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Many trematode groups have a long history of systematic revision, which can make parasite identification a difficult task. The trematode parasites of muskrats are no exception. Here, we highlight the systematic issues associated with trematodes of muskrats (Ondatra zibethicus). Then, we demonstrate the utility of using both morphological and molecular tools to identify these parasites. Morphological examinations of specimens from muskrats (n = 63) first suggested that at least 4 genera were present including Echinostoma, Wardius, Quinqueserialis, and Notocotylus. For the latter 3 groups, the 28S region verified this assessment. For echinostomes, ND1 sequences revealed at least 5 genetic lineages. A particular lineage, Echinostoma trivolvis lineage b, predominated in both prevalence and intensity of infection. Molecular sequences provided a more accurate estimate of echinostome diversity in the muskrats and further support the idea that E. trivolvis is a species complex. Future studies will focus on whether there are differences in host specificity among the E. trivolvis lineages. In addition, this study has provided initial sequences that will help verify the life cycles of Wardius, Quinqueserialis, and especially, Notocotylus. By linking molecular, morphological, and life history information, we can better understand parasite diversity.

Little attention has been given to the role that introgression and hybridization have played in the evolution of parasites. Most studies are host-centric and ask if the hybrid of a free-living species is more or less susceptible to parasite infection. Here we focus on what is known about how introgression and hybridization have influenced the evolution of protozoan and helminth parasites of animals. There are reports of genome or gene introgression from distantly related taxa into apicomplexans and filarial nematodes. Most common are genetic based reports of potential hybridization among congeneric taxa, but in several cases, more work is needed to definitively conclude current hybridization. In the medically important Trypanosoma it is clear that some clonal lineages are the product of past hybridization events. Similarly, strong evidence exists for current hybridization in human helminths such as Schistosoma and Ascaris. There remain topics that warrant further examination such as the potential hybrid origin of polyploid platyhelminths. Furthermore, little work has investigated the phenotype or fitness, and even less the epidemiological significance of hybrid parasites.

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.

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