Phylogenetic evidence for a single long-lived clade of crustacean cyclic parthenogens and its implications far the evolution of sex

The short-term advantages of sexual reproduction are unclear, but the exist ence of groups that are capable of producing either meiotic or ameiotic egg s (cyclic parthenogenesis, CP) might indicate that short-term advantages to sex exist. Alternatively, CP might be an unstable transitory stage between asexuality and sex, or a phylogenetically favoured life cycle (i.e. clade selection). The extensive knowledge of breeding systems and population gene tics in branchiopod crustaceans makes them a useful group to test phylogene tic predictions of these hypotheses. Several proponents favour the hypothes is that CP has evolved multiple times in five orders of branchiopod crustac eans. We inferred the first robust branchiopod phylogeny from nuclear rRNA sequence (small-subunit and large-subunit), morphology, and complex rRNA st em-loop structures to assess the phylogenetic distribution of cyclic parthe nogenesis. The sequence-based, structural rRNA and total evidence phylogeni es are concordant and suggest that cyclic parthenogenesis arose once in the branchiopods, that this clade is long-lived (at least since the Permian), and that it has radiated extensively into nearly every aqueous habitat with out reverting to strict sexuality and only rarely transforming to strict as exuality. These results are consistent with the clade selection hypothesis but inconsistent with the predictions of the hypothesis that CP is a transi tory stage that leads to strict sexual reproduction. The evidence also indi cates that clade selection for CP is a viable alternative explanation for t he maintenance of sex in CP life cycles.