Transmission of chloroplast alleles in somatic fusion products obtained from vegetative cells and/or 'Gametes' of Chlamydomonas reinhardi.

Somatic fusion between cells of Chlamydomonas containing complementing cell wall and auxotrophic mutations and bearing chloroplast markers for resistance to antibiotics (streptomycin or spectinomycin) have been performed to analyze the mode of chloroplast gene transmission in the fusion products. Prototrophic colonies developed from mitotic divisions of 'diploid' fusion products were isolated on minimal medium and analyzed for their resistance to antibiotics. Fusion was performed between vegetative or nitrogen-starved cells (non-flagellated gametes) of the same or of opposite mating type. In all cases, about one third of the fusion products (mt (+)/mt (+), 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 exclusively (uniparental, or UP fusion products). The results indicate that the preferential elimination of paternal chloroplast alleles (i.e. maternal inheritance) observed after sexual fusion does not occur following artificially induced cell fusion, and that heterozygosity at the mt locus is not sufficient to ensure a directionality in uniparental chloroplast gene transmission.When somatic fusions were made between vegetative cells and nitrogen-starved cells, preferential transmission of the chloroplast alleles of the vegetative parent was observed, independently of the mating type of the parent. The data can be interpreted in terms of differences in the input frequencies of parental chloroplast genomes at the time of cell fusion. The possible importance of flagellar contact between opposite mating types in determining patterns of chloroplast gene transmission is also discussed.