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Sex ratios and genetic variation in a functionally androdioecious species, Schizopepon bryoniaefolius (Cucurbitaceae)¹

²Center for Ecological Research, Kyoto University, Kyoto 606-8502, Japan; and ³Faculty of Human and Environment Studies, Kyoto University, Kyoto 606-8501, Japan

Androdioecy, coexistence of hermaphrodites and males, is an extremely rare breeding system in angiosperms. In the present study, Schizopepon bryoniaefolius (Cucurbitaceae) was confirmed to be functionally androdioecious based on observations of floral and pollen morphology and bagging experiments. Six out of the 11 studied populations consisted of only hermaphrodites, while the other five populations were androdioecious and the frequencies of males were consistently lower than those of hermaphrodites (5.5–28.3%). To understand the consequences of an androdioecious breeding system, genetic variation was estimated using four polymorphic allozyme loci. The degree of genetic differentiation among 11 populations was high (G_ST = 0.688). Inbreeding coefficients (F_IS) for all loci significantly deviated from zero. In particular, the F_IS values averaged across the polymorphic loci in hermaphrodite populations were close to unity, suggesting that hermaphrodites are predominantly selfing in the absence of males. A significant negative correlation was found between the frequencies of males and inbreeding coefficients, indicating that outcrossing rates depend on the population sex ratio.

Key Words: androdioecy • Cucurbitaceae • genetic differentiation • inbreeding coefficient • mating system • pollen morphology • Schizopepon bryoniaefolius • sex ratio

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