Maintenance of genetic diversity at ecologically-relevant loci may be important for allowing invasive populations to become established despite decreases in genomic diversity due to founder effects. To evaluate this prediction, we compared genetic diversity at an expressed MHC class IIß gene fragment to a 909 bp region of the neutral cytochrome b (cytb) locus from 20 populations of the American bullfrog (Rana catesbeiana) across its North American invasive and native ranges and quantified the presence of Batrachochytrium dendrobatidis (Bd), a pathogen, for which R. catesbeiana is a vector species. We recovered 28 unique MHC alleles and found that invasive populations had significantly higher Bd prevalence and intensity and significantly higher pairwise FST than native populations, but maintained similar levels of MHC diversity, contrasted by lower neutral cytb diversity. Across all populations the two most common alleles (LiCA_B & Rapi_33) were associated with a significant decreased risk of Bd infection, and we detected positive selection acting on three MHC peptide binding residues, indicating that positive selection drives MHC evolution, likely in part due to disease pressure. A cytb phylogenetic analysis indicated that invasive populations likely arose from a single founding population from somewhere in the American Midwest with a possible subsequent invasion. In contrast, MHC phylogenetic analyses revealed widespread allele sharing across native and invasive ranges, but limited trans-species polymorphism, indicating a unique MHC evolutionary history in R. catesbeiana that may play a key role in the species' high Bd tolerance. Overall, our data indicate that balancing selection maintains MHC diversity in invasive R. catesbeiana despite founder effects evident from the cytb dataset. This study suggests that maintenance of diversity at ecologically-relevant loci contribute to the successful establishment of invasive populations and highlights the importance of quantifying diversity at functional loci to assess the evolutionary potential of invasive populations.
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Master of Science (M.S.)
College of Sciences
Length of Campus-only Access
Masters Thesis (Open Access)
Lafond, Jacob, "Invasive Bullfrogs Maintain High Levels of Immune Gene Diversity Despite Elevated Bd Infection Relative to Native Populations" (2021). Electronic Theses and Dissertations, 2020-. 716.