Reproduction in fungi: Genetical and physiological aspects
It is now ahnost one hundred years since Klebs (1986)laid the foundation of a scientific approach to the study of reproduction in fungi. Since then an extensive literature has accumulated on reproduction in fungi. To present this information ill terms of principles and concepts and to highlight the immediate problems for study is a daunting task. C. G. Elliott has made a commendable attempt in presenting an overview of the genetical and physiological aspects of fungal reproduction. Fungi are remarkable for producing many kinds of reproductive structures in many ways. They produce spores both asexually and sexually. A single fungus produces a single kind of sexual spore: oospores in phycomycetes, ascospores in ascmnycetes and basidiospores in basidiomycetes. The underlying genetical and physiological mechanisms in the formation of spores are complex and we are only beginning to discern them. The introductory chapter examines the advantages of both asexual and sexual reproduction and the persistence of sex in the face of the apparent advantage of asexual reproduction. The reader is reminded that sexual reproduction in fungi not only serves to generate variability, but also provides a means of repairing damaged DNA. Moreover, it can give rise to propagules resistant to unfavourable environmental conditions. In the chapters that follow, one learns that superimposed on the basic event of nuclear fusion are a variety of regulating medhanisms, the effects of which are to determine a successful mating. Examples of some ascomycetous fungi illustrate the concepts of mating types and heterothallism. The division of individuals into two mating types, A and a, is determined by distinct gene sequences and is an incompatibility system. Each mating type is bisexual but self-incompatible. Perhaps the products of mating-type genes are involved in the attraction of the trichogyne to a cell of the opposite mating type by a diffusible mating-type-specific hormone. Ill the basidiomycetes, the situation is very complex--the mating-type determinants appear as multiple alleles at one (bipolar) or two (tetrapolar) loci, A and B. Mycelia are compatible only when both As and Bs differ, thus making :for a highly efficient outbreeding system ill the higher fungi. The A and B factors control several aspects o f dikaryon fornlation (nuclear migration, conjugate nuclear division and clamp connections) leading to the t'ormation of tile familiar agaric/mushroom fruit bodies. The most detailed insight into the mating process has come from studies with yeast. One chapter is devoted to the control of the mating-type phenotypes ill this organism. Tile reader is also introduced to tile mating systems in some basidiomycetes which allow mating cells to recognize self from nonself and to regulate a pathway of sexual development that leads to ineiosis and sporulation. Since tile writing of this book, molecular characterization of tile mating-type loci ill some of these fungi has been undertaken. These studies give some ideas on how in tile multiple-allelic