Transmission of chloroplast genes in crosses between Chlamydomonas reinhardtii diploids: Correlation with chloroplast nucleoid behavior in young zygotes.

In crosses between diploid strains of Chlamydomonas reinhardtii, the mode of chloroplast gene transmission was studied and correlated with the behavior of DAPI-stained chloroplast nucleoids in young tetraploid zygotes. Phenotypically plus (female) diploids with mating-type genotypes of mt (+)/mt (+) or mt (+)/0 (hemizygote) and phenotypically minus (male) diploids with genotypes of mt (+)/mt (-) or mt (-)/mt (-) or 0/mt (-) were obtained by either crosses or polyethylene glycol induced somatic fusions between haploid strains. When crosses were made between mt (+)/mt (+) or mt (+)/0 and mt (-)/mt (-) or 0/mt (-), the transmission of chloroplast genes occurred mostly from the female parent, indicating a typical maternal inheritance. In these cases the chloroplast nucleoids of one of the two parents disappeared within 2.5 h after mating. In contrast, when female diploids were crossed to mt (+)/mt (-) male diploids, the chloroplast genes from both parents were transmitted to the progeny at equal frequency. The chloroplast nucleoids derived from the two parents were conserved for at least 2.5 h after mating. It is concluded that the chloroplast genes derived from the heterozygous male (mt (+)/mt (-)) are protected from the degradation that takes place soon after zygote formation, by virtue of the presence of mt (+) gene although this allele is recessive with respect to the phenotypic expression of sexuality in diploids.