GENETIC-MAPPING OF MITOCHONDRIAL MARKERS BY RECOMBINATIONAL ANALYSIS IN CHLAMYDOMONAS-REINHARDTII

The mitochondrial genome of Chlamydomonas reinhardtii is a 15.8 kb lin ear DNA molecule present in multiple copies. In crosses, the meiotic p roducts 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 recombi national events between molecules of both origins frequently occur. Si x mitochondrial mutants unable to grow in the dark (dk(-) mutants) wer e crossed in various combinations and the percentages of wild-type dk( +) recombinants were determined in mitotic zygotes when all progeny ce lls had become homoplasmic for the mitochondrial genome. In crosses be tween strains mutated in the COB (apocytochrome b) gene and strains mu tated in the COX1 (subunit 1 of cytochrome oxidase) gene, the frequenc y of recombination was 13.7% (+/- 3.2%). The corresponding physical di stance between the mutation sites was 4.3 kb. In crosses between strai ns carrying mutations separated by about 20 bp, a recombinational freq uency of 0.04% (+/- 0.02%) was found. Two other mutants not yet charac terized at the molecular level were also used for recombinational stud ies. From these data, a linear genetic map of the mitochondrial genome could be drawn. This map is consistent with the positions of the muta tion sites on the mitochondrial DNA molecule and thereby validates the method used to generate the map. The frequency of recombination per p hysical distance unit (3.2% +/- 0.7% per kilobase) is compared with th ose obtained for other organellar genomes in yeasts and Chlamydomonas.