drosophila buzzatii and drosophila koepferae are two cactophilic sibling species , belonging to the repleta group , morphologically undistinguishable except by the male genitalia . they have an overlapping distribution in the arid zones of southern south america , although d. buzzatii has a wider geographical distribution compared with d. koepferae , which is mainly restricted to the oriental side of the andes mountains . moreover , they present a partial ecological isolation because d. buzzatii breeds and feeds on cacti from genus opuntia whereas d. koepferae prefers columnar cacti from cereus and trichocereus genus . although recent studies have provided evidences of recent gene flow between both species , we know they present reproductive isolation . hybrids have not been found in the wild yet , due probably to the absence of a molecular tool allowing their quick identification . in laboratory , interspecific crosses are possible as d. buzzatii males can mate d. koepferae females and female hybrid offspring can be backcrossed to d. buzzatii males . this potential of natural hybridization makes these species an excellent model for speciation studies by transposition , particularly because very early studies had already showed increases of transposition rates of osvaldo , the best characterized retrotransposon in d. buzzati , in hybrids compared with parental species . the most numerous and best documented cases of te mobilization after interspecific hybridization were reported in plants , where hybrids are easier to obtain and study than in animals in which the number of offspring is low making the crosses very difficult . however , now some examples begin to be available in animals : one of the best known cases is the kangaroo where increases of transposition and centromere expansion were observed in hybrids . the first direct observation of increases of transposition in hybrids between d. buzzatii and d. koepferae was reported by labrador and fontdevila with the retrotransposon osvaldo . these results were confirmed by a second experiment that included a larger sample size and a robust quantitative test . the next published work in drosophila referred to hybrids between d. melanogaster and d. simulans and showed a widespread de - repression of different te families in f1 hybrids . our recent published work , on d. buzzzatii and d. koepferae , analyzes by the first time the set of genomic elements by a genome - wide approach . the hybrids were followed by four successive generations ( f1 hybrids and three backcrosses ) and stocks used did not contain mutations that rescued sterility in f1 , reproducing the natural conditions of both species . we observed that a total of 33 tes belonging to 14 te families of class i and ii were mobilized in hybrid genomes compared with parental species , where only one te seemed to be mobilized . likely the number of mobilized elements is greater than observed , due to the low size of aflp bands and the lack of a reference sequenced genome , some elements could have been unnoticed . however , the number of elements in this study constitutes a representative part of the parental species genome . the detailed study of osvaldo , helena , and galileo , representative of ltr - retrotransposons , non - ltr - retrotransposons and transposons respectively , showed an increase of transposition rates in hybrids compared with parental species , reaching in some cases one order of magnitude greater . it is noteworthy that in these experiments differences were observed in the number of tes mobilized as well as in the transposition rates across de different generations and hybrid families ( hybrid crosses replicas ) . these results point first to the different mechanisms of regulation between elements and , second , that the portion of genome introgressed in hybrids could play an important role in te activation . it is important to note that the percentage of d. buzzatii genome introgressed in hybrids is increasing in each backcross with d. buzzatii parental males , and the region of genome is different between families . differences in hybrid instability or gene misexpression between generations of hybrids are not rare and have already been reported in plants and mouse . another important point is the trend to a higher increase of transposition rates in males than in females , even if the differences were only significant in one family . we ignore how the elements are awakened in hybrids or why that is especially relevant in males , as seen mainly for osvaldo retrotransposon . we think that most transposition events occurring in hybrids happened in female germline because a higher number of insertions are segregating along hybrid generations and it is hybrid females which are repeatedly crossed with d. buzzatii males . transpositions observed in hybrid males could correspond to events in somatic cells or / and in hybrid male germinal tissues that do not contribute to the next generation because hybrid males are sterile . out of the hybrid context , cases of copia transpositions or expression of 412 , 
 micropia , and 1731 have been reported in drosophila males suggesting regulation mechanisms different from females . the mechanism that induces transposition in hybrids is largely unknown and to date only an experimental study has addressed this issue in drosophila . but , the similarities between events ( sterility and transposition ) occurring during interspecific hybridization and hybrid dysgenesis suggest a possible parallelism . during the drosophila hybrid dysgenesis , by crosses between wild and laboratory strains , sterility and increases of transposition were observed in the f1 offspring as a consequence of the activation of male - derived tes . this happened because maternally contributed piwi - interacting rnas ( pirna ) were unable to repress tes paternally derived . in the same way , during interspecific hybridization between arabidopsis species , the parental copies of athila te , normally silenced in heterochromatin , were expressed in hybrids . the authors hypothesize that the activation of the element could be due to the insufficient production of small interference rnas ( sirnas ) in females or their low specificity because of the sequence divergence between the two arabidopsis species used in crosses . it is proposed that hybrid genetic dysfunctions occurred not only by the sequence divergence of genes in the two species but also by a large variety of mechanisms related with integrity and maintenance of chromatin . it is well known that the epigenetic control can affect gene expression and te mobilization , a phenomenon extensively studied in drosophila especially in the case of genes of the piwi pathway responsible for defending germline from te proliferation . kelleher et al.found both te misregulation and aberrant pirna production in interspecific hybrids between d. melanogaster and d. simulans , suggesting an adaptive divergence of pirna pathway genes rather than differences in pirnas derived from tes in the species under study . koepfere hybrids and raises de question about the involvement of te mobilization in hybrid sterility and the molecular causes . previous genetic work , with these same hybrids , proposes that male hybrid sterility results from the cumulative action of many interacting genes of minor effects . we ignore the direct impact of te derepression in fecundity of d. buzzatii / d . koepferae hybrids but the divergence between alleles of pirna pathway could contribute both to sterility and te mobilization . in drosophila , proteins encoded by these genes participate in pirna biogenesis , transposon silencing , and germline stem cell ( gsc ) self - renewal in both males an females and are important in the maintaining of fertility . mutations in aubergine gene ( a member of pirna pathway ) lead to accumulation of retrotransposons on ovaries and testes , and stellate transcripts in testes that are associated to male sterility . in view of the importance of these mentioned genes i suggest that the hybrid sterility is not directly caused by transposition but that could be a consequence of adaptive divergence of piwi pathway genes . however , we can not completely discard the possibility that te mobilization in hybrids affect sterility because of their capability of neighboring genes misregulation . on the other hand to explain te mobilization , it is known that pirnas silencing transposons come from heterochomatic te - rich regions named pirna clusters . for many tes the maternal deposition of te specific pirnas is critical for their propagation and , in the case of interspecific hybrid , if they do not inherited the pirna clusters , or the elements inserted inside have diverged between the two species , the amount of pirnas would not be sufficient to repress tes . this point could be the cause of differences in the amount of te mobilized through different hybridization generations and/or families observed in our work . to summarize , the figure  1 depicts putative scenarios leading to transposition in hybrids and their implications in speciation processes . shock  by interspecific hybridization leads to an epigenetic reprogramming and a te release associated to mutations and new gene regulatory ways . some of these mutations could be fixed by selection in a novel environment contributing to speciation processes . there is increasing evidences suggesting that tes had and have a promising role in evolutionary processes . transposition bursts ensuing hybridization have been suggested as makers of rapid genome reorganizations and source of evolutionary innovations , notwithstanding more empirical work will be necessary to address these questions conclusively . the recent advances in the knowledge of te silencing associated to interference rnas ( rnai and pirna ) could open new ways to the understanding of te activation in hybrids . provided that the genomic context could affect epigenetic regulation , a way to disentangle the mechanisms implicated in tes deregulation in hybrids could consist of comparing the small rna content of hybrids and parental species . this study could be completed with the analysis of the expression of pirna pathway genes in order to detected putative expression patterns altered in hybrids compared with parental species .
transposable elements ( tes ) are dna sequences able to be mobilized in host genomes . 
 they are currently recognized as the major mutation inducers because of their insertion in the target , their effect on neighboring regions , or their ectopic recombination . 
 a large number of factors including chemical and physical factors as well as intraspecific crosses have traditionally been identified as inducers of transposition . besides environmental factors , interspecific crosses have also been proposed as promoters of transposition of particular tes in plants and different animals . 
 our previous published work includes a genome - wide survey with the set of genomic tes and shows that interspecific hybridization between the species drosophila buzzatii and drosophila koepferae induces genomic instability by transposition bursts . 
 a high percentage of this instability corresponds to tes belonging to classes i and ii . 
 the detailed study of three tes ( osvaldo , helena , and galileo ) , representative of the different te families , shows an increase of transposition in hybrids compared with parental species , that varies depending on the element . 
 this study suggests ample variation in te regulation mechanisms and the question is why this variation occurs . 
 interspecific hybridization is a genomic stressor that disrupts the stability of tes probably contributing to a relaxation of the mechanisms controlling tes in the drosophila genome . in this commentary paper 
 we will discuss these results and the molecular mechanisms that could explain these increases of transposition rates observed in interspecific drosophila hybrids .