10 Pheromone Action in the Fungal Groups Chytridiomycetes and Zygomycetes and in the Oophytes

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In the three organismic groups considered, one noticeable observation concerning pheromone action recurs. The sexual interactions in large systematic units are mediated by the same or very similar substances at least at the level of genera and probably families in chytridiomycetes and oophytes and even in a whole class in zygomycetes. Nevertheless, different levels of specificity exist within each group, enabling recognition of a compatible mating partner belonging to the same species. In oophytes and zygomycetes, species specificity is, at a first level of recognition, realised by using different derivatives and isomers of a common basic compound and, subsequently and most probably more specifically, via interactions of individual surface components. Genetic analysis of the pheromone systems has made considerable progress for zygomycetes. The other two organismic realms are lagging behind with respect to genetic analysis of mating-type communication, but recognising the phytol derivatives in Phytophthora as chemical basis for communication between mating types in oophytes must be appreciated as major achievement in understanding recognition and sexual differentiation in this organismic group. It will be rewarding to reveal the functional connection between these substances and the mating-type locus. The regulation of several genes involved in pheromone biosynthesis is basically known for zygomycetes, and there are good reasons for interpreting the homeodomain transcription factor genes, identified in many zygomycetes, as the functional principle in the mating-type loci, sexM and sexP . Information on the targets of these transcription factors, however, is still missing. The transcription factors must not necessarily be associated with regulating the communication system at the level of trisporoids, but could be responsible for later steps during the sexual differentiation programme, starting at the level of surface interactions. Current knowledge on the structure of trisporoid binding proteins, together with the properties of other apocarotenoid binding proteins, will enable us to identify the intracellular events, initiated and regulated by trisporoids. Possibly, the high similarity between various apocarotenoids with regulatory potential, abscisic acid, retinoids and trisporoids will facilitate the definition of functional properties of trisporoid binding proteins and ideally trisporoid receptors. In these respects, regulation mediated by trisporoids could resemble the retinoid situation. These compounds function as intra- and intercellular communication systems in vertebrates, especially in developmental regulatory programmes. Biologically active retinoids bind to nuclear retinoid receptors belonging to the steroid/thyroid hormone nuclear receptor superfamily. Ligand-bound receptor complexes function as transcription factors by binding to specific DNA targets, the retinoid response elements. Comparable effects of intracellularly localised trisporoids, triggered by binding directly to nuclear receptor proteins and thus acting as transcription factors, are conceivable. It will be interesting to check the products of the sexM / sexP loci for interactions with trisporoids. According to signal transduction models along this line, membrane-associated receptors would not necessarily be involved in trisporoid-mediated regulation of sexual reactions in zygomycetes.

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