Harvard University, Department of Molecular and Cellular Biology
"Are There Any Ancient Asexual Eukaryotes? Evidence of Sexuality and Atypical Meiosis in Bdelloid Rotifers"
Abstract: The relatively few eukaryotic species that appear to be completely asexual generally have close sexual relatives, implying that the loss of sex leads to early extinction. Against this generalization the bdelloid rotifers, common freshwater invertebrates of ancient origin, have appeared to be a major exception. Despite much observation, neither males, hermaphrodites nor mating have ever been documented, reproduction being from eggs produced by mitosis, posing a problem for the large body of theory seeking to explain the evolutionary advantage of sex. Recently, however, evidence has been obtained for sexual reproduction and an atypical form of meiosis in a mitochondrial clade of the bdelloid morphospecies Macrotrachela quadricornifera. It is suggested that bdelloids are cyclic parthenogens and that a principal advantage they derive from outcrossing is the restoration of heterozygosity lost by gene conversion, while lines that fail to outcross suffer early extinction.
Signorovitch, et al. 2015 Allele Sharing and Evidence for Sexuality in a Mitochondrial Clade of Bdelloid Rotifers. Genetics 200: 581-590.