London, Jan 10 : Researchers at the Duke University Medical Center have found 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.
Lead by Joseph Heitman, M.D., the research team has isolated sex-determining genes from one of the oldest known types of fungi, Phycomyces blakesleeanus,
Fungi have sex determining sequences of DNA called 'mating-type loci,' not the entire sex chromosomes, like the familiar X and Y-chromosomes that determine sexual identity in humans.
Mating-type loci have been discovered in a number of higher-level fungal species, and display an unusual amount of diversity. These differences take place even among similar fungal species leading researchers to think how they evolved.
The researchers hypothesized that the sex-determining arrangement identified 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," Nature quoted Heitman, as saying.
In order to find the mating-type loci in Phycomyces, Heitman and his colleagues 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," Heitman said.
Within this stretch of DNA, the researchers 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 genes are able to mate with each other.
Both sexM and sexP encode for a single protein called a high mobility group (HMG)-domain protein that leads to sex differentiation through an unknown process.
This protein is like the one encoded by the human Y chromosome, called SRY, that when turned on leads a developing foetus to exhibit male characteristics.
According to Heitman, this resemblance suggests that HMG-domain proteins may mark the evolutionary beginnings of sex determination in both fungi and humans.
The researchers propose that sexM and sexP were once the same gene that went through a mutation process called inversion. The new versions then evolved into two separate sex genes.
He suggests that the same process is most likely responsible for the evolution of the male Y chromosome.
The findings appear in the Jan. 10 issue of Nature.