Biogeographic range expansion into South America by Coccidioides immitis mirrors New World patterns of human migration

Long-distance population dispersal leaves its characteristic signature in g enomes, namely, reduced diversity and increased linkage between genetic mar kers. This signature enables historical patterns of range expansion to be t raced. Herein, we use microsatellite loci from the human pathogen Coccidioi des immitis to show that genetic diversity in this fungus is geographically partitioned throughout North America. in contrast, analyses of South Ameri can C. immitis show that this population is genetically depauperate and was founded from a single North American population centered in Texas. Varianc es of allele distributions show that South American C. immitis have undergo ne rapid population growth, consistent with an epidemic increase in postcol onization population size. Herein, we estimate the introduction into South America to have occurred within the last 9,000-140,000 years. This range in crease parallels that of Home sapiens. Because of known associations betwee n Amerindians and this fungus, we suggest that the colonization of South Am erica by C. immitis represents a relatively recent and rapid codispersal of a host and its pathogen.