Genetic compatibilities, outcrossing rates and fitness consequences across life stages of the trematode Diplostomum pseudospathaceum

Many parasitic helminths exhibit mixed mating systems, and switches between self-fertilization and 24 outcrossing may be influenced by environmental conditions and parasite demography. While inbreeding 25 depression selects against the development of purely self-fertilizing populations, genetic compatibility 26 may contribute to stabilizing mixed strategies. Here we study the effects of inbreeding and genetic 27 compatibility on offspring fitness in the digenean trematode Diplostomum pseudospathaceum, a parasite 28 with a three-host life cycle. Hatching rates and infection success in two intermediate hosts, the freshwa- 29 ter snail Lymnaea stagnalis and the three-spined stickleback, Gasterosteus aculeatus, were used as proxies 30 for parasite fitness. Single trematode clones and combinations of two and three different clones were 31 allowed to reproduce sexually using naïve herring gulls (Larus argentatus) as definitive hosts. The hatched 32 larvae were used to assess the proportion of selfed and outcrossed miracidia by means of microsatellite 33 genotyping. These results were matched with hatching rates and infection success of inbred and 34 outcrossed trematodes in both intermediate hosts. Inbreeding effects were obscured by differences in 35 clone performance. In addition, clones outcrossed to a lesser extent than expected in some experimental 36 pairings, indicating the importance of genetic compatibility