Extrachromosomal inheritance in Schizosaccharomyces pombe

Summary

Primary and secondary spore clones were analyzed from two- and three-factor crosses involving the mitochondrial markers conferring resistance to antimycin (A ^R), chloramphenicol (C ^R), and erythromycin (E ^R). As in zygote clones (Seitz-Mayr et al., 1978), transmission of markers is higher in two-factor trans-crosses than in cis-crosses. Except transmission of C ^Rin the cross A ^RC^RE^RxA ^S C ^{S E S}, no significant differences between cis- and trans-configuration were observed in three-factor crosses. In contrast to zygote clones, in spore clones transmission rates of the two or three markers in a given cross are roughly equal. 18 out of 20 secondary spore clones of different mitochondrial phenotypes appeared to be homoplasmic, whereas 2 still continued to segregate. One of these spore isolates was analyzed, and segregation was found to continue for more than 150 generations after spore germination. Whereas up to more than 80% of zygote clones in certain crosses were uniform, only 2 out of 91 tetrads were uniform, i.e. all four spores were homoplasmic for the same mitochondrial genotype. Presence or absence of recombinant mitochondrial phenytypes among secondary spore clones from tetrads indicated, whether, cytoplasmic mixing had occurred in the original zygote or not. Within an ascus, the number of spores containing recombinant genotype(s) is a direct measure for the extent of cytoplasmic mixing in the zygote. In 82 tetrads analyzed, the number of tetrads with 0, 1, 2, 3, and 4 spores containing recombinant genotype(s) were 25, 37, 14, 5, and 1, respectively. In conclusion, the extent of cytoplasmic mixing at the cell stage before forespore membrane formation is highly variable.