Cross-Contamination Explains “Inter and Intraspecific Horizontal Genetic Transfers” between Asexual Bdelloid Rotifers

Citation

Wilson CG, Nowell RW & Barraclough TG (2018) Cross-Contamination Explains “Inter and Intraspecific Horizontal Genetic Transfers” between Asexual Bdelloid Rotifers. Current Biology, 28 (15), pp. 2436-2444.e14. https://doi.org/10.1016/j.cub.2018.05.070

Abstract
Summary 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, 4, 5, 6, 7, 8, 9]. However, males are famously unknown in bdelloid rotifers, a class of microscopic invertebrates comprising hundreds of species [10, 11, 12]. Bdelloid genomes have acquired an unusually high proportion of genes from non-metazoans via horizontal transfer [13, 14, 15, 16, 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 et al. [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 more highly 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 et al. [18] provide no reliable support for their conclusions that DNA is transferred horizontally between or within bdelloid species.

Keywords
DNA contamination; scientific experimental error; data accuracy; reproducibility of results; horizontal gene transfer; homologous recombination; polymerase chain reaction; genomics; molecular evolution; asexual reproduction

Journal
Current Biology: Volume 28, Issue 15

• Wilson 2018 Curr Biol - Cross-contamination Explains 'Inter Intraspecific Horizontal Genetic Transfers' Asexual Bdelloid Rotifers