Evolution of the sexes: What a fungus c...( DURHAM N.C. -- Fungi don't exactly c...)

DURHAM, N.C. -- Fungi don't exactly come in boy and girl varieties, but they do have sex differences. In fact, a new finding from Duke University Medical Center shows that some of the earliest evolved forms of fungus contain clues to how the sexes evolved in higher animals, including that distant cousin of fungus, the human.

A team lead by Joseph Heitman, M.D. has isolated sex-determining genes from one of the oldest known types of fungi, Phycomyces blakesleeanus, findings which appear in the Jan. 10 issue of Nature.

Fungi do not have entire sex chromosomes, like the familiar X and Y chromosomes that determine sexual identity in humans. Instead, they have sex determining sequences of DNA called "mating-type loci."

Mating-type loci have been found in a number of higher-level fungal species, and exhibit an unusual amount of diversity. These differences occur even among similar fungal species leading scientists to wonder how they evolved.

Heitman's group hypothesized that the sex-determining arrangement found in one of earliest forms of fungi might reveal the ancestral structure of mating-type loci, serving as a sort of molecular fossil.

"Fungi are good model systems for the evolution of human sexual differentiation because the genetic sequences responsible for sex are smaller versions of chromosomal sex-determining regions in people," Heitman said.

To identify the mating-type loci in Phycomyces, the researchers used a computer search to compare known mating-type loci in the genomes of other fungal lineages and then genetic mapping. "We employed a usual-suspects approach, comparing proteins between fungal types before identifying a candidate that appeared related in all lineages," says Heitman.

Within this stretch of DNA, they were able to isolate two versions of a gene that regulates mating, which they dubbed sexM, (sex minus) and sexP (sex plus). Strains of fungi with opposite versions of the sex gen
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