Evolution of Pathogen-Induced Gene Expression in the House Finch, Carpodacus mexicanus
Type of Degreedissertation
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Bacterial pathogens impact the survival, reproduction and fitness of their hosts making them extremely well-suited for experimental studies intended to understand adaptive phenotypic evolution of wild populations of vertebrate hosts. Pathogen-driven host phenotypic changes may occur from non-genetic effects like developmental plasticity or maternal effects, or from adaptive evolutionary processes like natural or sexual selection. When infectious diseases emerge and spread rapidly, they can have major consequences on host population dynamics, potentially resulting in rapid evolution of host phenotypes. The outbreak and spread of the pathogenic bacterium Mycoplasma gallisepticum (MG) in house finches (Carpodacus mexicanus) represents an excellent system for studying the evolution of such adaptive phenotypes in the wild. House finches are passerine songbirds ranging naturally across most of western North America. They were introduced to New York in 1940 and have since expanded throughout the entire eastern US. Male house finch ornamental plumage color is an important criterion in female mate choice, and infection with pathogens during molt can have significant effects on color expression. MG is a common pathogen of poultry that causes upper respiratory and eye conjunctivitis infections. It was first detected in house finches in the Washington, D.C. metro area in 1994, after which it spread across the entire eastern population in just a few years. MG had not, however, reached populations of the US Southwest when the infection experiments described hereafter were conducted. In this dissertation I use experimental and molecular approaches to investigate and identify molecular candidates associated with house finch phenotypic evolution, as driven by history of infection with MG. I develop a custom cDNA microarray designed to study the expression of genes in house finches infected with MG. I then derive a set of candidate genes from results of microarray studies and develop a highly multiplexed qRT-PCR assay for house finch immune tissues. Using these tools, I show that population-level phenotypic differences in resistance are associated with differences in gene expression in the spleen. Furthermore, expression responses of western US birds to experimental infection were more similar to eastern US birds studied in 2000 than to eastern birds in 2007. These results support the hypothesis that resistance has evolved in eastern birds over only 12 years. I further show that phenotypic differences in plumage color are also associated with differences in gene expression in both the spleen and the trachea. Together, these results contribute to our understanding of the potential for rapid vertebrate phenotypic evolution when populations are under strong selective pressures by pathogens, and they provide a basis for the continued study of gene expression in house finches as evidence of evolution by natural and sexual selection.