Fitness consequences of selfing and outcrossing in the cestode Schistocephalus solidus

Mixed-mating, that is reproduction by both self-fertilization and cross-fertilization is common in hermaphroditic parasites. Its maintenance poses, however, a problem for evolutionary biology. The tapeworm Schistocephalus solidus Müller 1776, served as a model to study experimentally the consequences of selfing and outcrossing in its 2 consecutive intermediate hosts, a copepod (Macrocyclops albidus Jurine) and the three-spined stickleback fish (Gasterosteus aculeatus). Size-matched tapeworms were allowed to reproduce either alone or in pairs in an in vitro system that replaced the definitive bird host's gut. Selfed eggs from singletons had a 4 times lower hatching success than outcrossed eggs from pairs. Outcrossed offspring achieved both a higher infection success and a higher weight in the copepod, and a higher number of parasites per host in both intermediate hosts, but only under competition. Outcrossed offspring were generally more successful. If a S. solidus plerocercoid has a partner in the bird's gut, they should outcross unless they differ in size and thus cannot solve the Hermaphrodite's Dilemma cooperatively. Using microsatellite markers, the proportion of selfed offspring and the total reproductive output of each worm within pairs varying in mean weight and in weight difference was measured. Worms produced more selfed offspring not only with increasing weight difference as expected but also with decreasing total weight of the pair. If small worms were selfed, they have already purged deleterious mutations and would thus be better selfers in a year with low parasite density when worms cannot find partners. To maintain this advantage they should self a higher proportion of their eggs even with a partner. Here I review recent exprimental evidence.