Summary 1. Sexual populations are expected to perform better in fluctuating environments than asexuals because recombination provides the potential to adapt to changing environments due to increased genetic variation. Nevertheless, some asexual species show comparably high levels of genotypic diversity. Such diversity might be achieved through gene flow between coexisting sexual and asexual populations or through sexual events within asexual populations. 2. Evidence for occasional sex in the flatworm Schmidtea polychroa was previously found at one specific site that is inhabited by parthenogenetic forms. There, varying rates of sex between subpopulations, reaching up to 12%, were observed. Past recurrent sexual processes left a significant genetic signature in the population genetic structure of this population. In the present study, we examined the population genetic structure of six independent metapopulations (lakes) of the freshwater planarian flatworm S. polychroa, to confirm the presence of occasional sex and that its population genetic consequences can be generalised. 3. Using microsatellites, we found varying rates of occasional sex among subpopulations. Metapopulations showed medium to high levels of genotypic diversity that correlated with the rate of sex. 4. We conclude that occasional sex has considerable consequences for population genetic structure of parthenogenetic species and promotes diversity that might allow response to the particular type of selection that is usually predicted to favour sexual reproduction. This reproductive strategy provides genetic characteristics required for selection to act on, and might, therefore, explain the success of this parthenogenetic species.

Most heat shock proteins help to cope with stress in organisms ranging from bacteria to vertebrates. Many stress types acting on the intensity of intracellular protein can induce expression of heat shock proteins. Here, we studied changes in expression level of heat shock protein 70 (Hsp70), one of the best investigated stress proteins, in response to five potential stress factors in the planarian flatworm Schmidtea polychroa: (1) homogenized planarian tissue, which releases an alarm substance that signals predation injury, (2) physical damage by puncturing, (3) a simulation of ecological competition by adding a mixture of naturally co-occurring species: one Dendrocoelum and two Polycelis flatworms, one Asellus water louse and one leech, and (4) magnesium chloride, which inhibits regeneration ability. We found that alarm substance (1), physical harm (2), and magnesium chloride (4) led to increased expression of Hsp70, while interspecific competition (3) did not result in elevated Hsp70 expression. There was no difference between the experimental negative control and two temporal controls immediately after collection and just before the experiment. Results show that Schmidtea polychroa is not sensitive to sampling and lab maintenance. However, planarian homogenate, magnesium chloride and physical harm all caused Hsp70-inducing stress. We conclude that Hsp70 quantification is appropriate to study the current stress level in planaria in response to specific conditions.

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