Transmission, recombination and conversion of mitochondrial markers in relation to the mobility of a group I intron in Chlamydomonas.

Mitochondrial DNA transmission has been analyzed in diploids produced from sexual crosses or artificial fusions between Chlamydomonas strains which differ by several genetic markers: a group I intron (Cs cob. 1 or alpha intron), three restriction sites (Nh, Nc and H markers) located 0.5-5 kb from the insertion site of the intron, and a MUD2 point mutation (27 bp from the insertion site) conferring resistance to myxothiazol. Recombination between mitochondrial markers is a general property of all crosses and fusions analyzed. In crosses between two intron-containing (alpha+) strains or two intron-less (alpha-) strains, the transmission is preferentially paternal (mt-), with a preponderance depending on the nature of the parental genomes. In crosses between alpha+ and alpha- strains, the conversion of intron-less molecules intron+ is frequent when the alpha+ parent is maternal (mt+) and nearly absolute when the alpha+ parent is paternal (mt-). In 94% of cases, the conversion is accompanied by the co-conversion of the MUD2 marker. In both crosses and artificial fusions, the conversion of alpha- into alpha+ also influences the transmission of the more distant Nh, Nc and H markers. It is hypothesized that the more frequent transmission of the genome containing the intron results from the elimination of alpha- molecules, as a result of a double-strand cut which is induced by an endonuclease encoded by the intron.