NUCLEOCYTOPLASMIC MALE-STERILITY AND ALTERNATIVE ROUTES TO DIOECY

Population-genetic models of nucleo-cytoplasmic gynodioecy are shown t o allow invasion of males and conversion to dioecy in a single cytotyp e. Pleiotropic effects of restorer alleles on fertility through male o r female function can maintain a cytoplasmic polymorphism in a populat ion that prevents evolution to dioecy regardless of the pollen fertili ty of males. However, a cytoplasmic polymorphism has little effect on, and may even reduce, the minimum pollen fertility required for the sp read of males into an equilibrium gynodioecious population. Where the thresholds for dioecy are similar, the presence of males during a tran sient preequilibrium high frequency of females can accelerate evolutio n to dioecy by more than 50 times relative to nuclear male sterility. However, the appearance of a nonrestorable male-sterile cytotype gener ally eliminates males from both subdioecious and dioecious populations , converting them to purely cytoplasmic gynodioecy. These models contr adict the previously suggested notion that nucleo-cytoplasmic gynodioe cy represents a ''stable'' intermediate breeding system and instead sh ow that such gynodioecy can generally evolve to subdioecy, and often t o dioecy, as easily as nuclear gynodioecy.