Geographic and host-mediated population genetic structure in a cestode parasite of the three-spined stickleback

Abstract

Comparative studies of genetic diversity and population structure can shed light on the ecological and evolutionary factors that influence host-parasite interactions. Here we examined whether geography, time and genetic variation in Alaskan three-spined stickleback (Gasterosteus aculeatus Linneaus) hosts shape the population genetic structure of the diphyllobothridean cestode parasite Schistocephalus solidus (M€ uller, 1776). Host lineages and haplotypes were identified by sequencing the mitochondrial cytochrome b gene, and host population structure was assessed by Bayesian clustering analysis of allelic variation at 11 microsatellite loci. Parasite population structure was characterized according to allelic variation at eight microsatellite loci. Mantel tests and canonical redundancy analysis were conducted to evaluate the proportion of parasite genetic variation attributable to time and geography vs. host lineage, haplotype, and genotypic cluster. Host and parasite population structure were largely discordant across the study area, probably reflecting differences in gene flow, environmental influences external to the host, and genomic admixture among host lineages. We found that geography explained the greatest proportion of parasite genetic variation, but that variation also reflects time, host lineage, and host haplotype. Associations with host haplotypes suggest that one parasite genotypic cluster exhibits a narrower host range, predominantly infecting the most common host haplotypes, whereas the other parasite cluster infects all haplotypes equally, including rare haplotypes. Although experimental infection trials might prove otherwise, distributional differences in hosts preferentially infected by S. solidus could underlie the observed pattern of population structure.

Population genetic structure and demographic history ofStreptococcus mutans(Bacteria: Streptococcaceae)

“…Considering all of the above information, it can be concluded that Japan and Thailand have populations derived from migrations of different Asiatic people, and that Finland has two different human historical patterns, although none reach the level of admixture that occurred in Argentina, which would be reflected in the high levels of admixed ancestry detected in S. mutans. On the other hand, studies about hostparasite relationships have shown that the host's geographical distribution and behaviour, as well as the environmental conditions where the host lives, could predict the population genetic structure of the parasite (Lagrue et al, 2015;Strobel et al, 2016). Accordingly, Didelot & Maiden (2010) suggested that recombination in bacteria can be high during the period of adaptation to new environments or lifestyles, followed by low recombination levels once adapted.…”

Estimating effective population size for a cestode parasite infecting three-spined sticklebacks

“…We combined existing microsatellite data for infected three-spined stickleback fish and corresponding S. solidus parasites collected from 2006 to 2012 (Sprehn et al ., 2015; Strobel et al ., 2016) with new data on parasites obtained from stickleback hosts collected from 2013 to 2015. Following specimen collection protocols in Sprehn et al .…”

“…We used data from 11 microsatellite loci (Strobel et al ., 2016) from 157 fish collected between 2010 and 2012 to characterize host genetic diversity and contemporary N e . To ensure consistency with work done on S. solidus parasites, we recalculated expected heterozygosity ( H e ) in Arlequin v. 3.5.2.2 (Excoffier and Lischer, 2010) and average number of alleles ( N a ), rarefied allelic richness ( A R ) and Shannon diversity ( S H ) in MSAnalyzer v. 4.05 (Dieringer and Schlštterer, 2003).…”

“…Consistent with this, experimental cross infection trials between sympatric and allopatric pairs of three-spined stickleback and S. solidus have yielded evidence of reciprocal evolution of host resistance and parasite virulence (Kalbe et al ., 2016). Evidence of discordant patterns of geographic differentiation suggests, however, that genetic variation – and thus N e – in S. solidus may instead be largely governed by factors external to stickleback hosts (Strobel et al ., 2016).…”

“…In this study, we leveraged and extended prior assessments of genetic variation (Sprehn et al ., 2015; Strobel et al ., 2016) to characterize contemporary N e in S. solidus . Building on prior findings that S. solidus parasites exhibit much weaker population genetic structure than three-spined stickleback hosts across a suite of Alaskan lakes (Sprehn et al ., 2015; Strobel et al ., 2016), our aim was to determine the range and magnitude of contemporary N e exhibited by S. solidus in comparison to stickleback hosts. We also set out to test the hypothesis that factors external to stickleback hosts exert greater influence on parasite N e than do host attributes (e.g.…”