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A number of parthenogenetic organisms appear to have survived for extended periods in spite of the clonal inheritance factor that usually increases the risk of the extinction of the clonal population. One reason for this could be that parthenogens sometimes reproduce through sex-like processes and to better understand the mechanism involved a study of the sex-like processes in the planarian flatworm 'Schmidtea (Dugesia) polychroa was undertaken.

An analysis of small-scale and large-scale genetic structure within and between natural lake populations is put forth. The results show that individual dispersal of Schmidtea polychroa is limited but there is gene flow between subpopulations from the same lake.

Hermaphroditism can outcompete gonochorism when mating opportunities are unpredictable and rare, resulting in diminishing returns for the male function. But what happens to hermaphrodites with frequent mating chances? Empirical data from frequently mating, internally fertilising hermaphrodites suggest that such systems often show copulatory mechanisms that are very costly for the sperm-receiver. Here, I ask whether hermaphrodites are inherently prone to escalations of this kind, and whether costly matings make a transition to gonochorism more likely. Analytical and individual-based models reveal four effects. (1) The chance of obtaining male fitness makes hermaphrodites 'mate like males'. As a result, mating rates are generally higher in hermaphrodites than in gonochorists under comparable conditions because in the latter females usually limit male mating rates. (2) Higher willingness to mate makes hermaphrodites accept higher mating costs. (3) If higher paternity can be obtained by harming a sperm-receiving partner, hermaphrodites develop higher levels of harm than males do. (4) Increased male harm suppresses mating rates in both hermaphrodites and gonochorists. In hermaphrodites it leads to a female biased sex allocation. As a result, hermaphrodites may still produce more eggs than gonochorists, despite higher losses of resources due to male harm. These results can explain escalated mating mechanisms in various hermaphroditic systems, but also suggest that the effect of male harm on the transition to gonochorism may be weak. I shall present results in which transitions between hermaphrodites and gonochorists are simulated with and without male harm under various mating scenarios.

Hermaphrodites may have sexual conflicts because of opposing sexual conflicts within individuals, rather than between them. It is thought that physically damaging sex, which occurs in the marin flatworm Pseudoceros bifurcus, may be more favoured in hermaphrodites than in separate sex species. Hermaphrodites inject sperm under the partner's skin, as this offers direct access to eggs, although females bear the cost of would healing and have not control over fertilization. Studies of P. bifurcus indicate that rearing up has specifically evolved to interact with a sexually responsive partner.