MBE Advance Access published online on July 26, 2006
Molecular Biology and Evolution, doi:10.1093/molbev/msl070
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1 Department of Molecular Genetics and Microbiology Duke University Medical Center, Durham, NC, United States of America
* To whom correspondence should be addressed. The availability of the whole-genome sequence from the two known varieties of the human pathogenic fungus Cryptococcus neoformans provides an opportunity to study the relative contribution of divergence and introgression during the process of speciation in a genetically tractable organism. At the genomic level these varieties are nearly completely syntenic, share approximately 85-90% nucleotide identity, and are believed to have diverged
Accepted June 23, 2006
Research Article
Recent Evolution of the Human Pathogen Cryptococcus neoformans by Inter-Varietal Transfer of a 14 Gene Fragment
Laura A. Kavanaugh 1, James A. Fraser 1, and Fred S. Dietrich 1 *
Fred S. Dietrich, E-mail: dietr003{at}mc.duke.edu
Abstract 
18 million years ago. Via a comparative genomic approach we identified a 14 gene region (40 kb) that is nearly identical between the two varieties that resulted from a non-reciprocal transfer event from var. grubii to var. neoformans 2 million years ago. The majority of clinical and environmental var. neoformans strains from around the world contain this sequence obtained from var. grubii. This introgression event likely occurred via an incomplete inter-varietal sexual cycle creating a hybrid intermediate where mobile elements common to both lineages mediated the exchange. The subsequent duplication in laboratory strains of a fragment of this same genomic region supports evolutionary theories that instabilities in subtelomeric regions promote adaptive evolution through gene amplification and subsequent adaptation. Along with a more ancient predicted transfer event in C. neoformans and a recently reported example from Saccharomyces cerevisiae, these data indicate that DNA exchange between closely related sympatric varieties or species may be a recurrent theme in the evolution of fungal species. It further suggests that while evolutionary divergence is the primary force driving speciation, rare introgression events also play a potentially important role.
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