Floral Biology and Evolution of Gynodioecism in Daphne kamtchatica var. jezoensis

Number of flowers was monitored until fruit maturation of Daphne kamtchatica var. jezoensis, a gynodioecious shrub species in a deciduous broad-leaved forest in Hokkaido, northern Japan, during three seasons from 1986 to 1988. No pollen grains were found in the anthers of female flowers. Pollen grain numbers in the anthers of hermaphrodite flowers ranged from 1000 to 7000 per anther. Fruit sets or ratios of number of ovaries in June to number of flowers in May were significantly higher in females than hermaphrodites. Results of experimental manipulation and hand pollination showed that emasculation with hand pollination as well as emasculation alone raised significantly the fruit set of hermaphrodite flowers. Bagged hermaphrodite flowers produced no fruit, implying self incompatibility. Hand pollination significantly increased the fruit set of female flowers. This indicates that pollination is the limiting factor in fruit set. Only three pollen grains per stigma on an average were found on the stigma of female flowers. More than 200 pollen grains, almost all of which are assumed to be shed by the anthers of own flower, covered the stigma surface of the hermaphroditic flowers. The phenomenon, or clogging by pollen grains of own flower is assumed to be a main factor driving the evolution towards gynodioecy in this species. A model is proposed in which fitness of the female organ inversely decreases while that of the male organ asymptotically increases with the increase of pollen grains, resulting in a bimodal pattern of total fitness explaining evolution of gynodioecism.