On 2016 May 18, MATTHEW MESELSON commented:

Meiotic Sex in Bdelloid Rotifers

Debortoli et al. conclude that the patchwork pattern of sequences shared within the group of three isolates of the bdelloid Adineta vaga they sequenced is “…unlikely to arise in cases of PTH (Oenothera-like) meiosis since haplotypes are transferred as entire blocks…” and therefore that “Genetic exchange among bdelloid rotifers is more likely due to horizontal gene transfer than to meiotic sex”. But this assumes without justification that homologous HGT cannot occur in species with Oenothera-like meiosis, for which we have reported evidence in the bdelloid Macrotrachela quadricornifera (Signorovitch et al. 2015 Genetics 200: 581-590). And it does not take account of the possibility that gene conversion followed by outcrossing would also contribute to such a patchwork pattern, even in Oenothera-like systems.

Moreover, the set of three individuals studied by Debortoli et al., in which the shared sequences are considerably diverged, is not well suited to the detection of sex in an outcrossing population that may include numerous distinct Oenothera-like complexes. For that purpose, one should select individuals whose shared sequences are identical or nearly so in order to enrich for direct descendants of the F1 from a particular cross. Otherwise, further crossing is likely to replace shared complexes with others, removing the evidence for the transfer of entire haplotypes. It is therefore important to note that the shared sequences in the three individuals we studied were either identical or very nearly so, allowing us to observe the specific and unusual pattern of sharing expected for Oenothera-like meiosis.

Debortoli et al. suggest that HGT of very long fragments, rather than sexual transfer of entire haplotypes, may explain the pattern of sharing we observed. Considering the large size of the M. quadricornifera genome, some 1500 mb, and the fact that there are 10 chromosomes, it is exceedingly unlikely that the sequences from all four regions we studied reside on one of the horizontally transferred segments of DNA required by their suggestion and that the four allelic sequences reside on the other. Moreover, the results of FISH in other bdelloid species suggests that at least three and quite possibly all four regions we studied reside on separate chromosomes.

While awaiting full genome sequencing of the allele-sharing isolates of M. quadricornifera, present evidence argues strongly for the occurrence of sexual reproduction with Oenothera-like meiosis.

Ana Signorovitch, Jae Hur, Eugene Gladyshev, Matthew Meselson

On 2016 May 18, MATTHEW MESELSON commented:

Meiotic Sex in Bdelloid Rotifers

Debortoli et al. conclude that the patchwork pattern of sequences shared within the group of three isolates of the bdelloid Adineta vaga they sequenced is “…unlikely to arise in cases of PTH (Oenothera-like) meiosis since haplotypes are transferred as entire blocks…” and therefore that “Genetic exchange among bdelloid rotifers is more likely due to horizontal gene transfer than to meiotic sex”. But this assumes without justification that homologous HGT cannot occur in species with Oenothera-like meiosis, for which we have reported evidence in the bdelloid Macrotrachela quadricornifera (Signorovitch et al. 2015 Genetics 200: 581-590). And it does not take account of the possibility that gene conversion followed by outcrossing would also contribute to such a patchwork pattern, even in Oenothera-like systems.

Moreover, the set of three individuals studied by Debortoli et al., in which the shared sequences are considerably diverged, is not well suited to the detection of sex in an outcrossing population that may include numerous distinct Oenothera-like complexes. For that purpose, one should select individuals whose shared sequences are identical or nearly so in order to enrich for direct descendants of the F1 from a particular cross. Otherwise, further crossing is likely to replace shared complexes with others, removing the evidence for the transfer of entire haplotypes. It is therefore important to note that the shared sequences in the three individuals we studied were either identical or very nearly so, allowing us to observe the specific and unusual pattern of sharing expected for Oenothera-like meiosis.

Debortoli et al. suggest that HGT of very long fragments, rather than sexual transfer of entire haplotypes, may explain the pattern of sharing we observed. Considering the large size of the M. quadricornifera genome, some 1500 mb, and the fact that there are 10 chromosomes, it is exceedingly unlikely that the sequences from all four regions we studied reside on one of the horizontally transferred segments of DNA required by their suggestion and that the four allelic sequences reside on the other. Moreover, the results of FISH in other bdelloid species suggests that at least three and quite possibly all four regions we studied reside on separate chromosomes.

While awaiting full genome sequencing of the allele-sharing isolates of M. quadricornifera, present evidence argues strongly for the occurrence of sexual reproduction with Oenothera-like meiosis.

Ana Signorovitch, Jae Hur, Eugene Gladyshev, Matthew Meselson