Adaptation in the forest deer mouse: evolution, genetics, and development

Abstract

Variation in the shape, size, and number of segments along the vertebral column underlies a vast amount of vertebrate diversity. Although the molecular pathways controlling vertebrate segmentation during normal development are well understood, the genetic and developmental underpinnings responsible for the tremendous variation in size and number of vertebrae are relatively unexplored. The main goal of this dissertation is to explore the genetic and developmental mechanisms influencing naturally occurring variation in the vertebral column. To this end, I focus on intraspecific skeletal variation, with an emphasis on tail length, in the deer mouse, Peromyscus maniculatus. In Chapter 1, I employ a phylogeographic framework to show that longer tails have evolved independently in different populations of forest-dwelling mice. Closer investigation of the underlying morphology shows that long-tailed mice have both (1) a greater number of tail vertebrae and (2) individually longer vertebrae, compared to ancestral short-tailed mice. Chapter 2 explores the genetic basis of tail length variation. I use quantitative trait locus mapping to uncover six loci that influence differences in total tail length (3 associated with vertebral length and 3 with vertebrae number). Finally, in Chapter 3 I combine comparative data from quantitative measurements of tissue dynamics during somitogenesis in fixed embryos and ex vivo explant culture to show that embryos of forest mice make more segments because they produce more presomitic mesoderm, and not because of any significant difference in the timing of somitogenesis. Together, this work integrates phylogeographic, genetic, and developmental studies to pinpoint the ways that natural selection modifies development to produce the repeated evolution of an evolutionarily important trait, and suggests that there are a limited number of ways that long tails can evolve.