Chloroplast gene transmission in Chlamydomonas reinhardtii. A model for its control by the mating-type locus

Summary In sexual crosses of Chlamydomonas reinhardtii, genes residing in the chloroplast (cp) are most often transmitted from the mating-type plus (mt⁺) parent only. Galloway and Goodenough (1985) proposed a model in which the mt locus (linkage group VI) is a complex region containing several genes involved in the control of both gametic differentiation and cp inheritance. The mt⁺ locus contains: the sfu locus necessary for sexual fusion between gametes; the upp locus (uniparental plus) which controls cp gene inheritance and also perhaps zygote maturation; and the sad locus which functions in sexual adhesion. The mt^– locus also contains a sad locus as well as a regulatory element (mid) necessary for the minus dominance in mt⁺/mt^– diploid gametes. This model has been extended to include new genetic functions linked to the mt⁺ or mt^– locus. In this new system, there is a group of genes (maps for mating-type plus structure), present in both plus and minus strains, controlling some mt⁺ phenotypical traits as well as the synthesis of an activator of the cp DNA nuclease; two genes contained in the mt^– locus — one (cge for cp genomic elimination) coding for a nuclease (in an inactive form) located in the chloroplast and another (mid) coding for a repressor of maps; one gene (upp) contained in the mt⁺ locus, coding for a substance preventing the synthesis or the activity of the nuclease and perhaps also acting on the maps product. The model can be applied to explain the complex dominance/ recessivity relationships observed between the mt⁺ and mt^– alleles in heterozygous diploid gametes, the basis for mitotic vs meiotic zygote formation and the differences in transmission observed for these two physiologically distinct zygote classes. Application of the model for prediction of mutant phenotypes and the design of future experiments is also described.