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Cytoduction (Fig. 5.4A) is the transfer of subcellular organelles between cells without the transfer of nuclear genes (Spencer and Spencer, 1996); this process can be used to specifically transfer mitochondria and mitochondrial genes, or the virus-like-particles that cause the ‘killer' phenotype (reviewed by Bussey, 1991; Wickner and Edskes, 2015). The use of cytoduction to introduce the ‘killer' phenotype, as well as mitochondria, into ale and lager yeast strains has been reported (Hammond and Eckersley, 1984) with the eventual goal(s) of creating brewing yeasts that could kill contaminating yeasts, or display altered fermentation characteristics, respectively. The method works by fusing or mating a ‘recipient' strain carrying a mutation (in the KAR1 gene) that makes its nucleus unable to fuse with the ‘donor' cell's nucleus after mating between the two cells has occurred. Cytoplasmic contents from the donor cell can then be transferred into the cytoplasm of the recipient cell; additionally, single chromosomes can ‘leak out' from the nucleus of the donor cell and get taken up by the nucleus of the recipient strain. Nilsson- Tillgren et al. (1981) performed early experiments using this method with lager yeasts, transferring single lager yeast chromosomes into S. cerevisiae lab strains. This allowed some genetic characterization of the transferred lager chromosomes and also helped deduce the interspecific hybrid nature of lager yeasts; however this method is laborious and restricted to genetic characterization, and is thus not very useful in the development of new and improved brewing yeast strains.

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