Abstract
A few metazoan lineages are thought to have persisted for millions of years without sexual reproduction. If so, they would offer important clues to the evolutionary paradox of sex itself [1, 2]. Most "ancient asexuals" are subject to ongoing doubt because extant populations continue to invest in males [3-9]. However, males are famously unknown in bdelloid rotifers, a class of microscopic invertebrates comprising hundreds of species [10-12]. Bdelloid genomes have acquired an unusually high proportion of genes from non-metazoans via horizontal transfer [13-17]. This well-substantiated finding has invited speculation [13] that homologous horizontal transfer between bdelloid individuals also may occur, perhaps even "replacing" sex [14]. In 2016, Current Biology published an article claiming to supply evidence for this idea. Debortoli etal. [18] sampled rotifers from natural populations and sequenced one mitochondrial and four nuclear loci. Species assignments were incongruent among loci for several samples, which was interpreted as evidence of "interspecific horizontal genetic transfers." Here, we use sequencing chromatograms supplied by the authors to demonstrate that samples treated as individuals actually contained two or morehighly divergent mitochondrial and ribosomal sequences, revealing cross-contamination with DNA from multiple animals of different species. Other chromatograms indicate contamination with DNA from conspecific animals, explaining genetic and genomic evidence for "intraspecific horizontal exchanges" reported in the same study. Given the clear evidence of contamination, the data and findings of Debortoli etal. [18] provide no reliable support for their conclusions that DNA is transferred horizontally between or within bdelloid species.
Highlights
A [10] to 576 Eppendorf tubes for DNA extraction
They amplified a 0.6-kb region of mitochondrial cytochrome oxidase I by PCR and used this common molecular barcode [19] to delineate six molecular taxa [20], which they call ‘‘Adineta vaga species A–F.’’ They applied whole-genome amplification (WGA) to a subset of 82 samples representing a range of mtCO1 haplotypes from these ‘‘cryptic species.’’ Four nuclear marker loci were amplified again by PCR and directly Sanger sequenced
If horizontal genetic transfer (HGT) occurs so extensively among such diverse and mobile animals, it is remarkable that every donor species happened to be sampled in the same small area (300 m2) as its respective recipient, at the same time
Summary
A [10] to 576 Eppendorf tubes for DNA extraction They amplified a 0.6-kb region of mitochondrial cytochrome oxidase I (mtCO1) by PCR and used this common molecular barcode [19] to delineate six molecular taxa [20], which they call ‘‘Adineta vaga species A–F.’’ They applied whole-genome amplification (WGA) to a subset of 82 samples representing a range of mtCO1 haplotypes from these ‘‘cryptic species.’’ Four nuclear marker loci were amplified again by PCR and directly Sanger sequenced. For six samples (7.3%), sequences at different loci matched two or even three different species. The authors interpret this incongruence as ‘‘strong evidence’’ of ‘‘interspecific horizontal genetic transfers’’ from ‘‘donor species’’ to ‘‘recipient individuals’’ that ‘‘may be mediated by DNA repair through homologous recombination.’’ They claim to have discovered ‘‘an unexpected (and possibly unique). The authors interpret this incongruence as ‘‘strong evidence’’ of ‘‘interspecific horizontal genetic transfers’’ from ‘‘donor species’’ to ‘‘recipient individuals’’ that ‘‘may be mediated by DNA repair through homologous recombination.’’ They claim to have discovered ‘‘an unexpected (and possibly unique). ameiotic strategy of genetic exchange and recombination among asexual, morphologically female organisms,’’ which they ‘‘propose here to call ‘sapphomixis’ (from the name of the Greek lesbian poetess Sappho and mixis ‘mingling’).’
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