Mating type in Chlamydomonas is specified by mid, the minus-dominance gene

Diploid cells of Chlamydomonas reinhardtii that are heterozygous at the mating-type locus (mt⁺/mt^-) differentiate as minus gametes, a phenomenon known as minus dominance. We report the cloning and characterization of a gene that is necessary and sufficient to exert this minus dominance over the plus differentiation program. The gene, called mid, is located in the rearranged (R) domain of the mt^- locus, and has duplicated and transposed to an autosome in a laboratory strain. The imp11 mt^- mutant, which differentiates as a fusion-incompetent plus gamete, carries a point mutation in mid. Like the fus1 gene in the mt⁺ locus, mid displays low codon bias compared with other nuclear genes. The mid sequence carries a putative leucine zipper motif, suggesting that it functions as a transcription factor to switch on the minus program and switch off the plus program of gametic differentiation. This is the first sex-determination gene to be characterized in a green organism.