|Beschreibung (original):||In this thesis the reproductive decisions of S. solidus in the context of mating system evolution, genetic variability and gamete exchange was studied. Microsatellite markers were used to determine if progeny had been reproduced by self- or cross-fertilization. S. solidus individuals reproduce through both selfing and outcrossing even when mating in pairs (i.e. mixed-mating). The usually low densities in their final host force worms sometimes to self-fertilize as last resort. Selfed offspring of worm lineages which have always selfed a certain proportion of their eggs (despite the availability of a partner), may produce fitter progeny than do offspring from worms that have always outcrossed when both have to self in a year of low worm density. The investment in selfing and outcrossing was influenced by mating partner size as well as by partner heterozygosity. Moreover, worms adaptively adjusted their outcrossing rate depending on whether their selfed progeny had a high or a low fitness in terms of hatching rate. This gives them a selective advantage compared to individuals that would outcross and self at random. Genetic variability in a natural population of S. solidus shows a heterozygote deficiency which is in line with the mode of reproduction found in the lab namely mixed-mating. The S. solidus system is to date one of the best established model systems to study mixed-mating in animals in the lab as well as in the field.