High fecundity and low fertility in parthenogenetic planarians

In hermaphrodites, the cost of sexuality can favor the spread of parthenoge nesis in two ways. First, it can promote higher female fecundity in parthen ogens that have reduced allocation to the male function. Second, if parthen ogens have retained a fertile male function, they can spread genes for part henogenesis into a coexisting sexual population. We present evidence for bo th effects in a natural population of the flatworm Schmidtea polychroa. Par thenogens, which have a reduced male function, had 42% higher female fecund ity than coexisting sexuals. New, presumably parthenogenetic, triploids aro se out of the diploid sexual population at a frequency of 1.3%, probably as a result of gene flow from parthenogens to sexuals. However, we could also identify a strong compensating fitness benefit for sexuals: they had subst antially higher female fertility than coexisting parthenogens, both in term s of cocoon fertility (93% and 74% respectively) and offspring per fertile cocoon (3.6 and 2.8 respectively).