Fig. 7.

Five distinct mechanisms preventing mtA expression in mating type O. The schematic drawings summarize the state of the MAC mtA and mtB genes in mating type O clones for the seven maternal-inheritance species, and for the Mendelian species Paramecium tredecaurelia. In the former, developmental rearrangements disrupt either mtA or mtB, preventing the production of the wild-type mtA protein. Excision of the mtA promoter (gray box) is shared between Paramecium tetraurelia and the three most closely related species, but other rearrangements are unique to each species. In P. tredecaurelia and Paramecium sexaurelia, the mtB transcription factor was lost and the mtA promoter must be activated by some other factor; in P. tredecaurelia this is prevented by a point mutation (red bar) in the O-determining alleles of mtA.