Researchers at Duke University are reporting a finding that defies the most basic tenet of sexual reproduction--that in order to reproduce, an organism must mate with a member of the opposite sex. The scientists report in the April 21 edition of the journal Nature that they've discovered that the infectious fungus Cryptococcus neoformans can mate and produce offspring with members of the same sex.
The fungus has two types of "mating type loci" genes--called "a" and "alpha"--which determine the sex of the organism. But samples taken from people infected with the fungus show a preponderance of "alpha" type fungus, leading researchers to believe that the organism reproduced unisexually through an ongoing division of fungal cells. But scientists at Duke's Howard Hughes Medical Institute discovered that the fungus reproduces both unisexually and by releasing reproductive cells in a process called "fruiting" that include half the number of chromosomes needed for reproduction, similar to the way a human egg or sperm has half the chromosomes needed for reproduction. These cells from the "alpha" fungus merge with other "alpha" fungus cells to begin the reproduction process, essentially mingling genetic material from two same-sex organisms to produce offspring.
"Sex is generally beneficial as a means to produce offspring with different gene combinations that can adapt more rapidly to new environments," said lead investigator Joseph Heitman on the Web site dukemednews.org. "The findings suggest for the first time that the fungus has developed a novel type of sexual cycle, allowing sexual reproduction between members of the same mating type. That ability might confer an advantage for the fungus because patients infected with it predominantly harbor a single mating type, reducing the possibility of normal fungal sexual reproduction."
C. neoformans is a life-threatening illness common in people with compromised immune systems, including advanced HIV disease. A C. neoformans infection is considered an AIDS-defining illness in HIV-positive people. The researchers say their findings of how the fungus reproduces might help improve scientific understanding of how to effectively combat the organism.