THE CONTROL OF SEXUAL MORPHOGENESIS IN THE ASCOMYCOTINA

Abstract

Summary(1) A series of factors controls sexual morphogenesis in the Ascomycotina, a process involving the formation of novel structures such as ascocarps (fruit bodies) and asci (sacs containing spores) during sexual reproduction.(2) Environmental and genetic factors must be correct before Ascomycetes may sexually reproduce. Compatibility in many heterothallic species is under polygenic control, with the mating type loci and also other genetic factors determining the productivity of sexual crosses.(3) Classical genetic studies have shown that sexual morphogenesis involves the expression of a series of developmentally regulated genes, and this has been confirmed by recent molecular studies which have demonstrated changes in patterns of mRNA and protein synthesis during ascocarp formation.(4) Hyphal differentiation leading to the formation of mature fruit bodies occurs in response to a series of signals, which include various physical and chemical factors.(5) Chemical sex factors have been identified which are believed to have important regulatory or nutritional roles in sexual morphogenesis. These include the following.(a) Diffusible sex hormones which may regulate developmental switching between asexual and sexual modes of reproduction, including (i) pheromones involved with the induction of gametangia and gamete attraction, and (ii) sex morphogens involved with triggering particular stages of fruit body formation. (b) Sexual growth substances which are required as nutrients, and may be precursors for the production of sex hormones, or metabolites used in the synthesis of novel sexual structures. Most of these sex factors are lipids.(6) Certain sex morphogens and sexual growth substances have been shown to exhibit activity in a variety of fungal species, suggesting that fungi of related phylogenetic descent may utilize similar metabolites or signalling factors during sexual reproduction.(7) Phenoloxidase enzymes may catalyse hyphal aggregation in developing fruit bodies.(8) Initial stages of ascocarp development may occur independently of the events of the sexual cycle. However, a link(s) with the functional ascogenous hyphae is needed for the formation of morphologically mature ascocarps.(9) Suitable environmental conditions are sufficient to trigger sexual morphogenesis in homothallic Ascomycetes. However, an extra level of control is present in heterothallic species, with a compatible partner required to complete sexual reproduction. This may be partly because novel regulatory products, formed by the combined action of the mating type loci of different partners, are required for further ascocarp development.(10) Further research is required to identify more fungal chemical sex factors and to determine the role of environmental stress in controlling sexual morphogenesis, and how this may be related to temporal patterns in the expression of mating type genes.