Rapid Evolutionary Response to Infectious Cancer

Tasmanian Devil Facial Tumor Disease (DFTD) is a unique infectious cancer that threatens the persistence of Tasmanian devils (Sarcophilus harrisii). Tumor cells are transmitted by biting among reproductively mature individuals, and infected individuals rarely reproduce more than once before succumbing to the disease. However, long-infected populations have persisted, and there is recent evidence for population recovery and tumor regression in a few individuals. Understanding the genetic basis of resistance to DFTD could illuminate mechanisms of cancer resistance and potentially allow prediction of the future course of DFTD. We have used Restriction-site Associated DNA sequencing (RADseq) to scan the genome for candidate loci exhibiting a response to selection by DFTD, and to determine geographic structure of genetic variation, both across the genome and at candidate loci. We identified genomic regions with concordant signatures of selection in response to DFTD across three focal populations. These results suggest rapid evolution in response to strong selection imposed by DFTD, and the potential for genetic variation for resistance to the disease that could facilitate population recovery, despite the overall low levels of genetic diversity within the species. Our phylogeographic analysis confirms previous evidence for two major clusters of devil populations roughly divided into the eastern two-thirds and western third of Tasmania. The only remaining uninfected populations are in the northwest corner. Our results suggest that genetic distinctiveness in western devil populations may have implications for the future spread of the disease.