The molecular ecology of echinostome trematodes: Elucidating the phylogenetics and transmission dynamics of a freshwater helminth parasite

Abstract

This study explored the transmission dynamics of a parasite with a complex life cycle that exhibits varying degrees of host specificity at particular life stages. I focused on echinostome trematodes, which are a widely distributed, species-rich group of internal parasites that infect a wide array of hosts and are agents of disease in amphibians, birds, and mammals. The project utilized several molecular markers evolving at different rates as well as novel ecological approaches to determine the dynamics of parasite transmission. First, we utilized nuclear and mitochondrial DNA markers to clarify echinostome systematics and to understand patterns of morphology, host use and geographic distribution among several species groups. Second, we compared host colonization patterns in nature and the laboratory and suggested that although parasites are more compatible with certain host species in a laboratory, parasite-host encounter rates mask this compatibility in nature and therefore can be a significant determinant of infection. Third, we used microsatellite and mitochondrial markers to evince the colonization routes of larval parasites between the molluscan hosts (1st intermediate and second intermediate) in the parasite life cycle. We compared clonal diversity of infection for one parasite species, E. revolutum, as well as haplotype richness among echinostome species between first and second intermediate hosts. Compared to previous studies, low levels of clonal diversity were found in both types of hosts, however higher diversity was observed in second intermediate hosts. Similarly, echinostome haplotype richness was higher in second intermediate hosts as compared to first intermediate hosts. We suggest that low host vagility and low water flow contributed to lower levels of clonal diversity relative to other trematode-host systems. In addition, host recolonization and antagonistic parasite-parasite interactions may help to limit both clonal and haplotype diversity within simultaneous first and second intermediate hosts. Morphological similarity among echinostomes, especially the larval stages, demonstrates that a molecular ecology approach is essential to understanding their biology. This work will help clarify the role of echinostomes in amphibian decline, and provide disease epidemiology models with a better understanding of the role of intermediate hosts in the distribution and maintenance of parasite genetic diversity