Cytoduction: A tool for mitochondrial genetic studies in yeast

Summary A study has been made of the general applications of the nuclear-fusion mutation, kar1, to mitochondrial genetic research. Procedures were developed which are suitable for constructing new strains by transfer (cytoduction) of mitochondrial genomes containing drug ^r, mit ^-, syn ^- or rho ^- mutations.Several examples of crosses of the type KAR rho ⁺ drug ^r xkarl rho ^o drug ^s were carried out and the resulting zygotes and their first buds separated by micromanipulation. Clones derived from these were in most cases homogeneous for any of the following nuclear types: heterokaryon (a+), diploid (a/), haploid a nucleus or haploid nucleus. The term cytoductant is given to the haploid rho ⁺ segregant having the same type of nucleus as the rho ^o strain employed in the cross.As examples of cytoduction of mit ^- mutations, two different pho mitochondrial genomes were studied. Transfer of pho to rho ^o acceptors was achieved with little difficulty.Haploid segregants containing recombinant mitochondrial genomes were obtained by crossing a KAR rho ⁺ cap ^r oli ^j par ^r strain with a karl rho ⁺ cap ^s oli ^s par ^s strain. Studies of intraclonal distribution of alleles indicate that cytoplasmic mixing is restricted when the dikaryotic state is maintained. Haploid cytoductant clones are usually comprised of cells retaining the mitochondrial genome of only one parent, and commonly both first-bud and residual-zygote clones are homogeneous for the same parental genome (apparent uniparental inheritance). Less frequently mixed clones are found where the number of classes of mitochondrial recombinant type is limited compared to the progeny of zygotes in which nuclear fusion takes place.These observations are discussed in terms of number and form of mitochondria forming the chondriome in the yeast cell, together with its association with the nucleus.