Configurational changes of mitochondria and mitochondrial nucleoids (mt-nucleoids) during meiosis and sporulation in the yeast, Saccharomyces cerevisiae, were examined using the mitochondrial membrane-binding fluorescent dye, dimethyl aminostyrylmethylpyridiniumiodine (DASPMI) and the DNA-binding fluorescent dye, 4′,6-diamidino-2-phenylindole (DAPI). In zygotes just after mating, mt-nucleoids were observed as many small discrete light spots in the cytoplasm. During meiosis in zygotes, mt-nucleoids at first coalesced with each other into a long string and then separated into spherical nucleoids in four spores. These changes paralleled those in mitochondria observed using DASPMI. The use of spheroplasts allowed us to examine the behaviour of mt-nucleoids at higher resolution and to identify several distinct meiotic prophase stages of the cell nucleus during early sporulation. In diploid spheroplasts at the stationary phase, 50–70 of the mt-nucleoids were observed to be separated from each other and each spherical mitochondrion contained only one mt-nucleoid. At the later stage of premeiotic DNA synthesis, a single branched giant mitochondrion was formed as a result of complete mitochondrial fusion. All of the mt-nucleoids were arranged in an array on a giant mitochondrion and coalesced into a string-like network. Through meiosis I and II, strings of mt-nucleoids were observed close to the dividing nuclei. At late meiosis II, a ring of mt-nucleoids enclosing each daughter nucleus was formed. In ascospores, discrete small nucleoids were visible close to each spore nucleus with a ‘string-of-beads’ appearance. Many mt-nucleoids were excluded from the ascospores and remained in the residual cytoplasm of the ascus.

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