PI University of Michigan, Michigan, United States
Abstract: Understanding factors affecting parasite populations is critical for everything from conservation of endangered species to public health and agricultural policies. Of particular interest is the evolution of parasite genetic diversity, since different genotypes vary in key traits like transmission, virulence and replication rate. There is a growing body of research focused on changes in parasite population structure over the course of epidemics. However, this work has largely focused on parasite evolution in the context of single host systems, whereas most parasites exist in systems with multiple viable host species. Within these more complex communities, spillover of parasites from one host species to another is possible, which affects both epidemic dynamics and selection on the parasite population. However, there is little empirical evidence regarding how the presence of multiple hosts changes the population’s genetic structure. In this study, we aimed to understand how the presence of multiple hosts in a community changes both epidemic dynamics within that community and the parasite population structure. To address this question, we surveyed the Daphnia spp.- P. ramosa system across seven lakes in Southeast Michigan. Daphnia spp. are freshwater microcrustaceans which are distributed globally in lake communities of variable diversity. P. ramosa is a lethal castrating bacteria which infects hosts via environmentally transmitted spores and shows high host genotype specificity for infections.
Over seven years of sampling, eight Daphnia species were found across the seven lakes, of which six were observed infected with P. ramosa. Of those species infected, the highest infection prevalence across the dataset varied from 9% for Ceriodaphnia dubia to 77% for D. retrocurva. In each of the seven lakes, epidemics (infection prevalence over 1%) occured in at least two host species throughout the study period. Interestingly, epidemics within one host species did not always co-occur with epidemics in another, possibly indicating high host specificity where certain genotypes of P. ramosa can only infect certain host species. Additionally, in some cases, epidemics occurred only in non-dominant host species; if specificity did not exist, we would expect that the most abundant host species would be infected in tandem with less abundant host species. Future analyses will explore the genotypic structure of P. ramosa populations in these lake communities to provide genetic evidence of the bacteria’s degree of specificity. Overall, our study highlights the relevance of multi-host communities in the study of epidemics and the potential of those systems to alter parasite population structure.
