Conservative seed fertility maintains hermaphrodites in gynodioecious Daphne jezoensis: a test of the sex-differential plasticity hypothesis.

Gynodioecy is assumed to be an evolutionary transition from hermaphroditism to dioecy. However, if hermaphrodites can better flexibly regulate seed production depending on resource availability than females, i.e. sex-differential plasticity (SDP), gynodioecy can be a stable state. In the gynodioecious shrub Daphne jezoensis, hermaphrodites generally exhibit low seed fertility and largely act as males. We examined the SDP hypothesis and the cost of fruit production to clarify why D. jezoensis did not evolve into unisexual morphs. We evaluated the size and resource dependency of reproduction in field experiments by manipulating soil nutrient and light conditions. We compared the plant size and pollen production among females, fruiting hermaphrodites and non-fruiting hermaphrodites. We then analysed the effect of current fruit production on subsequent flower production, i.e. the cost of fruit production. The fruiting ability was independent of plant size and resource availability in both sexual phenotypes, indicating the absence of SDP in D. jezoensis. Hermaphrodites produced larger-sized pollen and allocated more resources to pollen production in the non-fruiting year than in the fruiting year. In contrast, the cost of fruit production was not revealed for either sexual phenotype, even in the absence of pollen limitation, and even when fruit production was maximized. SDP could not explain the maintenance of hermaphrodites in D. jezoensis. Alternatively, the lower cost of fruit production in hermaphrodites due to their potentially low fruiting ability may hinder the evolutionary shift to dioecy.