Comparative Population Genetics Among Trematode Parasites of Salmonids in the Pacific NorthwestEPA Grant Number: U916130
Title: Comparative Population Genetics Among Trematode Parasites of Salmonids in the Pacific Northwest
Investigators: Criscione, Charles D.
Institution: Oregon State University
EPA Project Officer: Packard, Benjamin H
Project Period: January 1, 2003 through January 1, 2006
Project Amount: $108,344
RFA: STAR Graduate Fellowships (2003) Recipients Lists
Research Category: Fellowship - Zoology , Academic Fellowships , Biology/Life Sciences
The overall objective of this research project is to compare the population genetic structure of four species of parasitic trematodes that infect salmon. Two species complete their life cycles entirely within a freshwater habitat (autogenic life cycle). Two species cycle through birds and mammals and have a greater opportunity to disperse between aquatic habitats (allogenic life cycle). The specific objective of this research project is to use mitochondrial and Internal Transcribed Spacer DNA sequence data to test if there is more gene flow and less population structure among populations of the allogenic species than of the autogenic species.
Population genetic structure (PGS) refers to the distribution of genetic variation within and among populations of a species. Genetic structure is largely controlled by the effective sizes of the populations and rates of gene flow (dispersal) among them. Consequently, an understanding of parasite PGS provides insight into evolutionary processes such as speciation, extinction, and adaptation to host defenses. From an environmental perspective, PGS is important for the dynamics of disease transmission and adaptive potential for drug resistance (Anderson and May, 1991). Nevertheless, there have been few studies on PGS in parasitic helminths (cestodes, trematodes, nematodes), and studies on the factors that influence PGS of helminth parasites are virtually nonexistent. This lack of data is surprising given the medical, veterinary, and economic (e.g., fisheries) importance of helminths (Roberts and Janovy, 2000).
My primary goal is to examine the factors that control parasite dispersal among populations. For parasitic helminths, life history characteristics of both the parasites and their hosts will influence dispersal capabilities. An ecological hypothesis put forth by Esch, et al. (1988) provides an integrated framework in which to test factors that affect parasite gene flow. In examining helminth community structure in freshwater fish, Esch, et al. (1988) distinguished between two modes of parasitic transmission: autogenic and allogenic. Autogenic parasites mature in fish and complete their life cycle entirely within an aquatic habitat. Allogenic parasites cycle through the freshwater environment, but mature in terrestrial hosts such as birds and mammals. The land and sea are ecological barriers that limit the dispersal of freshwater-dependent hosts utilized by autogenic parasites; therefore, allogenic species should have a greater dispersal potential because their terrestrial hosts may circumvent these barriers (Esch, et al., 1988). The same conceptual basis can be used to examine the effects of host movement on PGS within parasite species. Thus, the population genetics questions I will test are as follows:
1. Because of the vagility of their terrestrial hosts, do allogenic parasites have less subdivided populations (i.e., more gene flow) than autogenic parasites? I will use molecular markers from digenetic trematodes that parasitize salmonid fishes in the Pacific Northwest to test the autogenic/allogenic hypothesis. The proposed study organisms include two autogenic trematodes, Plagioporus shawi and Deropegus aspina, and two allogenic species, Nanophyetus salmincola and Apophallus donicus.
2. Are there cryptic species among the populations of proposed study organisms? Populations with restricted gene flow are most likely to be comprised of cryptic species (genetically distinct, but morphologically indistinguishable). A corollary prediction is that cryptic species are more likely to be found among parasites with autogenic life cycles.