Bdelloid rotifers revisited.

S exual reproduction is one of the most striking features of the biological world. The great majority of the animals and plants with which we are most familiar reproduce at least part of the time by means of sexual reproduction with meiosis, recombination, and fertilization. But many eukaryotes reproduce asexually part of the time, and some are exclusively asexual (1, 2). Two articles in a recent issue of PNAS (3, 4) answered some questions about one such group, the bdelloid rotifers, and raised some new questions. Rotifers are microscopic freshwater invertebrates. The class Bdelloidea (5, 6) consists entirely of females reproducing by apomixis, in which diploid eggs produced by mitotic division develop parthenogenetically into females. Their closest relatives are in the class Monogononta, which reproduce mostly by apomictic parthenogenesis, with an occasional one-generation sexual cycle. The bdelloids are believed to descend from a parthenogenetic female monogonont that lost the ability to enter a sexual cycle. Fossil evidence shows that the bdelloids are at least 35–40 million years old (7); the diversity of their gene sequences (8) suggests they are more than twice that age. In that time, the descendants of the first bdelloid diversified into 360 species and occupied nearly every freshwater habitat on every continent (5, 6).