Most fungal isolates taken from people infected by C. neoformans are of the alpha mating type, said study lead author and post-doctoral fellow Xiaorong Lin, Ph.D., also of Duke.
"Organisms, including fungi, usually exist in an approximately one-to-one sex ratio," Lin said. "Yet in Cryptococcus neoformans, one mating type predominates, leaving them with apparently few chances to mate. It's been a mystery."
In 1996, another group discovered that those alpha isolates could undergo fruiting and produce spores, a process that resembles sexual reproduction. However, researchers thought that the unisexual fruiting occurred strictly through the asexual division of cells into identical clones, Heitman said.
That left scientists with a conundrum. C. neoformans has a defined sexual cycle involving both mating types. "Yet, how can sexual reproductive potential be maintained in an organism with a largely unisexual population structure?" Heitman asked. "The fruiting of alpha strains provided a clue."
In their laboratory experiments, the researchers found that, rather than being an asexual process, the hallmarks of mating occur during fruiting of alpha isolates. Unisexual fruiting involves the fusion of cells followed by meiosis, enabling genetic exchange between members of the same sex, they reported.
Meiosis is the process whereby cells divide into two "haploid" cells, each with half the number of chromosomes. Unlike diploid animals and plants, C. neoformans normal state is haploid, Heitman explained.
Furthermore, the team showed, strains lacking components required for mating -- including pheromones, pheromone receptors and other genes with known roles in mating -- exhibited a defect in fruiting.
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