Transmission of chloroplast genes in triploid and tetraploid zygospores of Chlamydomonas reinhardtii: Roles of mating-type gene dosage and gametic chloroplast DNA content.

Diploid clones homozygous (mt(+)/mt(+) or mt(-)/mt(-)) or heterozygous (mt(+)/mt(-); phenotypically mt(-)) for the mating-type locus and homoplasmic for a chloroplast marker conferring resistance to an antibiotic were isolated by artificially induced cell fusion or sexual mating. These diploids were crossed with haploid or diploid strains of opposite mating type and carrying another chloroplast marker. The transmission of the chloroplast genes was analyzed in the triploid and tetraploid zygospores in comparison with diploid zygospores used as controls. The transmission was almost exclusively maternal (mt(+)) (>94%) in the crosses mt(+) x mt(-), mt(+)/mt(+) x mt(-), and mt(+)/mt(+) x mt(-)/mt(-). The transmission was preferentially maternal (>76%) in the crosses mt(+) x mt(-)/mt(-) whereas in the crosses mt(+) x mt(+)/mt(-), <50% of the zygospores transmitted the chloroplast allele of maternal (mt(+)) origin. The zygospores produced in crosses mt(+)/mt(+) x mt(+)/mt(-) transmitted the alleles from both parents in >60% of cases. The results show that (i) the presence of one mt(+) allele in the mt(+)/mt(-) (phenotypically mt(-)) diploid gametes and (ii) the higher amount of chloroplast DNA molecules (input) present in the diploid gametes versus the haploid ones favor the transmission of the chloroplast allele contributed by these gametes. Moreover, because the zygospores issued from crosses mt(+)/mt(+) x mt(-) and mt(+) x mt(+)/mt(-) were genotypically identical mt(+)/mt(+)/mt(-)) but behaved very differently in their chloroplast gene transmission, it was concluded that the molecular events leading to preferential elimination of paternal DNA copies must occur before the fusion of nuclei or chloroplasts in the newly formed zygotes.