Investigations on the genetics of yeast

Bevan, Edward A (1955) Investigations on the genetics of yeast. PhD thesis, University of Glasgow.

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The main object of the work reported was to investigate the possibilities of (a) carrying out in yeast a type of genetic analysis of the same kind as that of random sampling of gametes in higher organisms, and (b) using intact incomplete asci for this purpose, i.e. containing only one, two, or three instead of the normal four spores. The incomplete asci utilized were derived from three genotypically different diploid strains. One strain was heterozygous for adenine requirement (AD/ad), another for tryptophane requirement (TRYP/tryp), and the third for both these requirements. Each strain was heteroxygous for the mating type gene (a/a). A study of individual spores dissected from two- and three-spore asci showed that each spore asci showed that each spore was haploid. Intact one-spore asci germinated to give haploid colonies each of whose cells were all of one mating and nutritional phenotype. Intact two-spore asci gave either diploid colonies whose cells were all of one nutritional phenotype, of haploid colonies whose cells were either of one or two nutritional pheonotypes the haploid colonies arose when the two spores were of the same mating type, and the diploid colonies when they were of complementary mating type and fused at germination. Approximately one half of the intact three-spore asci gave colonies made up of both haploid and diploid cells. The latter arose following fusion of spores arose following fusion of spores of complementary mating type, and the former from the odd spore which was invariably of a mating type. Presumably the odd spores of a mating type failed to germinate. This conclusion is supported by evidence from the analysis of intact one- and two-spore asci where approximately 80% of such spores filed to germinate. The results show that both one-spore asci and haploid cells arising from two- and three-spore asci are analogus to random samples of gametes in higher organisms and can, therefore, the genetically analysed in the same manner. Such an analysis showed that the three genes AD/ad, TRYP/tryp and a/a freely recombined with one another. The possibility of using the diploid cells arising from intact two- and three-spore asci for genetic analysis was also studied. This type of analysis, however, required the working out of an appropriate theory and the first steps in the development of this are presented. Information obtained from this 'diploid' analysis confirmed the genetic results obtained from the analysis of one-spore asci and the haploid cells derived from two- and three-spore asci. In addition, it yielded information of linkage between the gene loci and their centromeres. When taken jointly, the two types of genetic analysis of yeast, demonstrated for the first time in the present work, achieve all the advantages of random sampling and tetrad analysis, with the exceptions of directly detecting any abnormalities of meiosis and providing evidence of chromatid interference. Both the 'total isolation' and the 'delayed enrichment after starvation' techniques were employed to provide genetically marked (auxotrophic) strains necessary for the above investigations. In reporting the results emphasis is laid on a comparison of the spectrum of strains obtained with those obtained by various workers from other species of fungi. This comparison revealed the following points of interest: the parallelism of yeast, Neurospora, Aspergillus, Ophiostoma and Penicillium in the high proportions of auxotrophs requiring arginine, lysine, and adenine; the high proportion of methionine requiring autotrophs common in yeast and Neurospora but not in Aspergillus and Ophiostoma where a correspondingly high proportion of parathiotrophic types have been isolated; and the occurrence in yeast of a greater variety of amino acid requiring types than occur in either Aspergillus or Ophiostoma. Types of auxotrophs isolated which have not hitherto been recorded as having arisen in some or all of the other four species include those requiring tyrosine, histidine, serine or glycine, arginine or ornithine (not citrulline), and tryptophane (only, i.e. not responding to any known precursor). Types of auxotrophs isolated which may be of particular interest to biochemists are those requiring tyrosine and that requiring arginine or ornithine. The former do not respond to phenylalanine and may therefore involve the blockage of a similar biochemical reaction as is supposed to be blocked in human phenylketonuries. The latter does not respond to eitrulline which is an intermediate between arginine and ornithine in the Krebs cycle. Also of interest may be the auxotrophs requiring arginine and adenine which are competitively inhibited by lysine and guanine respectively. The 'delayed enrichment after starvation' technique did not prove any more efficient than the 'total isolation' technique for the isolation of yeast auxotrophs. This was partly due to the vast amount of labour necessary to weed out the diauxotrophs from the slow growing strains which, presumably, were selected by this technique. The results of the present work do not exclude the possibility that this technique would be successful should a different monoauxotrophic strain be used.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: G Pontecorvo
Keywords: Genetics
Date of Award: 1955
Depositing User: Enlighten Team
Unique ID: glathesis:1955-73658
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jun 2019 08:56
Last Modified: 14 Jun 2019 08:56

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