Parasite survey of a Daphnia hybrid complex: host-specificity and environment determine infection

Hybridization between species is a common phenomenon in plants and animals. If parasites prevalence differs for hybrids and parental species (i.e. taxa) there may be large consequences for relative hybrid fitness. Some hybrid complexes have been investigated for infection, and complex-specific differences in parasites prevalence have been detected. Based on results from a field study of a hybridizing Daphnia population from a single lake it has been hypothesised that permanently over- or under-infected hybrids do not exist. The observed field-patterns are only temporal because taxa, in addition to single genotypes, might be a subject of parasite driven host frequency-dependent selection, thus parasites will track any common taxon within a hybrid complex. In the present study, hybridizing Daphnia populations from 43 lakes were screened for parasite infections to test for an indirect evidence for coevolutionary cycles. It was hypothesised that due to time lags between evolution of resistance in host and evolution of the parasite towards tracking common host taxon, the same Daphnia taxon will be over-infected in some lakes, while being under-infected in others. Two of the four parasite species were specialists: their prevalence differed among coexisting Daphnia taxa. Various infection patterns detected across spatially segregated hybridizing Daphnia populations are consistent with theoretical predictions for coevolutionary cycles. Thus the infection patterns, as observed under natural conditions, are temporal and unstable. Additionally, the spatial distribution of the four parasite species was analysed with respect to habitat differences. The results show that the presence of a particular parasite on a host taxon was determined not only by host-specificity of the parasite, but also by host-habitat relations.