Role of parasite transmission in promoting inbreeding: I. Infection intensities drive individual parasite selfing rates.

Among parasitic organisms, inbreeding has been implicated as a potential driver of host-parasite co-evolution, drug-resistance evolution and parasite diversification. Yet, fundamental topics about how parasite life histories impact inbreeding remain to be addressed. In particular, there are no direct selfing-rate estimates for hermaphroditic parasites in nature. Our objectives were to elucidate the mating system of a parasitic flatworm in nature and to understand how aspects of parasite transmission could influence the selfing rates of individual parasites. If there is random mating within hosts, the selfing rates of individual parasites would be an inverse power function of their infection intensities. We tested whether selfing rates deviated from within-host random mating expectations with the tapeworm Oochoristica javaensis. In doing so, we generated, for the first time in nature, individual selfing-rate estimates of a hermaphroditic flatworm parasite. There was a mixed-mating system where tapeworms self-mated more than expected with random mating. Nevertheless, individual selfing rates still had a significant inverse power relationship to infection intensities. The significance of this finding is that the distribution of parasite infection intensities among hosts, an emergent property of the transmission process, can be a key driver in shaping the primary mating system, and hence the level of inbreeding in the parasite population. Moreover, we demonstrated how potential population selfing rates can be estimated using the predicted relationship of individual selfing rates to intensities and showed how the distribution of parasites among hosts can indirectly influence the primary mating system when there is density-dependent fecundity.