Little to no inbreeding depression in a tapeworm with mixed mating

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 though O. javaensis has mixed mating. We discuss this result within the context of kin mating in O. 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_IS or identity disequilibrium) from nature-derived adult samples to provide context rather than relying on primary selfing rates alone.