(l to r) Matthew Meselson, Ana Signorovitch, and Eugene Gladyshev; (Jae Hur not shown)
Nearly all species of animals and plants reproduce sexually, either exclusively or alternating with parthenogenesis. The relatively few species that appear to be completely asexual have close sexually-reproducing relatives, indicating that asexual lines do not persist for long and that sexual reproduction is essential for long-term evolutionary success. An apparent exception to this conclusion is a class of common fresh-water invertebrates that arose tens of millions of years ago, the bdelloid rotifers. Despite a great deal of investigation in the laboratory and in the field, neither bdelloid males nor hermaphrodites have ever been documented, the only known means of bdelloid reproduction being via eggs produced by mitosis. Although citing alternative explanations, we have previously interpreted certain bdelloid characteristics as further support for asexuality. Nevertheless, the possibility remained that bdelloids reproduce sexually under unknown or inadequately investigated conditions.
Although there are definitive tests for sexual reproduction based on DNA sequence comparisons between individuals, they had not previously been applied to bdelloid rotifers. We therefor conducted such a test within a group of closely-related bdelloids collected at diverse sites in northeast United States. Comparing homologous genomic regions in different individuals, we found a pattern of sequence identity or near identity known as “allele sharing”, constituting clear evidence of sexual reproduction in the fairly recent ancestry of some of the individuals tested.
We also found evidence that during bdelloid sexual reproduction entire haploid genomes are passed intact from parents to progeny, without assortment of chromosomes and without crossing-over, indicating a non-standard form of meiosis, well known in certain plants, in which meiotic segregation is accomplished without requiring the existence of homologous chromosome pairs.
What accounts for the evolutionary advantage of sex is a major unsettled problem in biology and the existence of ancient asexuals would pose an additional complexity for hypotheses to explain it. The finding that bdelloid rotifers are not completely asexual therefore removes what had appeared to be a serious problem for evolutionary theory.
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