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Two mutations are described that alter the pattern of inheritance of chloroplast genes in Chlamydomonas. The mutant gene mat-1 linked to the mating type allele mt(-) greatly increases the frequency of exceptional zygotes, i.e., zygotes that transmit chloroplast genes from the mt(-) (male) parent. In some crosses, 80-90% of the zygotes are biparental, transmitting chloroplast genes from both parents. The mat-2 mutation, linked to mt(+), acts to decrease the frequency of exceptional zygotes below the spontaneous level. The mutant effects are discussed in terms of a DNA modification-restriction system, postulated to regulate the transmission of chloroplast DNA in zygotes.

The cytoplasmic linkage group of Chlamydomonas shows maternal inheritance, i.e. preferential transmission of cytogenes from the female (m t (+)) parent and loss of the corresponding male (m t (-)) genome in sexual crosses. The mechanism of this process is postulated to be enzymatic modification of chloroplast DNA of the female to protect it from a restriction enzyme which degrades the chloroplast DNA of the male parent in the zygote soon after fusion. Genetic, biochemical and physical data bearing on this hypothesis are summarized and discussed.

A mutation, car, determining resistance to several macrolide antibiotics, including carbomycin, has been identified in the alga Chlamydomonas as cytoplasmic, and mapped in the known cytoplasmic linkage group close to genes determining resistance to other antibiotics, including streptomycin, erythromycin, and spectinomycin. The effect of the car mutation on chloroplast ribosome function was demonstrated with an in vitro system incorporating amino acids especially developed to assess activity of 70S chloroplast ribosomes. In an S-30 extract containing both 70S chloroplast and 80S cytoplasmic ribosomes, low concentrations of Mg(++) and spermidine favored 80S ribosome activity, and high concentrations activated 70S ribosomes and reversibly inactivated the 80S component. Under conditions favoring chloroplast ribosome activity, carbomycin inhibited incorporation by an S-30 extract, and by purified 70S ribosomes from wild-type but not from car cells. These results show that cytoplasmic genes are directly involved in chloroplast ribosome function and they suggest that the car gene product is a ribosomal protein; the results further strengthen the evidence that the cytoplasmic linkage group is located in chloroplast DNA.

A group of eight non-Mendelian genes have been shown by recombination analysis to be linked into a linear structure or chromosome. Similar genetic maps of gene order and relative distances between genes have been constructed by two methods, one based on additivity of recombination frequencies, the other on frequency of reciprocal recombination with a postulated attachment point. The data indicate that the progeny are diploid for this linkage group, and that the strands are distributed in a precisely oriented manner at mitosis. Evidence is discussed in support of the view that this linkage group is located in chloroplast DNA.