The yeast cell, its genetics and cytology

Abstract

The yeast cell, with its apparently intranuclear vacuole and associated structures, has been variously interpreted and has for long presented a cytological problem. The genetics of yeast, however, is a more recent study, begun by Kruis and Satava who first demonstrated the distinction between haploid and diploid forms, and extended by Winge et al. who independently made the same discovery. Lindegren and his colleagues began their investigations of yeast genetics about 1940, and since that time have made a very large number of single spore isolations and controlled matings. Their findings have mainly been published immediately in scientific journals. The work under review represents a survey of their work during the last ten years. References are made here and there to the work of others, but the greater part of the book is devoted to the researches of the author and his colleagues, and it is evident that very great progress has been made by them in the study of yeast genetics. Detailed accounts are given of the techniques employed in handling yeasts, in particular of isolating single spores, preserving strains, inducing sporulation, etc., and in this respect the book will undoubtedly be of considerable use to those engaged in research on yeast. Experimental results are presented in detail and there are numerous photographs, tables and diagrams. These form a valuable record and will enable others to compare their findings with those of Lindegren. Following the work of Beadle and Tatum with Neurospora, Lindegren et al. have studied the inheritance of characters such as the ability to synthesize various vitamins or to ferment various sugars, using strains produced by hybridizing species which diner in their biochemical potentialities. Such characters have the great advantage that they can be analysed precisely, whereas morphological characters often obscure one another. This work, coupled with tetrad analysis, has enabled Lindegren to find several linked characters and hence to prepare the first chromosome maps of yeast. Moreover, this is the first time that centromeres have been mapped without distinguishing first and second-division segregation in individual tetrads, but the author has used a rather unsatisfactory graphical method for determining the position of the centromeres and has not made clear that tetrad analysis for three unlined loci is the essential requirement. The finding of normal Mendelian inheritance for a number of characters and the mapping of genes on chromosomes has clarified the cytological picture and enabled the deduction to be made tha t apparently normal chromosomes exist in yeast. The creditable achievements in the practical study of yeast genetics described in this book are unfortunately overshadowed by interpretations of the data which are often highly speculative or even based on faulty reasoning. Thus on p. 10 of chapter 13 (it is necessary to refer to the page in this manner since each chapter starts afresh at page 1), evidence for the existence of multiple alleles controlling mating-types in the Hymenomycetes is incorrectly used as evidence for a high mutation rate. On p. 7 of chapter 24, a six-point map of a yeast chromosome is given. There is very poor agreement among certain of the map distances and consequently the true sequence of the centromere and the loci ad (affecting adenine synthesis) and pn (affecting pantothenate synthesis) is uncertain. Nevertheless, Lindegren has drawn a precise map, the sequence having apparently been chosen arbitrarily. He even attempts to explain away the discrepancy in the figures by hinting at non-random strand relationships between successive cross-overs, and on p. 3 of chapter 25 the position and strand-relationship of cross overs are given regardless of the uncertainty as to the sequence of the loci. In chapters 25 and 26 certain abnormal tetrads showing non-Mendelian ratios are interpreted as the result of "gene conversion" through the influence in the heterozygote of one gene on its homologue. This may be the correct explanation, but most geneticists would be reluctant to accept it until alternative explanations such as mutation, heterocaryosis or cytoplasmic influence have been satisfactorily eliminated. The existing evidence against these hypotheses is not convincing. In the final chapter, in which Lindegren presents his views on the structure of the gene, "gene conversion" plays a large part. Until the existence of this phenomenon is satis- factorily established, this interpretation must be regarded as speculative. The general appearance of the book leaves much to be desired. The printing resembles typescript, the right hand margins of the print are uneven, and the numbering of the pages, figures and tables is by chapters instead of consecutively through the book. There are numerous typographical and other errors, giving the impression that the work was put together hurriedly. Thus the work of Emerson referred to on p. 6 of chapter 4 and of Kniep on p. 3 of chapter 12 are not given in the bibliography, and on page 15 of chapter 27 maize and Drosophila are referred to in consecutive sentences, but no reference is made to either by name. To conclude, valuable contributions to the cytology and particularly to the genetics of yeast are brought together in one volume for the first time, but the deductions made from the experimental data and the conclusions reached represent essentially a particular point of view and are not necessarily the only possible interpretation.