Sex and Repetitive Sequence Dynamics in Bacillus Stick Insects (Phasmida, Bacillidae)

Eukaryotic genomes are characterized by the wide occurrence of repetitive sequences; these play an important role in genome structure and variability and the reproductive biology of the host organism seems to affect their dynamics. The relationship between sex and the non-LTR retrotransposable element R2 is here studied in Bacillus stick insects. In this genus, in addition to gonochorism, reproduction is achieved through several non-canonical mechanisms. The analyses were performed in populations of the strictly gonochoric B. grandii, the facultative parthenogenetic B. rossius and the obligate parthenogenetic B. atticus, as well as in the offspring of either facultative (B. rossius) and obligate (B. atticus) parthenogenetic isolates or of crosses between gonochoric individuals of facultative parthenogenetic populations of B. rossius. Data here produced showed that R2 is present and active in all Bacillus species, irrespective of the reproductive strategy, counteracting Muller’s ratchet expectations. Moreover, the lowest values of R2 accumulation detected in the obligate unisexual B. atticus suggest that also mechanisms limiting R2 load are active. The higher R2 accumulations found in facultative parthenogenetics, with respect to the values scored in obligate ones, could be due to the different mechanism of parthenogenesis. Furthermore, models of parental-offspring inheritance evidenced that also recombination and selection appear to control R2 proliferation. These results highlighted the presence of unisexuals bearing transposable element (TE) insertions. Their survival may be allowed from mechanisms able to keep the TE load low which, in addition to the recombination and selection, could be represented from silencing mechanisms and selection of less virulent TEs. Muller’s ratchet predictions were counteracted also in the analysis conducted to determine the abundance of repetitive sequences in the genomes of B. grandii, B. rossius and B. atticus. In fact, in the latter TEs content was comparable to the one found in the other species.