Humans come as boys or girls, but some organisms do not and without being hermaphrodites, they still have sex. In fact, one of the most primitive types of sexual differentiation has just been described by a team from Duke University Medical Center lead by Dr. Joseph Heitman in the journal Nature.

The ancestral sex-determining genes were described in one of the oldest fungus species, Phycomyces blakesleeanus. Fungi do not possess sex chromosomes, like X and Y in humans and mammals, but sex determining DNA stretches named "mating-type loci." Mating-type loci are found in all fungi, in various locations and types, even among close fungi species. Sex-determining arrangement encountered in the primitive fungus could have explained how the ancestral mating-type loci emerged, like a 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.

The team detected the mating-type loci of Phycomyces by comparing via computer already known mating-type loci in the DNA of other fungal types.

"We employed a usual-suspects approach, comparing proteins between fungal types before identifying a candidate that appeared related in all lineages," he added.

They detected two variants of a gene controlling mating, baptized sexM, (sex minus) and sexP (sex plus). Only fungi bearing opposite variants could mate with each other. Both sexM and sexP encoded for a sole protein, named high mobility group (HMG)-domain protein, but how it determines sex differentiation is still unknown. The protein is similar to the SRY one, encoded by the human Y chromosome, and involved in the masculinization of the embryos.

"This similarity suggests that HMG-domain proteins may mark the evolutionary beginnings of sex determination in both fungi and humans," the researched also said.

The researchers believe that both sexM and sexP were once the same gene which experienced a mutation through inversion. "The same process is most likely responsible for the evolution of the male Y chromosome," Heitman added.

The team is going to look for the sex region in the fungus, Rhizopus oryzae, to see how HMG-domain proteins determines sex in fungi. Rhizopus' genes are easier to manipulate.

"Rhizopus can be used to understand the influences of certain genes in lesser studied fungi much in the way we use mice to understand genetic effects in humans," primary author Dr. Alexander Idnurm, assistant professor at the University of Missouri-Kansas City, said.

Some more evolved fungi species are devoid of HMG-domain proteins.

"These proteins have been replaced with alternative transcription factors, which are proteins that turn genes on and off," proposed Heitman.