DURHAM, N.C. -- Same-sex mating between two less harmful yeast strainsmight have spawned an outbreak of disease among otherwise healthypeople and animals on Vancouver Island, British Columbia, Howard HughesMedical Institute geneticists at Duke University Medical Center havereported. The fungus, Cryptococcus gattii, is normally restricted tothe tropics and subtropics.
The researchers said their findings provide important additionalinsight into the origin of the Vancouver Island outbreak, which beganin 1999. Moreover, the evidence that sex played an important role inthe pathogen's expansion may provide a useful model for the evolutionof infectious diseases and parasites more generally, they said.
The team reported its findings October 9, 2005, in an advancedonline publication of Nature. The work was supported by the NationalInstitute of Allergy and Infectious Disease.
After extensive genetic analysis of fungal samples, theresearchers suggest that mating between two less harmful fungal strainsof the same sex or "mating type" produced the more virulent form. Thatstrain has now taken hold and appears to be spreading -- perhaps drivenby unique conditions in the Vancouver area, they said.
"While the number of people infected so far does not approachthat of many other infectious diseases, this fungus is invading thecentral nervous systems of people who have no other apparent riskfactors except having taken a walk in the park on Vancouver Island,"said Joseph Heitman, M.D., Ph.D. "A year after infection, some of thesepeople still have not fully recovered.
"The fungus appears to have become entrenched in the VancouverIsland area," he added. "It is unlikely to disappear, and allindications are that it is spreading. Our findings suggest that sexplayed a role in the expanded geographic range for this pathogen."
Since it was first documented in 1999, C. gattii has infectedat least 100 people on Vancouver Island and the Canadian mainland andled to four deaths. The fungus, which lives in trees and soil, has alsoinfected a variety of domestic and marine animals, including dogs,cats, llamas and porpoises.
C. gattii is closely related to the more widespread infectiousyeast, Cryptococcus neoformans. The potentially life-threatening C.neoformans invades the central nervous system to cause disease. It mostcommonly affects immune-compromised patients such as organ transplantrecipients and cancer patients -- whose immune systems are crippled byimmunosuppressive drugs or chemotherapy -- and people with HIV/AIDS. Incontrast, C. gattii infects individuals with apparently normalimmunity. Symptoms include persistent headaches, coughing and nightsweats. In rare cases, C. gattii causes cryptococcal disease,pneumonia, meningitis or death.
While C. neoformans is found worldwide in association withpigeon droppings, the rarer C. gattii is normally restricted totropical and subtropical areas, often in association with Eucalyptustrees.
"It is suspected that the infectious propagules of Cryptococcusare airborne spores," Heitman said. "Such spores are produced duringsexual reproduction, though mating of the fungus has never beenobserved in nature."
In plants and animals, sexual identity is governed by sexchromosomes, Heitman explained. In fungi, however, sexual identity isdetermined by so-called "mating type loci," genes arrangedcontiguously, but which typically do not span an entire chromosome.Cryptococcus exists in two mating types, "a" and "alpha," determined bya single genetic region, or locus.
Earlier studies by the Duke team found that most VancouverIsland outbreak isolates are sexually fertile, but all are of one"sex," a trend that would seem to preclude the normal sexual cycle. Arecent laboratory study led by Heitman's group suggested a possibleexplanation: the related yeast C. neoformans can undergo same-sexmating between two alpha partners.
Among clinical and environmental isolates of the fungus fromBritish Columbia, the researchers identified two forms: an extremelyvirulent major strain, which accounted for 95 percent of all samples,and a less virulent and less common strain, which made up the otherfive percent.
By comparing select gene sequences that spanned the genomes ofthe Vancouver Island fungi to samples collected from around the world,the team traced the rarer type to identical isolates in Australia. Themajor form matched a sample taken from an infected person in Seattle 30years ago and another collected from a Eucalyptus tree in San Franciscoin 1992.
What's more, the Canadian strains shared approximately half oftheir genetic makeup, suggesting that the two might be related. Furtheranalysis confirmed this initial finding, suggesting that the two C.gattii strains in Vancouver Island are either siblings or that one isthe parent and the other the progeny.
"Given that the minor outbreak form also exists in multiplelocations in Australia, while the major outbreak form has only beenfound in the Pacific Northwest, we favor the hypothesis that the minortype represents one of two parental strains that gave rise to the majoroutbreak isolate," said study author James Fraser, Ph.D., also of Duke."The second parent strain remains to be discovered."
Additional examination of the mating type locus providedevidence that the major outbreak isolate may have resulted fromsame-sex mating, Heitman added. In a traditional sexual cycle, allalpha progeny inherit identical alpha genes from their alpha parent.However, the mating type locus of the two strains from Vancouver Island-- both of the alpha mating type -- differed at numerous sites, theyfound.
"Sex within the same mating-type may confer an evolutionaryadvantage when the opposite mating type is unavailable," Heitman said."Other human pathogens or parasites may harbor cryptic same-sex cyclesthat contribute to produce progeny with altered virulence, geographicor host range or other advantageous characteristics."
Collaborators on the study include Steven Giles, Emily Wenink,Scarlett Geunes-Boyer, Jo Rae Wright, Stephanie Diezmann, Andria Allen,Jason Stajich, Fred Dietrich and John Perfect, all of Duke.
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