Microbiologists from Duke University have unraveled the story behind the emergence of a virulent fungus strain that was responsible for an 1999 outbreak of meningoencephalitis on Vancouver Island, Canada. Two clonal lineages of the same 'sex' — one local strain found in the Pacific Northwest, and another that originated in Australia — combined through sexual reproduction to yield the hypervirulent recombinant genotype that was primarily responsible for the outbreak(Fig. 1).

Figure 1
figure 1

The origin of a Cryptococcus gattii outbreak. An original α mating event involving an avirulent α parent that likely originated in Australia (orange circle) yielded a virulent recombinant (yellow circle), and ongoing α mating could enable robust infectious spore production and outbreak expansion. Image adapted from Fraser et al. © (2005) Macmillan Magazines Ltd.

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Cryptococcus gattii is a yeast normally restricted to tropical and subtropical regions of the world. Identification of this pathogen as the causative agent of the Vancouver Island outbreak indicated a new and expanded geographical range for this microorganism, prompting questions about its origin and evolution. To address these questions, Joseph Heitman, James Fraser and colleagues undertook a large-scale genealogical analysis of C. gattii isolates that were linked to the outbreak and compared them with each other and with the global C. gattii population. This analysis revealed that the outbreak isolates comprised two distinct genotypes — a major genotype (95% of isolates) that is hypervirulent and a minor genotype (5% of isolates) that is less virulent and has an identical genotype with fertile isolates from an Australian recombining population. This analysis also revealed evidence of sexual reproduction between the outbreak genotypes; however, instead of the classical fungal a–α sexual cycle, Fraser et al. found evidence that the major-outbreak hypervirulent genotype descends from a same-sex mating event involving two α mating-type parents.

Further studies will be required to ascertain the prevalence of same-sex C. gattii mating in nature and whether this process facilitated the production of infectious fungal spores. However, these findings do establish the importance of same-sex reproduction in fungi by allowing the expansion of a pathogen into a new geographical region, ultimately resulting in an infectious-disease outbreak in humans. The earlier demonstration of same-sex mating in Cryptococcus neoformans, a sibling species of C. gattii, and the authors' assertion that other parasites, including Trypanosoma cruzi , Leishmania species and Plasmodium falciparum , could harbour a same-sex cycle that produces progeny with altered characteristics, has fascinating implications for our understanding of parasite pathogenicity and host range.