Viable meiotic segregants were isolated from a bottom fermenting brewing yeast transformed by IME1 using a high copy number plasmid. These transformants sporulated under sporulation conditions and viable meiotic segregants were obtained from the spore asci. Flow cytometry showed that the meiotic segregants had a lower DNA content per cell than the parental strains, indicating meiosis had proceeded normally. Chromosomal observation by pulsed-field gel electrophoresis and array comparative genomic hybridization (array-CGH) showed that meiotic segregants, like the parent strains, contained two types of chromosome: the S. cerevisiae type and the S. bayanus type. In certain meiotic segregants, some chromosomes were missing, and the chromosome copy number changed. Isolation of meiotic segregants resulted in a ploidy reduction, which can be applied in the breeding of yeast strains.
DNA microarray for comparative genome hybridization (CGH) of bottom-fermenting yeast was performed based on our in-house DNA sequence data. Aneuploidy, copy number variation and unique chromosomal structures were observed among bottomfermenting yeast strains. Our array experiments revealed a correlation between copy number variation and mRNA expression levels. Chromosomal structures in a Saccharomyces carlsbergensis-type strain and in a S. monacensis-type strain that both belong to S. pastorianus phylogenetically differed greatly from those in contemporary industrial bottom-fermenting yeast strains. The knowledge gained in this study contributes to a more precise genomic characterization of bottom-fermenting yeast strains.
Aim: To determine the structure of the chimeric chromosome X of bottom‐fermenting yeasts.
Methods and Results: Eight cosmid clones carrying DNA from chromosome X of bottom‐fermenting yeasts were selected by end‐sequencing. Four of the cosmid clones had Saccharomyces cerevisiae (SC)‐type and Saccharomyces bayanus (SB)‐type chimeric ends, two had SC‐type ends and two had SB‐type ends. Sequencing revealed that the bottom‐fermenting yeast strains in this study had four types of chromosome X: SC–SC, SC–SB, SB–SC and SB–SB. The translocation site in the chimeric chromosome is conserved among bottom‐fermenting yeast strains, and was created by homologous recombination within a region of high sequence identity between the SC‐type sequence and the SB‐type sequence.
Conclusions: Existing bottom‐fermenting yeast strains share a common ancestor in which the chimeric chromosome X was generated.
Significance and Impact of the Study: The knowledge gained in this study sheds light on the evolution of bottom‐fermenting yeasts.
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