EVOLUTION OF DIOECY - CAN NUCLEAR-CYTOPLASMIC INTERACTIONS SELECT FORMALENESS

A model of evolution of reproductive systems, when sex is determined b y both nuclear and cytoplasmic genes, is presented. Such a control of sex is known to facilitate the occurrence of female individuals in her maphroditic populations, thus leading to gynodioecy. A two-cytotypes t wo-nuclear loci (two alleles at each nuclear locus) model for gynodioe cy has been developed previously. Such gynodioecious systems are usual ly considered as stable, i.e. not leading to dioecy. In order to find out if the presence of females can select for male individuals when se x determination is nuclear-cytoplasmic, we followed the evolution of a lleles responsible for female sterility. These alleles can be at the p receding loci or at a third locus. We show that male individuals can b e selected. Dioecy evolves in less restrictive conditions than under n uclear sex determination. The same also holds for trioecy (coexistence of females, hermaphrodites and males). Nuclear-cytoplasmic polymorphi sm can be maintained in these reproductive systems.