Paternal sex in parthenogenetic planarians: a tool to investigate the accumulation of deleterious mutations

Clonally reproducing organisms are expected to accumulate slightly deleteri ous mutations, and this has been demonstrated in RNA viruses, bacteria and unicellular algae. In this paper we present evidence for increased embryo m ortality in obligate parthenogenetic strains of the freshwater flatworm Sch midtea polychroa, possibly indicating the action of deleterious mutations. The inheritance of this fitness defect was tested by crossing parthenogens with sexuals. This is possible because both forms are simultaneous hermaphr odites that produce fertile sperm. The resulting sexual offspring showed si gnificantly increased embryo mortality in comparison to offspring of a sexu al x sexual cross. Alternatives to a mutation explanation of these results, like the degeneration of male traits in parthenogens, are being discussed. In conclusion, these results lend support to the hypothesis that sex is ad vantageous to a multicellular organism because it prevents the accumulation of deleterious mutations.