Chloroplast gene inheritance studied by somatic fusion in Chlamydomonas reinhardtii

Somatic fusion between strains of Chlamydomonas containing complementing cell-wall and auxotrophic mutations, having the same mating-type (mt) and bearing chloroplast markers, have been performed to study the mode of chloroplast gene inheritance in the fusion products. About one third of the fusion products (mt (+)/mt (+) or mt (-)/mt (-)) transmitted chloroplast markers from both parents (= biparental fusion products). The rest of the population was equally distributed between fusion products transmitting the chloroplast marker of one parent or the other (uniparental fusion products) exclusively. Incubation of the fusion products in the dark for 48 hours, immediately after the fusion, decreases the frequency of biparental fusion products. The results indicate that the general process of elimination of chloroplast alleles is independent of the presence of both mt (+) and mt (-) alleles in the cell. In contrast, directional elimination (i.e. preferential elimination of paternal chloroplast alleles) does appear to depend upon heterozygosity at the mt locus. These results are discussed in relation to the models which have been proposed to explain the maternal inheritance of chloroplast genes in Chlamydomonas.