Morphological specialization (production of differentiated gametangia) has little relevance to mating in fungi based on the haploid phase. Indeed, specialized cells for mating are found only in filamentous Ascomycotina and even here a single individual produces both male and female structures. Nonetheless, most species have genetic barriers to self-fertilization and only individuals with different mating types can engage in sexual reproduction. Ascomycotina have just two mating types but Basidiomycotina may have several hundred mating types. The mating type loci encode proteins and different mating types bring compatible versions together. Only proteins from different mates that interact with one another can activate sexual development and promote outcrossing. Sexual reproduction guarantees genetic variability in a species. The genes that determine fungal mating type are located at one or more complex loci.
U. maydis exemplifies several features of sexual development also found in the familiar mushroom fungi. In the absence of morphological differentiation of gametangia, recognition of conjugants is based on pheromone-based recognition. There are hundreds of different genetically determined mating types (individuals). The a locus has two known mating types with each allele containing two genes, one for a pheromone polypeptide and one for a pheromone receptor. The necessary condition for a successful mating is that two conjugant partners must be complimentary. Self-nonself recognition is at the level of protein dimerization. The significance of fertilization (diploidy) being postponed in favor of an extended dikaryotic phase is elusive. Despite the lack of morphological differentiation in fusing partners, fertilization and meiosis are still involved. Sexual fusion reprograms development—the mating of two compatible haploid sporidia switches to a filamentous form that is pathogenic. Elucidating the cascades of the signal transduction pathway into distinct cellular responses, such as the decision to grow as yeast, engage in mating, or colonize a maize plant, is the focus of current work.
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