TY - JOUR T1 - Evidence for “inter- and intraspecific horizontal genetic transfers” between anciently asexual bdelloid rotifers is explained by cross-contamination JF - bioRxiv DO - 10.1101/150490 SP - 150490 AU - Christopher G. Wilson AU - Reuben W. Nowell AU - Timothy G. Barraclough Y1 - 2017/01/01 UR - http://biorxiv.org/content/early/2017/06/20/150490.abstract N2 - Bdelloid rotifers are a diverse group of microscopic invertebrates that are believed to have evolved for tens of millions of years without sexual reproduction. They have attracted the attention of biologists puzzled by the maintenance of sex and recombination among nearly all other eukaryotes. Bdelloid genomes show a high proportion of non-metazoan genes, acquired by horizontal transfer. This well-substantiated finding has invited speculation that homologous horizontal transfer between rotifers also may occur, perhaps at rates sufficient to “replace” the functions of sex in bdelloids. In 2016, Debortoli and colleagues claimed to supply evidence for this hypothesis (Current Biology 26, 723-32). They sampled individuals of the bdelloid genus Adineta from natural populations, extracted DNA and sequenced five marker loci. For several samples, the assignment of haplotypes to species was incongruent among loci, which the authors interpreted as evidence of “interspecific horizontal genetic transfers”. Here, we use sequencing chromatograms supplied by the authors to demonstrate that samples treated as single individuals actually contained mitochondrial and ribosomal haplotypes from two or even three animals. We also show that the putatively transferred DNA molecules share only 75% sequence identity, which is not compatible with known mechanisms of homologous recombination, or with established features of bdelloid genomes. We argue that these and other patterns are parsimoniously explained by cross-contamination of animals or DNA between sample tubes, and therefore that the study offers no reliable evidence for the hypothesis that genes are transferred either within or between these bdelloid species. In light of this and other recent cases, we recommend that work considering horizontal gene transfer in microscopic animals be conducted and evaluated with special caution in future, owing to the risk of contamination. ER -