PATTERNS OF MITOCHONDRIAL SORTING IN YEAST ZYGOTES

Inheritance of mitochondrial DNA (mtDNA) in Saccharomyces cerevisiae is usually biparental. Pedigree studies of zygotic first buds indicate limited mixing of wild-type (rho+) parental mtDNAs: end buds are frequently homoplasmic for one parental mtDNA, while heteroplasmic and recombinant progeny usually arise from medial buds. In crosses involving certain petites, however, mitochondrial inheritance can be uniparental. In this study we show that mitochondrial sorting can be influenced by the parental mtDNAs and have identified intermediates in the process. In crosses where mtDNA mixing is limited and one parent is prelabeled with the matrix enzyme citrate synthase 1 (CS1), the protein freely equilibrates throughout the zygote before the first bud has matured. Furthermore, if one parent is rho0 (lacking mtDNA), mtDNA from the rho+ parent can also equilibrate; intracellular movement of mtDNA is unhindered in this case. Surprisingly, in zygotes from a rho0 CS1+ X rho+ CS1- cross, CS1 is quantitatively translocated to the rho+ end of the zygote before mtDNA movement; subsequently, both components equilibrate throughout the cell. This initial vectorial transfer does not require respiratory function in the rho+ parent, although it does not occur if that parent is rho-. Mouse dihydrofolate reductase (DHFR) present in the mitochondrial matrix can also be vectorially translocated, indicating that the process is general. Our data suggest that in zygotes mtDNA movement may be separately controlled from the movement of bulk matrix constituents.