DISSERTATION RESEARCH: The conflicting effects of gene flow in a geographic mosaic of predator-prey coevolution

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This research will test the conflicting effects of gene flow in the co-evolutionary arms race between common garter snakes and their toxic prey, Pacific newts. The structure and complexity of ecological communities is shaped by the joint evolution of interacting species. Negative interactions, like predator-prey relationships, are often characterized by sustained and rapid adaptation; each species must constantly adapt and counter-adapt to the co-evolving defenses of its natural enemy. Theory predicts that migration and the exchange of genetic material among populations (a process called gene flow) will have two important, yet conflicting, effects on local adaptation to antagonistic interactions. Gene flow is generally thought to homogenize genetic variation among populations and hinder local adaptation. Conversely, gene flow and migration may play a critical role to spread beneficial mutations to new populations and promote rapid adaptive change. This research will form a foundation for understanding the genetic impetus behind sustained, and often rapid, evolutionary changes that are responsible for the tremendous diversity of complex traits that span predator-prey, host-parasite, and plant-animal systems. In addition, this study will contribute to the training of graduate and undergraduate students.In western North America, garter snakes (Thamnophis sirtalis) have evolved resistance to tetrodotoxin (TTX), an otherwise lethal neurotoxin found in the newts. Highly resistant snake populations tend to co-occur with highly toxic newts (Taricha species), as coevolution has apparently driven the escalation of armaments in both predator and prey. Toxin resistance in the garter snake is largely due to key amino acid replacements in the NaV1.4 sodium channel protein. This project will use next-generation DNA sequencing to test whether snake gene flow promotes adaptation to toxic newts through the spread of TTX-resistance mutations in the NaV1.4 sodium channel. The researchers will generate two separate genetic datasets from garter snake populations across western North America: (1) genetic variation at the NaV1.4 locus, and (2) background genetic variation, unlinked to NaV1.4, in neutral Single Nucleotide Polymorphisms (SNPs) from double digest Restriction Associated DNA sequencing (ddRADseq). Population structure and gene flow will then be estimated separately for each dataset. Discordant patterns of gene flow between the two datasets will indicate where selection has altered the spread of NaV1.4 haplotypes in relation to neutral expectations of the SNPs. For example, in populations where gene flow and selection favor the spread of TTX-resistance mutations, gene flow at the NaV1.4 locus should exceed that of background variation in the neutral SNPs.