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Sex Played A Vital Role For Evolution Of Infectious Diseases

Same-sex mating between two less harmful yeast strains might have spawned an outbreak of disease among otherwise healthy people and animals on Vancouver Island, British Columbia, Howard Hughes Medical Institute geneticists at Duke University Medical Center have reported//. The fungus, Cryptococcus gattii, is normally restricted to the tropics and subtropics.

The evidence that sex played an important role in the pathogen's expansion may provide a useful model for the evolution of infectious diseases and parasites more generally, they said.

After extensive genetic analysis of fungal samples, the researchers suggest that mating between two less harmful fungal strains of the same sex or mating type produced the more virulent form. That strain has now taken hold and appears to be spreading perhaps driven by unique conditions, they said.

While the number of people infected so far does not approach that of many other infectious diseases, this fungus is invading the central nervous systems of people who have no other apparent risk factors except having taken a walk in the park on Vancouver Island, said Joseph Heitman, M.D., Ph.D. A year after infection, some of these people still have not fully recovered.

The fungus appears to have become entrenched is unlikely to disappear, and all indications are that it is spreading. Our findings suggest that sex played a role in the expanded geographic range for this pathogen.

The fungus, which lives in trees and soil, has also infected a variety of domestic and marine animals, including dogs, cats, llamas and porpoises.

The potentially life-threatening Cryptococcus neoformans invades the central nervous system to cause disease. It most commonly affects immune-compromised patients such as organ transplant recipients and cancer patients -- whose immune systems are crippled by immunosuppressive drugs or chemotherapy -- and people with HIV/AIDS. In contrast, C. gattii infects indi viduals with apparently normal immunity. Symptoms include persistent headaches, coughing and night sweats. In rare cases, C. gattii causes cryptococcal disease, pneumonia, meningitis or death.

While C. neoformans is found worldwide in association with pigeon droppings, the rarer C. gattii is normally restricted to tropical and subtropical areas, often in association with Eucalyptus trees.

It is suspected that the infectious propagules of Cryptococcus are airborne spores, Heitman said. Such spores are produced during sexual reproduction, though mating of the fungus has never been observed in nature. In fungi, however, sexual identity is determined by so-called mating type loci, genes arranged contiguously, but which typically do not span an entire chromosome.

This initial finding, suggests that the two C. gattii strains in Vancouver Island are either siblings or that one is the parent and the other the progeny.

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 cycles that contribute to produce progeny with altered virulence, geographic or host range or other advantageous characteristics.

Source: Eureka alert
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