Transmission of mitochondrial DNA in crosses involving diploid gametes homozygous or heterozygous for the mating-type locus in Chlamydomonas

Summary The two interfertile algal species Chlamydomonas reinhardtii and C. smithii possess physically distinct mitochondrial (mit) genomes. Recently, use was made of this difference to demonstrate that sexual zygotes transmit the mit DNA from the mating-type minus (mt ^-, or paternal) parent exclusively. Diploid clones homozygous or heterozygous for the mt locus and carrying the mit genome of either of the two species were constructed by sexual crosses or artificially induced fusions. Haploid x diploid and diploid x diploid crosses were performed in order to analyze the role of both the mt locus and ploidy on the mode of transmission of mit DNA to the meiotic progeny. The inheritance of the mit DNA was determined by use of two molecular probes which hybridize to different regions of the organelle genomes. The mt ^u+/mt ^- gametes, which behave as mt ^- in the mating reaction, usually transmit their mit genome to the meiotic progeny, as do mt ^- or mt ^-/mt ^- gametes, regardless of the ploidy of the mt ⁺ gametes. In the cross mt ⁺ x mt ⁺/mt ^- however, 2 zygospore clones (out of 14) transmitted recombinant DNA molecules containing a large segment of the C. reinhardtii mit genome and a 1 kb fragment typical of C. smithii. It can thus be concluded that, contrary to what was observed earlier for chloroplast gene transmission: (1) mt ^- is dominant to mt ⁺with regard to mit DNA transmission, and (2) nuclear ploidy has little, if any, effect on mit DNA transmission.