Identification of the sex genes in an early diverged fungus

Sex determination in fungi is controlled by a small, specialized region of the genome in contrast to the large sex- specific chromosomes of animals and some plants. Different gene combinations reside at these mating- type ( MAT) loci and confer sexual identity; invariably they encode homeodomain, alpha-box, or high mobility group ( HMG)- domain transcription factors(1). So far, MAT loci have been characterized from a single monophyletic clade of fungi, the Dikarya ( the ascomycetes and basidiomycetes)(2), and the ancestral state and evolutionary history of these loci have remained a mystery. Mating in the basal members of the kingdom has been less well studied, and even their precise taxonomic interrelationships are still obscure(3,4). Here we apply bioinformatic and genetic mapping to identify the sex- determining ( sex) region in Phycomyces blakesleeanus ( Zygomycota), which represents an early branch within the fungi. Each sex allele contains a single gene that encodes an HMG- domain protein, implicating the HMG- domain proteins as an earlier form of fungal MAT loci. Additionally, one allele also contains a copy of a unique, chromosome- specific repetitive element, suggesting a generalized mechanism for the earliest steps in the evolution of sex determination and sex chromosome structure in eukaryotes.