The comparative genetic structure of hosts and their parasites has important implications for their coevolution, but has been investigated in relatively few systems. In this study, we analysed the genetic structure and diversity of the New Zealand intertidal snail Zeacumantus subcarinatus (n = 330) and two of its trematode parasites, Maritrema novaezealandensis (n = 269) and Philophthalmus sp. (n = 246), using cytochrome c oxidase subunit I gene (COI) sequences. Snails and trematodes were examined from 11 collection sites representing three regions on the South Island of New Zealand. Zeacumantus subcarinatus displayed low genetic diversity per geographic locality, strong genetic structure following an isolation by distance pattern, and low migration rates at the scale of the study. In contrast, M. novaezealandensis possessed high genetic diversity, genetic homogeneity among collection sites and high migration rates. Genetic diversity and migration rates were typically lower for Philophthalmus sp. compared to M. novaezealandensis and it displayed weak to moderate genetic structure. The observed patterns likely result from the limited dispersal ability of the direct developing snail and the utilization of bird definitive hosts by the trematodes. In addition, snails may occasionally experience long-distance dispersal. Discrepancies between trematode species may result from differences in their effective population sizes and/or life history traits.
SUMMARY Interactions among different parasite species within hosts can be important factors shaping the evolution of parasite and host populations. Within snail hosts, antagonistic interactions among trematode species, such as competition and predation, can influence parasite abundance and diversity. In the present study we examined the strength of antagonistic interactions between 2 marine trematodes ( Maritrema novaezealandensis and Philophthalmus sp.) in naturally infected Zeacumantus subcarinatus snails. We found approximately the same number of snails harbouring both species as would be expected by chance given the prevalence of each. However, snails infected with only M. novaezealandensis and snails with M. novaezealandensis and Philophthalmus sp. co-occurring were smaller than snails harbouring only Philophthalmus sp. In addition, the number of Philophthalmus sp. rediae was not affected by the presence of M. novaezealandensis sporocysts and the within-host clonal diversity of M. novaezealandensis was not influenced by the presence of Philophthalmus sp. Our results suggest that antagonistic interactions may not be a major force influencing the evolution of these trematodes and that characteristics such as host size and parasite infection longevity are shaping their abundance and population dynamics.
SUMMARY Host specificity is a fundamental component of a parasite's life history. However, accurate assessments of host specificity, and the factors influencing it, can be obscured by parasite cryptic species complexes. We surveyed two congeneric species of intertidal snail intermediate hosts, Zeacumantus subcarinatus and Zeacumantus lutulentus , throughout New Zealand to identify the number of genetically distinct echinostome trematodes infecting them and determine the levels of snail host specificity among echinostomes. Two major echinostome clades were identified: a clade consisting of an unidentified species of the subfamily Himasthlinae and a clade consisting of five species of the genus Acanthoparyphium . All five Acanthoparyphium species were only found in a single snail species, four in Z. subcarinatus and one in Z. lutulentus . In contrast, the Himasthlinae gen. sp. was found in both hosts, but was more prevalent in Z. lutulentus (97 infections) than Z. subcarinatus (10 infections). At least two of the Acanthoparyphium spp. and the Himasthlinae gen. sp. are widespread throughout New Zealand, and can therefore encounter both snail species. Our results suggest that host specificity is determined by host–parasite incompatibilities, not geographic separation, and that it can evolve in different ways in closely related parasite lineages.
Widespread losses in river connectivity and habitat degradation have led to rapid declines in migratory freshwater fishes. Large, connected tributary systems are likely critical to the conservation of fluvial species when they provide access to life-cycle-dependent habitats. The Blue Sucker (Cycleptus elongatus) is a large-bodied migratory catostomid, endemic to the large rivers of North America and declining in abundance across much of its range. Blue Suckers occupying the Wabash River (Illinois/Indiana, USA), a large tributary within the Mississippi River basin, may be one of few remaining robust populations of this species. To understand the characteristics of a successful Blue Sucker population, we analyzed data from ten years of electrofishing surveys conducted in the lower Wabash River (2010–2019, n = 563 Blue Suckers). We found Blue Sucker presence probability increased at sites with snags and with increasing surface water velocity. The length–weight regression was comparable to other populations, and the mean relative weight was 94.27. Maturation was estimated to occur at a minimum of 422 mm total length, around 2–3 years of age. We found support for variable individual spawning preparedness evidenced by inconsistent gonadal development among pre-winter adults and found support for intermittent reproductive success evidenced by a multi-modal population age structure. Genetic analysis supported the presence of a panmictic population throughout the Wabash River system, with no barrier to separate this population from the greater Mississippi River basin metapopulation. The effective population size was estimated to be 632.8 (95% CI = 318.4–4,492.2), indicating the population is not at immediate risk of inbreeding depression but should continue to be monitored. The characteristics of this resilient lower Wabash River Blue Sucker population can inform the management and conservation of this imperiled species throughout its range.
ABSTRACT The pulmonate freshwater snail Promenetus exacuous (Planorbidae) has a widespread patchy distribution throughout much of North America, including New York State and the surrounding areas. Minimal life history information exists for the species in this region and information on its genetic diversity and structure is currently lacking for any portion of its range. We examined the species’ reproductive behaviour, genetic diversity and population structure throughout New York and western Connecticut using cytochrome c oxidase subunit I (COI) haplotypes (241 snails) and 10 microsatellite loci (312 snails). Throughout the region, P. exacuous is a single, primarily outcrossing species with relatively high within-population genetic diversity. Populations are genetically differentiated and composed of divergent COI lineages. The region was colonized after the Wisconsin glacial retreat by snails from multiple, historically isolated populations and Pleistocene events played a major role in the historical diversification of lineages. Subsequent dispersal has likely been facilitated by birds and humans, but contemporary gene flow is low, resulting in genetic differentiation even among geographically proximate sites. Our data reveal that complex interactions between historical and contemporary processes contribute to the overall patterns of genetic diversity in freshwater snails.
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