Edited by David B. Wake, University of California, Berkeley, CA, and approved January 31, 2001 (received for review August 24, 2000)
Long-distance population dispersal leaves its characteristic signature in genomes, namely, reduced diversity and increasedlinkage between genetic markers. This signature enables historicalpatterns of range expansion to be traced. Herein, we use microsatelliteloci from the human pathogen Coccidioides immitis to show thatgenetic diversity in this fungus is geographically partitionedthroughout North America. In contrast, analyses of South AmericanC. immitis show that this population is genetically depauperateand was founded from a single North American population centeredin Texas. Variances of allele distributions show that South AmericanC. immitis have undergone rapid population growth, consistentwith an epidemic increase in postcolonization population size.Herein, we estimate the introduction into South America to haveoccurred within the last 9,000-140,000 years. This range increaseparallels that of Homo sapiens. Because of known associationsbetween Amerindians and this fungus, we suggest that the colonizationof South America by C. immitis represents a relatively recentand rapid codispersal of a host and itspathogen.