Evolution of the Sexes

Vive la différence: When it comes to determining sex, humans and fungi may share common origins

Adam and Eve, from the Sagars altarpiece. Spanish School, (12th century). The Bridgeman Art Library

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

A team lead by Joseph Heitman, James B. Duke Professor of molecular genetics and microbiology, has isolated sex-determining genes from one of the oldest known types of fungi, Phycomyces blakesleeanus. The findings of their study appear in the journal 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," which exhibit an unusual amount of diversity among species. Heitman's group hypothesized that the sex-determining arrangement found in one of the earliest forms of fungi might reveal the ancestral structure of mating-type loci, thereby serving as a sort of molecular fossil.

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. 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 genes are able to mate with each other.

Both versions of the gene, 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 very similar to one encoded by the human Y chromosome, called SRY, that when turned on leads a developing fetus to exhibit male characteristics. Heitman says this similarity suggests that HMG-domain proteins may mark the evolutionary beginnings of sex determination in both fungi and humans.

Heitman's team proposes 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. The same process is most likely responsible for the evolution of the male Y chromosome, he says.