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Applied and Environmental Microbiology, April 2007, p. 2486-2490, Vol. 73, No. 8
0099-2240/07/$08.00+0     doi:10.1128/AEM.02431-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Analysis of Homothallic Saccharomyces cerevisiae Strain Mating during Must Fermentation{triangledown}

Jesús Ambrona and Manuel Ramírez*

Departamento de Microbiología, Facultad de Ciencias, Universidad de Extremadura, 06071 Badajoz, Spain

Received 17 October 2006/ Accepted 13 February 2007

Genetic instability and genome renewal may cause loss of heterozygosity (LOH) in homothallic wine yeasts (Saccharomyces cerevisiae), leading to the elimination of the recessive lethal or deleterious alleles that decrease yeast fitness. LOH was not detected in genetically stable wine yeasts during must fermentation. However, after sporulation, the heterozygosity of the new yeast population decreased during must fermentation. The frequency of mating between just-germinated haploid cells from different tetrads was very low, and the mating of haploid cells from the same ascus was favored because of the physical proximity. Also, mating restriction between haploid cells from the same ascus was found, leading to a very low frequency of self spore clone mating. This mating restriction slowed down the LOH process of the yeast population, maintaining the heterozygote frequency higher than would be expected assuming a fully random mating of the haploid yeasts or according to the Mortimer genome renewal proposal. The observed LOH occurs because of the linkage of the locus MAT to the chromosome III centromere, without the necessity for self spore clone mating or the high frequency of gene conversion and rapid asymmetric LOH observed in genetically unstable yeasts. This phenomenon is enough in itself to explain the high level of homozygosis found in natural populations of wine yeasts. The LOH process for centromere-linked markers would be slower than that for the nonlinked markers, because the linkage decreases the frequency of newly originated heterozygous yeasts after each round of sporulation and mating. This phenomenon is interesting in yeast evolution and may cause important sudden phenotype changes in genetically stable wine yeasts.

* Corresponding author. Mailing address: Departamento de Microbiología (Antiguo Rectorado), Facultad de Ciencias, Universidad de Extremadura, 06071 Badajoz, Spain. Phone: 34 924289426. Fax: 34 924289427. E-mail: mramirez{at}unex.es

{triangledown} Published ahead of print on 23 February 2007.

Applied and Environmental Microbiology, April 2007, p. 2486-2490, Vol. 73, No. 8
0099-2240/07/$08.00+0     doi:10.1128/AEM.02431-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

  • Ramirez, M., Ambrona, J. (2008). Construction of Sterile ime1{Delta}-Transgenic Saccharomyces cerevisiae Wine Yeasts Unable To Disseminate in Nature. Appl. Environ. Microbiol. 74: 2129-2134 [Abstract] [Full Text]  


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