Effects of seed size, inbreeding and maternal sex on offspring fitness in gynodioecious Plantago coronopus

Summary Male steriles (MS) must have a fitness advantage relative to hermaphrodites (H) if they are to be maintained in gynodioecious species. We report experiments in which we disentangle the relative contributions of seed size, inbreeding and maternal sex to the fitness advantage of male steriles in Plantago coronopus L. Seed size effects were observed throughout growth experiments in the glasshouse and were reflected in all biomass measurements. In the field, seed size effects resulted in a fourfold increase in standardized seed production per initial buried seed after 2 years between small (mean weight = 0.13 mg) and large (0.20 mg) seeds. Inbreeding depression, calculated from seed to seed was δ = 0.37 after one generation of selfing and δ = 0.93 after the second generation of selfing. Regression of log(1 − δ) on inbreeding level suggested synergistic epistasis in fitness. Even after taking into account the effects of seed size and inbreeding level, the offspring of a male sterile mother had a 16% advantage over a hermaphrodite, but this disappeared when the progeny sex ratio (the ratio of MS : H individuals among the offspring) was taken into account. In the field, offspring of large seeds had both a higher overall incidence of flowering, and a higher probability of flowering in their first year, thus generating an extra cohort of individuals. The high inbreeding depression in fitness after two generations of selfing was also due to a very low incidence of flowering among the S2 individuals. Flowering probability therefore appears to be a critical trait in this system. In the field, the contributions of seed size variation (15%) and inbreeding (9%) combine with 48% higher seed production to give a total fitness advantage of 70% of male steriles relative to hermaphrodites. This is probably sufficient for maintenance of gynodioecy under nuclear‐cytoplasmic inheritance of male sterility. Both inbreeding effects (as a consequence of the sexual system) and pleiotropic effects (of the genes coding for male sterility) play a role in the maintenance of gynodioecy in this species, with an apparently greater role for the latter.