Uniparental inheritance of chloroplast genes is a phenomenon common to both higher plants and eukaryotic algae1. In the latter case, as exemplified by the heterothallic green alga Chlamydomonas reinhardtii, chloroplast gene transmission is controlled by the nuclear mating-type locus2,3. When the stable opposite mating types are crossed, chloroplast markers carried by the mating-type plus (mt+) gamete are transmitted to the zygote progeny, while the markers (and chloroplast DNA) of the mating-type minus (mt-) gamete are not, in most cases 4-7. Here we report the isolation of a uniparentally inherited erythromycin-resistant mutant (ery-u1) of the homothallic (self-mating) alga, Chlamydomonas monoica. The inheritance pattern of the ery-u1 mutant can be interpreted in terms of a general model for homothallism which assumes that each cell of a clonal population carries genetic information analogous to the mt+ and mt- alleles of heterothallic species, and that self-mating results from differential gene expression within the clonal population.
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