolutionary history of insects, this antiviral protection system does not require co-option of a specific TE copy that would become stationary and evolve under purifying selection like a regular gene. Thus, while in conflict with their hosts because of the overall deleterious consequences of transposition, insect LTR retrotransposons may become beneficial when host individuals incur viral infections.

Even more remarkably, insect retrotransposons have recently been shown to catalyse vDNA integration into piRNA clusters of an Aedes aegypti cell line, leading to formation of endogenous viral elements (EVE) (132, 143). Such EVEs are co-transcribed with other TE remnants from piRNA clusters, leading to the production of anti-sense viral piRNAs that provide antiviral immunity. Thus, in addition to their role in the amplification and systemic spread of antiviral immunity, insect retrotransposons and their piRNA-mediated host surveillance system have the potential to confer transgenerational antiviral immunity to their hosts (132, 143). The finding that LTR retrotransposons may be essential to insect antiviral immunity opens new perspectives on the way we conceive insect/TE relationships.

Much like other TEs such as TBE DNA transposons in the ciliate Oxytricha trifillax (136) or the telomeric non-LTR retrotransposons in Drosophila flies (118), and much like cellular endosymbionts such as Wolbachia and Cardinium [START_REF] Gavotte | Costs and benefits of Wolbachia infection in immature Aedes albopictus depend upon sex and competition level[END_REF][START_REF] Joshi | Wolbachia strain wAlbB confers both fitness costs and benefit on Anopheles stephensi[END_REF]142), the position of insect LTR retrotransposons on the continuum of host/symbiont interactions may vary in unexpected ways between conflict and cooperation, depending on context [START_REF] Cosby | Host-transposon interactions: conflict, cooperation, and cooption[END_REF].

-3 Aging

Aging is typically associated with the progressive alteration of genome regulation, notably due to the loss of repressive heterochromatin marks (145). Several studies have shown that this dysregulation is associated with an increase of TE expression in various eukaryotic organisms including yeasts [START_REF] Maxwell | Retrotransposition is associated with genome instability during chronological aging[END_REF], nematodes [START_REF] Dennis | C. elegans Germ Cells Show Temperature and Age-Dependent Expression of Cer1, a Gypsy/Ty3-Related Retrotransposon[END_REF], mice [START_REF] De Cecco | Transposable elements become active and mobile in the genomes of aging mammalian somatic tissues[END_REF] and human cell lines [START_REF] De Cecco | Genomes of replicatively senescent cells undergo global epigenetic changes leading to gene silencing and activation of transposable elements[END_REF]. Thus, it has been proposed that expression and transposition of TEs that are silenced in young individuals may strongly affect cell and genome integrity in old individuals, triggering or reinforcing aging symptoms [START_REF] Driver | Transposable elements as a factor in the aging of Drosophila melanogaster[END_REF]145). A series of elegant experiments performed in D. melanogaster seemingly support this model. In particular, it was shown that Dicer2-mutant individuals had more DNA double-strand breaks, presumably caused by activated TEs, than controls in the nuclei of fat body cells (146). This is consistent with the known role of Dicer2 in the formation of repressive heterochromatin around repeated DNA (including TEs) via the recruitment of the Su(var)3-9 histone methyl-transferase [START_REF] Fagegaltier | The endogenous siRNA pathway is involved in heterochromatin formation in Drosophila[END_REF]104). Importantly, decreasing TE activity in these Dicer2 mutants through the addition of a reverse-transcriptase inhibitor led to reduced transposition and increased lifespan. Thus, TE activity rather than the mere deterioration of chromatin structure could be directly implicated in aging in D. melanogaster (146). In apparent agreement, the lower lifespan of male versus female D. melanogaster flies has been attributed to a higher amount of heterochromatin in males, which possess a highly repeat-and heterochromatinrich Y chromosome [START_REF] Brown | The molecular basis of I-R hybrid Dysgenesis in drosophila melanogaster: Identification, cloning, and properties of the I factor[END_REF][START_REF] Marais | Sex gap in aging and longevity: can sex chromosomes play a role?[END_REF].

