Spatial, ecological and genetic correlates of the geographic expansion of an infectious disease, white-nose syndrome in bats
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Infectious disease dynamics are inherently shaped by the distribution, ecology, and genetic variation of hosts. Conversely, pathogens exert powerful influences on hosts through demographic processes and natural selection. These tenets of disease ecology and evolutionary biology are illustrated in the case of white-nose syndrome (WNS), an emerging infectious disease of hibernating bats. WNS first emerged in 2006 and spread rapidly throughout eastern North America, causing massive declines in bat populations. To understand how host ecology and spatial distribution influence the spread of WNS, I evaluated risk models of colony-level correlates, including bat colony size, species composition, behavior, and gene flow. WNS was more likely to emerge in large colonies first, and species composition and behavior were also significant predictors of risk. Spatial spread was predicted by population genetics of little brown myotis (Myotis lucifugus), indicating coupling of host gene flow and pathogen dispersal, and potential for the application of landscape genetics to predict future spread. To guide management and evaluate pre-existing genetic diversity, I assessed population genetic structure of little brown myotis using restriction site-associated DNA sequencing (RAD-seq). RAD-seq data revealed two populations divided by the Rocky Mountains, with high gene flow between the distributions of putative subspecies. Demographic analyses and genome scans suggest adaptive genetic variation, variation that may be threatened by WNS in eastern North America. Drastic declines from WNS have likely imposed strong selection, and recent stabilization of populations near the disease epicenter suggests that resistance may have evolved in the host population. I generated whole genome sequence data for bats sampled before and after declines to test for demographic changes and natural selection. Average genomic differentiation and nucleotide diversity indicated little demographic change between the two periods, but preliminary analyses suggest genomic regions of differentiation combined with decreased nucleotide diversity in post-WNS relative to pre-WNS samples, hinting at a pattern of natural selection. Additional samples and in-depth analyses are necessary to robustly test these patterns; however, identification of signatures of selection in the bat genome would be an exciting indication of a rapid evolutionary response to an introduced disease.