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Inbreeding avoidance has been proposed as a key factor in the evolution and maintenance of separate sexes in flowering plants, but requires the controversial coexistence of high selfing rate and high inbreeding depression in hermaphrodites. Subdioecy (males, females and hermaphrodites in populations) provides an ideal system for exploring the role of inbreeding avoidance because the 3 sexual morphs can be compared. In subdioecious 'W. dioica' populations, we examined selfing rates, inbreeding depression and relative performance, using open-pollinated and experimentally pollinated hermaphrodites (selfed and crossed) and females (crossed). Open hermaphrodites produced 62% fewer seeds and exhibited higher selfing rates than females (0.60 to 0.25). Hermaphrodite inbreeding depression was high (> 0.85) and parental inbreeding coefficients were zero, indicating that selfed progeny are unlikely to reproduce. Relative performance of open hermaphrodites to females (0.27) was sufficiently low to maintian females at observed frequencies in populations, assuming nuclear genes control male sterility. By experimentally increasing outcrossing, we improved hermaphrodite performance (0.80) to levels that should exclude females from populations. Our findings demonstrate that intermediate selfing rates and high inbreeding depression reduce hermaphrodite fitness, and confirm the role of inbreeding avoidance in maintaining gender dimorphism in 'W. dioica'. |
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