J. Genet., Vol. 73, Number 1, April 1994, pp. 55-56. (~) Printed in India.
BOOK REVIEW
Reproduction in Fungi: Genetical and Physiological Aspects By CHARLES G. ELLIOTT; Chapman & Hall, London, 1994; 310 pages; s 35.00
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 of 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 55 56 Ramesh Maheshwari mating system heteroallelic polypeptide combinations can be active in determining sexual development. The discussion of this subject in the book has become somewhat outdated because of considerable work in the last three or four years. Elliott's treatment of reproductive processes in fungi is quite broad. He has included a discussion of the developmental mutants that have been identified in both sexual and asexual reproductive pathways. Molecular analysis supports a model that shows that some developmentally expressed genes have a regulatory function, The treatment of this aspect also is superseded by more recent work on isolation and characterization of these genes and the elucidation of their regulatory interactions. In contrast to the studies on ascomycetous and basidiolnycetous fungi, genetic analysis of reproduction in the oomycetous (zygomycetous) fungi has not developed, presumably because of the difficulty in germinating their sexual spores. However, these fungi have been used in classical experiments which have revealed that mating responses are controlled by specific chemical substances that can elicit responses in very low doses. It is well known that a variety of nutritional conditions affect reproduction. However, the mechanisms underlying these effects are in no case well known. Finally, the author discusses the role of light in the induction of the fruit bodies in agarics. It is to be hoped that in future we will learn about the nature of the photoreceptors and the signal transduction leading to morphogenetic response. The book serves as an overview of the extensive literature on reproduction in fungi. It serves to remind mycologists that for a full appreciation of the beauty of fungal reproductive structures they must contemplate the underlying genetic controls. It draws the attention of professional geneticists to the potential offered by a variety of fungi in studying developmental gene regulation.
Department of Biochemistry RAMESH MAHESHWARI Indian Institute of Science Bangalore 560 012