Mating systems in ascomycetes: a romp in the sac

The ascomycetous fungi choose between sexual and asexual reproduction; it is only when appropriately stressed that most resort to sexual development. Many of the proteins known to regulate development in animals (homeodomain proteins, HMG-box proteins) are also central to the control of sexual differentiation in the ascomycetes. Most ascomycete mating-type loci are structurally complex idiomorphs (large regions of nonhomologous DNA), in that they encode multiple products of probable independent evolutionary origin. The products of the mating-type genes are the master regulators of sexual development; we are just beginning our search for factors involved in the signal transduction pathway downstream from these master regulators.