| The dearth of asexual taxa above the level of species suggests that species are unable to persist and form higher-order taxa in the absence of sexual reproduction and genetic exchange. The Class Bdelloidea of the Phylum Rotifera, by far the largest metazoan taxon in which males and hermaphrodites are unknown, has long been considered a possible exception to this generalization. If bdelloid rotifers have evolved without sexual reproduction or genetic exchange they would offer a unique opportunity to test theories explaining why these processes are nearly universally maintained and to examine the consequences of evolving without them.;Here I present the results of a molecular-genetic test of the possibility that bdelloid rotifers have evolved without sexual reproduction or genetic exchange. The test is based on the expectation that the loss of these processes, in preventing the segregation of haplotypes, would result after many millions of years in genomes that lack the nearly identical alleles characteristic of sexually-reproducing species; instead, descendants of former alleles, if still present, should be highly divergent. I examined regions of several protein-coding genes, present in single copy in all invertebrates examined to date, in individual genomes of four species of bdelloid rotifers and in seven species representing the other three classes of the phylum, in which sexual reproduction is known. I found that the genomes of bdelloid rotifers, unlike the genomes of other rotifers, are characterized by highly divergent copies of genes and by the absence of nearly identical copies. For the two genes that could be analyzed phylo-genetically, all copies could be assigned to one of two lineages that began to diverge after Bdelloidea separated from the rest of the phylum but before the bdelloid radiation. Both the degree and the pattern of divergence of the lineages suggest that sexual reproduction and genetic exchange were abandoned in Bdelloidea tens of millions of years ago. |
