FIGURE 1. Evolutionary model of the Aspergillus mating genes.

In a bisexual common ancestor, the two mating-type genes, alpha (red) and HMG (blue), were fused head to tail on the same chromosome and share a similar flanking, regulatory region (green). In the bisexual A. nidulans, the chromosome is broken and the two mating-type genes end up on different chromosomes (with their flanking regions). In the ancestor of A. oryzae and A. fumigatus, the two mating-type genes dissociate in different strains, but remain flanked by similar genes. After speciation, both A. fumigatus and A. oryzae become fully heterosexual, with some isolates having only the alpha mating type and others only the HMG mating type, both being in similar chromosomal environments.