Minimum sample size, population genomics and morphological variation in the terrestrial gastropod Webbhelix multilineata (Mollusca, Polygyridae)

Patterns in evolutionary and ecological biology help shape our view of the physical world both in the present and its past. Biogeographical studies in particular afford understanding of how biological systems have been shaped by processes over geological time, both genetically and morphologically. Some systems, like terrestrial gastropod mollusks, are well suited for these types of studies owing to their near ubiquitous occupation of terrestrial habitats, while also individually being very locally restricted and having poor reproductive dispersal. A notable species is the striped white-lip snail Webbhelix multilineata, a morphologically charming and biogeographically interesting study system because of its somewhat unusually broad geographic range spanning large regions of North America previously glaciated and unglaciated during the Pleistocene. It is also quite restricted to moist, shaded riparian woodlands, a historically profuse habitat that has been largely lost and become highly fragmented due to human development. These features and others make Webbhelix an enticing system for biogeographical and population study, and the chosen focus of the studies presented here. While there are many different molecular and morphological approaches that can be taken to pursue such studies, this dissertation focused primarily on the generation and use of genomic data for a non-model system, and secondarily on generating and utilizing morphological data, to accomplish three major aims: (1) test for a minimum sample size to accurately estimate basic population genomic parameters in Webbhelix multilineata; (2) generate the first genomic dataset for Webbhelix and utilize it for population genomic assessment, as well as coalescent simulations to assess models of post-glacial refugial expansion; and (3) test variation in Webbhelix shell size and brightness within the context of established ecogeographic patterns. Addressing the first aim using SNPs generated through a ddRADseq approach showed that as few as 6 individuals per sampled locality was sufficient to accurately estimate observed and expected heterozygosity, inbreeding coefficient, and pairwise differentiation in Webbhelix. Pursuit of the second aim netted an 8,480 SNP dataset composed of 18 sample localities from across the species range, and its assessment for population genomic structure revealed 4 population clusters: two Midwestern clusters, a southern Mississippi River cluster, and an eastern cluster on the shores of Lake Erie in Ontario. Estimates of genetic diversity found it was highest in the southern-most sampled localities and generally decreased further north and away from the Mississippi River, indicative of serial founder effects resulting from range expansion. Analysis of coalescent simulations suggested that post-glacial range expansion in Webbhelix occurred along major river corridors rather than openly in all directions, and that expansion either began as early as some 20,000 years ago around the LGM or a more northerly refugia existed for this species than initially considered. Lastly, work toward the third aim resulted in a moderately strong negative correlation being found between shell size and latitude in Webbhelix, indicating a pattern described by a reverse Bergmann cline. Additionally, a weak positive correlation was found between shell brightness and latitude, suggesting (with heavy caveats) that Webbhelix may exhibit the pattern described by Gloger’s ecogeographic hypothesis.

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