Genetic mapping of mitochondrial markers by recombinational analysis in Chlamydomonas reinhardtii

The mitochondrial genome of Chlamydomonas reinhardtii is a 15.8 kb linear DNA molecule present in multiple copies. In crosses, the meiotic products only inherit the mitochondrial genome of the mating type minus (paternal) parent. In contrast mitotic zygotes transmit maternal and paternal mitochondrial DNA copies to their diploid progeny and recombinational events between molecules of both origins frequently occur. Six mitochondrial mutants unable to grow in the dark (dk^? mutants) were crossed in various combinations and the percentages of wild-type dk⁺ recombinants were determined in mitotic zygotes when all progeny cells had become homoplasmic for the mitochondrial genome. In crosses between strains mutated in the COB (apocytochrome ) gene and strains mutated in the COX1 (subunit 1 of cytochrome oxidase) gene, the frequency of recombination was 13.7% (± 3.2%). The corresponding physical distance between the mutation sites was 4.3 kb. In crosses between strains carrying mutations separated by about 20 bp, a recombinational frequency of 0.04% (± 0.02%) was found. Two other mutants not yet characterized at the molecular level were also used for recombinational studies. From these data, a linear genetic map of the mitochondrial genome could be drawn. This map is consistent with the positions of the mutation sites on the mitochondrial DNA molecule and thereby validates the method used to generate the map. The frequency of recombination per physical distance unit (3.2% ± 0.7% per kilobase) is compared with those obtained for other organellar genomes in yeasts and Chlamydomonas.