Abstract
The nature of gene flow in parasites with complex life cycles is poorly understood, particularly when intermediate and definitive hosts have contrasting movement potential. We examined whether the fine-scale population genetic structure of the diphyllobothriidean cestode Schistocephalus solidus reflects the habits of intermediate threespine stickleback hosts or those of its definitive hosts, semi-aquatic piscivorous birds, to better understand complex host-parasite interactions. Seventeen lakes in the Cook Inlet region of south-central Alaska were sampled, including ten in the Matanuska-Susitna Valley, five on the Kenai Peninsula, and two in the Bristol Bay drainage. We analyzed sequence variation across a 759 bp region of the mitochondrial DNA (mtDNA) cytochrome oxidase I region for 1,026 S. solidus individuals sampled from 2009-2012. We also analyzed allelic variation at 8 microsatellite loci for 1,243 individuals. Analysis of mtDNA haplotype and microsatellite genotype variation recovered evidence of significant population genetic structure within S. solidus. Host, location, and year were factors in structuring observed genetic variation. Pairwise measures revealed significant differentiation among lakes, including a pattern of isolation-by-distance. Bayesian analysis identified three distinct genotypic clusters in the study region, little admixture within hosts and lakes, and a shift in genotype frequencies over time. Evidence of fine-scale population structure in S. solidus indicates that movement of its vagile, definitive avian hosts has less influence on gene flow than expected based solely on movement potential. Observed patterns of genetic variation may reflect genetic drift, behaviors of definitive hosts that constrain dispersal, life history of intermediate hosts, and adaptive specificity of S. solidus to intermediate host genotype.
Highlights
We examined whether the fine-scale population genetic structure of the diphyllobothriidean cestode Schistocephalus solidus reflects the habits of intermediate threespine stickleback hosts or those of its definitive hosts, semi-aquatic piscivorous birds, to better understand complex host-parasite interactions
This study tested the hypotheses that the genetic structure of S. solidus reflects the habits of its intermediate host or, alternatively, those of its definitive hosts
We evaluated spatial and temporal patterns of genetic variation and found evidence of significant population genetic structure in S. solidus infecting threespine sticklebacks in lakes throughout south-central Alaska
Summary
Gene flow can be a key determinant of evolutionary potential, for organisms engaging in interactions shaped by adaptation Landscape Genetics of Schistocephalus Parasites potential can be influenced, for example, by dispersal promoting the influx of new alleles affecting interactions between a highly specialized parasite and its host [5]. Barriers to gene flow can influence the nature of host-parasite interactions by impeding the influx of adaptive or maladaptive alleles. Adaptive interactions may reciprocally affect gene flow, which in turn can give rise to dynamic changes in evolutionary potential. Characterizing gene flow in parasites, which is often understudied relative to their hosts, can help identify factors shaping adaptive interactions
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