First case of R2 non-LTR retrotransposon horizontal transfer: data from Bacillus stick-insects genome (Insecta Phasmida)

Horizontal transfer is the passage of genetic material between reproductively isolated, even distantly related species. R2 is one of the best-studied non-LTR retrotransposons and inserts sequence-specifically into 28S rDNA. Non-LTR retrotransposons have been maintained in diverse taxonomic lineages and extensive studies carried out on the site-specific non-LTR retrotransposons R1 and R2 had shown that they have been vertically transmitted for long periods of time. Evidences of horizontal transfer emerge when the TEs and host species phylogenies are not congruent and TEs are more similar than expected on the basis of host divergence. We isolated and sequenced eight R2 elements from the stick-insects species Bacillus rossius, Bacillus grandii and Bacillus atticus. A Maximum Likelihood tree on all R2 elements compared with the phylogeny of the host species showed that all stick- insects R2s, except one isolated from Bacillus grandii grandii genome, grouped in a monophyletic cluster within the D clade. Here, Bacillus rossius, Bacillus grandii maretimi and Bacillus grandii benazzii elements exhibit an unusually low divergence. The age versus divergence analysis statistics confirmed the possibility of horizontal transfer between Bacillus rossius and the two Bacillus grandii subspecies. This is the first evidence of horizontal transfer for elements of the R2 clade and the past history of interspecific hybridisation known for these stick-insects would likely explain these data.