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 hermaphrodites. Knowledge of the relative contribution of inbreeding depression in hermaphrodites and maternal sex effects to the female fertility advantage and the genetic basis of variation in female fertility advantage is central to our understanding of the evolution of gender specialization. In this study we examine the relative contribution of inbreeding and maternal sex to the female fertility advantage in gynodioecious Thymus vulgaris and quantify whether there is genetically based variation in female fertility advantage 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 pollination. Inbreeding depression on viable seed production by hermaphrodites occurred in two populations, where inbreeding had been previously detected. Biparental inbreeding depression on viable seed production occurred in three of four populations for females, but in only one population for hermaphrodites. Whereas the maternal sex effect may consistently enhance female fertility advantage, inbreeding effects may be limited to particular population contexts where inbreeding may occur. A significant family X maternal sex interaction effect on viable seed production was observed, illustrating that the extent of female fertility advantage varies significantly among families. 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 populations of T. vulgaris, gynodioecy is a stable polymorphism, suggesting that strong genetic and/or ecological constraints influence the stability of this polymorphism.