MILINSKI, M; Max-Planck-Institute of Limnology, Pl�n, Germany: Fitness consequences of selfing and outcrossing in the cestode Schistocephalus solidus

S. solidus is a simultaneous hermaphroditic cestode, which passes through two sequential intermediate hosts, i.e. a copepod and the three-spined stickleback fish, until it reproduces in the gut of its definitive host, i.e. any fish-eating bird, which can be mimicked by an in vitro system in the lab. When alone in the �bird� the cestode produced self-fertilized (selfed) eggs. With a partner mostly cross-fertilized (outcrossed) eggs were produced. However, all worms in pairs engaged in some self-fertilization, although they had the opportunity to outcross, i.e. they adopted a mixed-mating strategy. Selfed eggs produced by singleton worms had a much lower hatching success than outcrossed eggs. When copepods were exposed singly to selfed and outcrossed parasites simultaneously, outcrossed parasites (coracidia of S. solidus) had a higher infection success and faster development in the host. A similar result was obtained, when single juvenile sticklebacks were simultaneously exposed to two experimentally infected copepods, one harboring outcrossed the other selfed parasites (procercoids of S. solidus): infected fish contained significantly more outcrossed than selfed parasites. Thus, outcrossed parasides have higher fitness prospects. Why did all pairs of S. solidus nevertheless produce some selfed offspring?