Abstract
Horizontal transfer (HT) is an event in which the genetic material is transferred from one species to another, even if distantly related, and it has been demonstrated as a possible essential part of the lifecycle of transposable elements (TEs). However, previous studies on the non-LTR R2 retrotransposon, a metazoan-wide distributed element, indicated its vertical transmission since the Radiata-Bilateria split. Here we present the first possible instances of R2 HT in stick insects of the genus Bacillus (Phasmida). Six R2 elements were characterized in the strictly bisexual subspecies B. grandii grandii, B. grandii benazzii and B. grandii maretimi and in the obligatory parthenogenetic taxon B. atticus. These elements were compared with those previously retrieved in the facultative parthenogenetic species B. rossius. Phylogenetic inconsistencies between element and host taxa, and age versus divergence analyses agree and support at least two HT events. These HT events can be explained by taking into consideration the complex Bacillus reproductive biology, which includes also hybridogenesis, gynogenesis and androgenesis. Through these non-canonical reproductive modes, R2 elements may have been transferred between Bacillus genomes. Our data suggest, therefore, a possible role of hybridization for TEs survival and the consequent reshaping of involved genomes.
Highlights
Mechanisms leading to HT are still unclear, different ways have been observed/suggested: from Transposable elements (TEs) intrinsic features to vector-mediated transmission and to host-parasite and/or trophic relationship[6,12,14,15,16,17]
One R2 element was found in B. atticus (R2Ba); it is 3507 bp long, excluding the 3′terminal poly-(A) tail of nine nucleotides, with an open reading frame (ORF) of 3177 bp encoding a protein of 1058 amino acids
The analysis of the two proteins suggests they are part of a 3243 bp long ORF that encodes a protein of 1079 amino acids in which a frame shift mutation occurred
Summary
Mechanisms leading to HT are still unclear, different ways have been observed/suggested: from TEs intrinsic features (such as the presence of retrovirus-like proteins) to vector-mediated transmission and to host-parasite and/or trophic relationship[6,12,14,15,16,17]. R2 is subject to significant changes in copy number, even within a single species, due to its rapid turnover[24,25] This may account for the overall phylogenetic incongruence with host species: an R2 phylogenetic analysis across metazoan, suggested that paralogous lineages replacements and extinctions during the long-term evolution of this non-LTR element may explain its distribution, without assuming horizontal transfer events[22]. Bacillus species show a wide range of reproductive strategies, from strict bisexuality (in B. grandii) to facultative (B. rossius) or obligatory (B. atticus) parthenogenesis. These species gave origin to inter-specific hybrid taxa, reproducing either through obligate parthenogenesis or hybridogenesis[26]. The analyses of the evolutionary history of all retrieved Bacillus R2 lineages suggest the occurrence of HT events that can be explained taking into account the hybridogenetic mechanism
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