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Many parasite species use multiple host species to complete development; however, empirical tests of models that seek to understand factors impacting evolutionary changes or maintenance of host number in parasite life cycles are scarce. Specifically, Brown et al.’s (2001) mating system model, which posits multi-host life cycles are an adaptation to prevent inbreeding in hermaphroditic parasites and thus, preclude inbreeding depression, remains untested. The model assumes loss of a host results in parasite inbreeding and predicts host loss can only evolve if there is no parasite inbreeding depression. We provide the first empirical tests of this model using a novel approach we developed for assessing inbreeding depression from field-collected, parasite samples. The method compares genetically-based, selfing-rate estimates to a demographic-based selfing rate, which was derived from the closed mating system experienced by endoparasites. Results from the hermaphroditic trematode Alloglossidium renale, which has a derived 2-host life cycle, supported both the assumption and prediction of the mating system model as this highly inbred species had no indication of inbreeding depression. Additionally, comparisons of genetic and demographic selfing rates revealed a mixed mating system that could be explained completely by the parasite’s demography, i.e., its infection intensities. 

Abstract Meta‐studies on hermaphrodites have found a negative relationship between primary selfing rates and levels of inbreeding depression (ID) and, thus, generally support purging in inbred systems. However, in plants, high among‐taxa variance in ID results in no difference in the mean ID between outcrossing and mixed‐mating taxa. Selective interference likely explains high ID among mixed‐mating taxa, whereas low levels of ID among mixed‐mating taxa are not as stressed. Among animal hermaphrodites, primarily molluscs, there are little data on mixed‐mating systems. To fill a taxonomic and mating system gap, we tested for ID in a mixed‐mating tapeworm,Oochoristica javaensis. We provide a direct estimate of ID across infection of an intermediate host by comparing selfing rates at two life history stages. We found little to no evidence for ID, and the level of ID falls in line with what is reported for highly selfing species even thoughO. javaensishas mixed mating. We discuss this result within the context of kin mating inO. javaensis. Our results emphasize that primary selfing rates alone may be insufficient to classify the inbreeding history in all species when testing for a relationship to ID. Mixed‐mating taxa, and possibly some outcrossing taxa, may exhibit low levels of ID if biparental inbreeding is also driving purging. We advocate that ID studies report estimates of inbreeding history (e.g.F_ISor identity disequilibrium) from nature‐derived adult samples to provide context rather than relying on primary selfing rates alone.