River systems are unique, dendritic ecological networks (DENs) characterized by unidirectional stream flow. The structural characteristics of river systems are known to influence the ecology and evolution of their inhabitants at multiple scales, however little is known about how these systems influence the dispersal and transmission of parasites with complex life cycles. For parasites with complex life cycles, the most mobile host in the life cycle is predicted to exert the most influence on population connectivity. Thus, hosts with high dispersal abilities may negate the influence of stream drift on trematode parasites by facilitating upstream dispersal. We examine the effect of DENs and unidirectional stream drift on the population structure on Renifer aniarum, a trematode parasite with a complex life cycle that infects the mouths of watersnakes as a definitive host. Surveys were conducted at 13 sites distributed along 400 km of three reaches within the river system. Collection sites were selected to encompass locations upstream of a major confluence, their two tributaries, and downstream of the confluence. Three hundred and sixty nine watersnakes were captured and 531 parasites were genotyped at 12 microsatellite markers and used in a preliminary analysis for genetic diversity and population structure.
Results/Conclusions
Genetic diversity of R. aniarum was high for both observed heterozygosity and allelic richness (HO ranged from 0.638 ± 0.044 to 0.724 ± 0.041; AR ranged from 9.516 to 10.85). Genetic diversity was not significantly different between upstream and downstream locations, which is consistent with the dispersal abilities of the host. However, multiple analyses of spatial genetic variation suggest there is genetic subdivision among the populations, which is contrary to the expected population structure of a parasite with a highly mobile host. The results of the STRUCTURE analysis indicate three primary genetic clusters: (1) northern Colorado and Elm Creek, (2) Northern Colorado downstream of Elm Creek, and (3) southern Colorado and the Concho River. Thus, there is significant admixture between the lower Colorado and Concho Rivers but limited parasite gene flow between the northern and southern Colorado, despite a shorter distance between collection sites in this area. FST values are also highest between northern Colorado collection sites and the Concho River, Concho tributary and Southern Colorado. The prolonged periods of no water flow experienced by the northern-most sites in the Colorado River may be driving the pattern of structure in the parasite, despite the dispersal ability of the definitive host.