evolution, homothallism, MAT-locus, sexual differentiation, zygomycetes
Sexual processes are believed to represent the most efficient mechanisms for maintaining genetic variability in fungi. Although reproduction does not normally depend on meiotic spore formation, but instead in most cases on various mitotic spore forms, the ability to undergo sexual differentiation via nuclear fusion followed by meiosis is surprisingly frequently found. Sexual processes are strictly regulated at various levels, mainly at the level of recognition between complementary mating partners. Fungi can be heterothallic or homothallic. Sexual communication and interaction in heterothallic fungi take place between genetically different, but compatible mycelia. Very often, the two individuals do not differ morphologically from each other. Physiologically, however, the mating types differ considerably. Specific pheromones followed by species and mating type specificity at the hyphal surface mediate recognition exclusively between compatible mating types. Genetically, these characteristics are accomplished at two levels, on the one hand the mating type-specific expression of the characteristic proteins and other compounds, and on the other hand by control of the system via mating type-specific transcription factors that are ascribed to a master regulator locus, normally designated mating type locus (MAT) or sex locus (sex). Zygomycetes range among evolutionary old fungi. Especially under this aspect, the structures of MAT-loci in homothallic and heterothallic species are of special interest. In this review, the sequenced sex loci and the related genes of homothallic and heterothallic Mucorlike zygomycetes are compared.