The finding that TEs largely contribute to aging in D. melanogaster has recently been complemented by studies in social insects. These insects are ideal models in aging research because individuals from a given colony sharing an identical genotype show large differences in lifespan, depending on the caste they belong to. In the termite Macrotermes bellicosus for example, reproductive castes (kings and queens) can live for as long as 20 years whereas the lifespan of major and minor workers never exceeds a few months. Elsner et al. (36) have shown that, while overall TE and gene expression levels in heads were stable through the lifetime of kings and queens, several hundreds of TE copies were expressed at higher levels in old versus young major workers. The higher TE expression in old workers was associated with down-regulation of four genes involved in the piRNA pathway, suggesting that aging in major workers may be due to a decrease in TE suppression. Since these genes were expressed at equal levels in old and young reproductive castes, it was proposed that the longer lifespan of kings and queens evolved thanks to cooption of the piRNA pathway (36) to suppress TEs in their somatic tissues (here heads). Though interesting, this hypothesis does not explain why piRNA genes are expressed (and TEs suppressed) in heads of young major workers [START_REF] Lucas | New explanation for the longevity of social insect reproductives: Transposable element activity[END_REF].

In fact, somatic expression of the piRNA pathway has been inferred to be ancestral to all arthropods, with only few species (including D. melanogaster), having lost this pathway in the soma [START_REF] Lewis | Pan-arthropod analysis reveals somatic piRNAs as an ancestral defence against transposable elements[END_REF]. Thus, rather than cooption of the piRNA pathway in heads of M. bellicosus reproductives, observations may be more consistent with a secondary loss of this pathway in heads of old major workers.

-Conclusions and perspectives

Major breakthroughs in our understanding of eukaryote genome regulation and evolution have been made through the study of insect TEs. Our knowledge of TE landscapes and their impact on insect genomes and phenotypes has rapidly increased over the last ten years thanks to the availability of an increasing number of genome sequences. Current large-scale surveys of insect TE landscapes must be interpreted cautiously as TEs have been thoroughly mined only in a handful of insect genomes and new TE families are still being discovered even in the best annotated genomes (148). Yet, global trends emerge, suggesting that while TE landscapes can be highly variable both within and across insect orders, phylogenetic relatedness generally correlates with similarity in TE content overall [START_REF] Lower | Genome Size in North American Fireflies: Substantial Variation Likely Driven by Neutral Processes[END_REF]105,122,130,148), even in cases where most extant TEs are not inherited from shared ancestors. Vertical inheritance, horizontal transfer and the ecology/physiology of hosts are likely involved in shaping insect TE landscapes, and it will be interesting to assess their relative contribution along the insect phylogeny.

Because of their high diversity and the wide spectrum of molecular mechanisms through which they can generate evolutionary innovations (including insertion, deletion, rearrangement, dispersion of cis-regulatory elements and TE protein sequences), TEs appear to be involved in a variety of adaptive events in many insect species. Thus, insects may be good models to quantify the respective contribution of TE-mediated genomic change versus other mutations in adaptation.

The finding that retrotransposons are at the heart of antiviral response in some insects emerges as one of the most important discoveries made in invertebrate immunity in the last few years [START_REF] Miesen | PIWIs Go Viral: Arbovirus-Derived piRNAs in Vector Mosquitoes[END_REF]. Yet, the range of viruses that are targeted by TE-mediated small RNA responses in the wild remains to be defined and it is unclear to what extent retrotransposons play a role in suppressing viruses beyond D. melanogaster and Aedes mosquitoes. Whether and how retrotransposons involved in this response vary among insects is also unknown. In addition, it will be interesting to further decipher the mechanisms by which insect retrotransposons selectively recognize and reverse-transcribe viral RNAs over cellular RNAs.

Monitoring and manipulating insect TE expression in old versus young insects uncovered a likely causal role for TE activity in aging, in agreement with the retrotransposon theory of aging [START_REF] Driver | Transposable elements as a factor in the aging of Drosophila melanogaster[END_REF]145).

In social insects, it will be interesting to assess whether TE activity in old workers is associated with a deterioration of heterochromatin structure, as observed in D. melanogaster. Furthermore, a general prediction of the retrotransposon theory of aging is that individuals with higher heterochromatin loads should be shorter lived than individuals with low amounts of heterochromatin. While this hypothesis seems to hold in D. melanogaster [START_REF] Brown | The molecular basis of I-R hybrid Dysgenesis in drosophila melanogaster: Identification, cloning, and properties of the I factor[END_REF], testing it could also take advantage of species in which B chromosomes, known to be largely heterochromatic, are present at substantial frequencies [START_REF] Hanlon | B Chromosomes in the Drosophila Genus[END_REF]. More generally, these results call for a detailed evaluation of the impact of somatic transposition on insect biology.

Figure legend

Figure 1 .

 1 Figure 1. Proportions of DNA transposons, LTR, LINE and SINE retrotransposons in insect genomes. TEs were automatically annotated in 195 insect species (103). Because of space constraints, only one

Figure 1

 1 Figure 1