Genetic Exchange among Bdelloid Rotifers Is More Likely Due to Horizontal Gene Transfer Than to Meiotic Sex

Although strict asexuality is supposed to be an evolutionary dead end, morphological, cytogenetic, and genomic data suggest that bdelloid rotifers, a clade of microscopic animals, have persisted and diversified for more than 60 Myr in an ameiotic fashion. Moreover, the genome of bdelloids of the genus Adineta comprises 8%-10% of genes of putative non-metazoan origin, indicating that horizontal gene transfers are frequent within this group and suggesting that this mechanism may also promote genetic exchanges among bdelloids as well. To test this hypothesis, we used five independent sequence markers to study the genetic diversity of 576 Adineta vaga individuals from a park in Belgium. Haplowebs and GMYC analyses revealed the existence of six species among our sampled A. vaga individuals, with strong evidence of both intra- and interspecific recombination. Comparison of genomic regions of three allele-sharing individuals further revealed signatures of genetic exchanges scattered among regions evolving asexually. Our findings suggest that bdelloids evolve asexually but exchange DNA horizontally both within and between species. Debortoli et al. analyze patterns of allele sharing to delineate cryptic species in the bdelloid rotifer Adineta vaga. They find evidence of inter- and intraspecific genetic exchanges interspersed with chromosome regions bearing signatures of asexual evolution, suggesting that bdelloids exchange DNA horizontally rather than via meiotic sex.