Year of Award

2017

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Organismal Biology and Ecology

Department or School/College

Division of Biological Sciences

Committee Chair

Jeffrey Good

Commitee Members

Creagh Breuner, Zac Cheviron

Subject Categories

Evolution | Genetics | Genomics

Abstract

Environmental seasonality presents many challenges for survival. In response, organisms from across the tree of life exhibit adaptive phenology, which can facilitate survival from season to season. In particular, many species are white in the winter and brown in the summer where winter snow cover alters the visual environment. There is an extensive body of literature describing the physiological and cellular regulation of this trait in the Siberian dwarf hamster (Phodopus sungorus), but there are major shortcomings in our understanding concerning the evolution and molecular regulation of mammalian skin phenology. Notably, little is known about the genetic basis or the transcriptional regulation of seasonal coat color change and previous quantification of coat color during the molt to the winter pelage has substantial qualitative bias, rendering comparative frameworks futile. Resolving these gaps in knowledge will illuminate how adaptive phenology evolves and how seasonal coat color change is regulated in the skin. I used two species of dwarf hamster to address these issues: P. sungorus, a species that expresses a white winter coat, and the sister species P. campbelli, which expresses phenology similar to P. sungorus, but does not molt to a white winter coat. I conducted a series of crosses between P. campbelli and P. sungorus to dissect the genetic basis of pelage phenology. I also quantified pelage reflectance on an objective, continuous scale in both species, and their F1 and backcross hybrids. Measuring coat color on this scale allows for unbiased, direct comparisons between genotypes. Finally, I used RNA-seq to further understand the regulation of molt phenology in the skin of both species, and in hybrid individuals. In our backcross experiment, F1 hybrids expressed a winter pelage phenotype more similar to P. campbelli than the white phenotype in P. sungorus, indicating that molting to a white winter coat is generally recessive. There was also considerable variation in terminal coat color in backcross individuals, suggesting that mutations in relatively few genes underlie the evolution of seasonal coat color change. Phodopus sungorus exhibited more robust patterns of phenology in the skin than what was found in P. campbelli or F1 hybrids, which brings into question the validity of direct phenology comparisons made in the past between the two parental species. Our crossing experiment demonstrates that evolution of a complex adaptive trait, like seasonal coat color change, may only require changes in relatively few loci. Despite the fact that these two species share many seasonal changes, gene expression changes during seasonal molts appear fairly divergent. Phodopus sungorus shows weak temporal patterning in gene expression changes while P. campbelli and F1 hybrids do not. The hybrids, in particular, showed dampened expression changes, potentially reflecting hybrid incompatibilities that disrupt seasonal phenology.

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© Copyright 2017 Zachary Clare-Salzler