The female advantage in natural populations of gynodioecious Plantago coronopus: seed quantity vs. offspring quality

In gynodioecious plant species, females can only persist when they have a reproductive advantage in comparison with hermaphrodites. However, several studies have shown that females do not necessarily produce more seeds than hermaphrodites, since seed production can be affected by population characteristics, such as female frequency or population size. The aim of this study was to quantify the female advantage across a large number of natural populations, examine its relationship with population sex ratio and size, and to assess the role of competition on the magnitude of the female advantage. We sampled 27 populations of Plantago coronopus (nuclear-cytoplasmic gynodioecy) along the Belgian and Dutch coast. In each population, we estimated population sex ratio and size, and assessed seed production per flower and seed production per plant. Subsequently, germination, growth, and competition experiments were performed in the greenhouse to determine the female advantage regarding offspring quality. Females produced fewer seeds per plant than hermaphrodites (FA = 0.90), and seed production was negatively related to female frequency. Since both sex morphs were equally affected by pollen availability, the female advantage was not related to population sex ratio. On the other hand, offspring of females showed higher germination and growth rates, resulting in higher competitive abilities when seeds of a female and a hermaphrodite were grown together. Overall, these results indicate that differences in competitive abilities between the offspring of females and hermaphrodites may have contributed to the maintenance of females in relatively high frequencies in populations of this short-lived gynodioecious plant species.