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Cover   How many mitochondria are there in a yeast cell? For Saccharomyces cerevisiae, the answer varies depending upon growth conditions and the developmental stage of the cell. However, although the morphology discerned by microscopic snapshots changes, yeast mitochondria are dynamic structures whose fusion and fission apparently generate the equivalent of a single compartment. Indeed, yeast cells can contain a single mitochondrion as a reticulated network when growing logarithmically.  This view has emerged from a variety of studies. Stevens reconstructed 34 cells from electron micrographs of serial sections (The Molecular Biology of the Yeast Saccharomyces, Life Cycle and Inheritance, ed. J.N. Strathern, E.W. Jones, and J.R. Broach, Cold Spring Harbor, NY: Cold Spring Harbor Laboratory, 471-504, 1981). She found that all of these cells contained a giant mitochondrion and a variable number (as low as zero) of smaller organelles: the greatest fragmentation was in the stationary phase. The reconstruction of one of these giants is the top figure on the cover. Hoffmann and Avers (Science 181, 749-751, 1973) first reported a single organelle based on similar studies. The lower figure is a stereo image of a living cell whose mitochondrion, labeled with GFP, appears to be a single peripherally located network (Nunnari, et al., Mol. Biol. Cell 8, 1233-1242, 1997).  Fusion can be observed after the start of mating (Azpiroz and Butow, Mol. Biol. Cell 4, 21-36, 1993; Nunnari et al. Mol. Biol. Cell 8, 1233-1242, 1997), or at the beginning of meiosis (Miyakawa et al., J. Cell. Sci. 66, 21-38, 1984; Smith et al., Mol. Biol. Cell 6, 1381-1396, 1995). Interestingly, in the former case mitochondrial matrix proteins are distributed rapidly throughout the zygote, whereas mtDNA is usually relatively immobile.Thomas D. Fox