Exploring the genetic basis and proximate causes of female fertility advantage in gynodioecious Thymus vulgaris

In many gynodioecous species, females produce more viable seeds than hermap hrodites. Knowledge of the relative contribution of inbreeding depression i n hermaphrodites and maternal sex effects to the female fertility advantage and the genetic basis of variation in female fertility advantage is centra l to our understanding of the evolution of gender specialization. In this s tudy we examine the relative contribution of inbreeding and maternal sex to the female fertility advantage in gynodioecious Thymus vulgaris and quanti fy whether there is genetically based variation in female fertility advanta ge for plants from four populations. Following controlled self and outcross (sib, within population, and between-population) pollination, females had a more than twofold fertility advantage (based on the number of germinating seeds per fruit), regardless of the population of origin and the type of p ollination. Inbreeding depression on viable seed production by hermaphrodit es occurred in two populations, where inbreeding had been previously detect ed. Biparental inbreeding depression on viable seed production occurred in three of four populations for females, but in only one population for herma phrodites. Whereas the maternal sex effect may consistently enhance female fertility advantage, inbreeding effects may be limited to particular popula tion contexts where inbreeding may occur. A significant family X maternal s ex interaction effect on viable seed production was observed, illustrating that the extent of female fertility advantage varies significantly among fa milies. This result is due to greater variation in hermaphrodite (relative to female) seed fertility between families. Despite this genetic variation in female fertility advantage and the highly female biased sex ratios in po pulations of T. vulgaris, gynodioecy is a stable polymorphism, suggesting t hat strong genetic and/or ecological constraints influence the stability of this polymorphism.