Adaptive evolution, sex-linkage, and gene conversion in the voltage-gated sodium channels of toxic newts and their snake predators

Abstract

Understanding how genetic changes ultimately affect morphology and physiology is essential for understanding and predicting how organisms will adapt to environmental changes. Although most traits are complex and involve the interplay of many different genetic loci, some exceptions exist. These include the convergent evolution of tetrodotoxin resistance in snakes, which has a simple genetic basis and can be used as a model system to investigate the genetic basis of adaptive evolution. Tetrodotoxin is a potent neurotoxin used as a chemical defense by various animals, including toxic newts. Snakes have evolved resistance through mutations in voltage-gated sodium channels, the protein targets of tetrodotoxin, sparking an evolutionary arms race between predator and prey. In this dissertation, I describe how genomic rearrangements have led to sex-linkage of four of the voltage-gated sodium channel genes in snakes and compare allele frequencies across populations and sexes to make inferences about how sex linkage has influenced the evolution of resistance in garter snakes. By measuring gene expression in different snake tissues, I show that three of these sex-linked sodium channel genes are dosage compensated in embryos, adult muscle, and adult brain. In contrast, two channels show sexual dimorphism in their expression levels in the heart, which may indicate differences in dosage compensation among tissues. I then use comparative genomics to track the evolutionary history of tetrodotoxin resistance across all nine sodium channel genes in squamate reptiles and show how historical changes have paved the way for full-body resistance in certain snakes. Finally, I use targeted sequence capture to obtain the sodium channel sequences of salamanders and show evidence that tetrodotoxin self-resistance in toxic newts was likely accelerated through gene conversion between resistant and non-resistant sodium channel paralogs. Together, these results illustrate parallelism in evolutionary mechanisms and processes contributing to the appearance of an extreme and complex trait that arose independently in two distinct taxa separated by hundreds of millions of years.