Among clinical and environmental isolates of the fungus from British Columbia, the researchers identified two forms: an extremely virulent major strain, which accounted for 95 percent of all samples, and a less virulent and less common strain, which made up the other five percent.
By comparing select gene sequences that spanned the genomes of the Vancouver Island fungi to samples collected from around the world, the team traced the rarer type to identical isolates in Australia. The major form matched a sample taken from an infected person in Seattle 30 years ago and another collected from a Eucalyptus tree in San Francisco in 1992.
What's more, the Canadian strains shared approximately half of their genetic makeup, suggesting that the two might be related. Further analysis confirmed this initial finding, suggesting that the two C. gattii strains in Vancouver Island are either siblings or that one is the parent and the other the progeny.
"Given that the minor outbreak form also exists in multiple locations in Australia, while the major outbreak form has only been found in the Pacific Northwest, we favor the hypothesis that the minor type represents one of two parental strains that gave rise to the major outbreak 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 provided evidence that the major outbreak isolate may have resulted from same-sex mating, Heitman added. In a traditional sexual cycle, all alpha 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, they found.
"Sex within the same mating-type may confer an evolutionary advantage when the opposite mating type is unavailable," Heitman said. "Other human pathogens or parasites may harbor cryptic same-sex c
Source:Duke University Medical Center