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

Abstract

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.

What does the geography of parthenogenesis teach us about sex?

“…Alternatively, the generation of new asexual genotypes can still be ongoing, for instance via mutation [64], continuing hybridization of, or with, the sexual parents [65], contagious asexuality via endosymbiont transmission [66] or rare crossings with sexuals (e.g. [42] in hermaphrodite flatworms; [67] in Daphnia and [68] in Artemia owing to rare parthenogenetic sons), or forms of 'parasex' [69] such as horizontal gene transfer between individuals (bdelloid rotifers, see [70]) or introgression of environmental DNA (anhydrobiotic rotifers or tardigrades [71]). …”

Condition-dependent sex: who does it, when and why?

“…Many organisms that were considered strictly asexual were recently discovered to be exchanging DNA, most notably the bdelloid rotifers (the ancient asexual scandal [64]). After many years of intensive research, recent genomic tools allowed the identification of massive genetic exchange [65] through atypical meiosis [66] and/or horizontal gene transfer [67]. Thus, bdelloid rotifers are in a sense facultative sexuals.…”

General environmental heterogeneity as the explanation of sexuality? Comparative study shows that ancient asexual taxa are associated with both biotically and abiotically homogeneous environments

“…Alternative ways of exchange of genetic information could theoretically substitute sexual reproduction and thus were repeatedly proposed as the key adaptation to asexuality (Boschetti, Pouchkina‐Stantcheva, Hoffmann, & Tunnacliffe, 2011; Butlin, Schön, & Griffiths, 1998; Debortoli et al., 2016; Gladyshev & Meselson, 2008; Schwander, 2016). However, we identified this factor only once in the AAs included in our study (i.e., in one of eight cases), namely in Bdelloidea that experience intensive horizontal gene transfer (Boschetti et al., 2011; Debortoli et al., 2016; Gladyshev & Meselson, 2008).…”

“…However, we identified this factor only once in the AAs included in our study (i.e., in one of eight cases), namely in Bdelloidea that experience intensive horizontal gene transfer (Boschetti et al., 2011; Debortoli et al., 2016; Gladyshev & Meselson, 2008). Another mechanism of genetic exchange, parasexuality (sensu Pontecorvo, 1954), was proposed in some contested ancient asexuals—Glomales (Croll & Sanders, 2009), Tricholomataceae and Lepiotaceae (Mikheyev, Mueller, & Abbot, 2006), and certain protists (Birky, 2009).…”