High male fertility in males of a subdioecious shrub in hand-pollinated crosses

Female reproductive success in females versus hermaphrodites has been well documented. However, documenting a potential advantage in male fertility of male versus hermaphrodite individuals in subdioecious species is also essential for understanding the evolutionary pathway toward dioecy from hermaphroditism via gynodioecy. Siring success in terms of fruit set, fruit mass, number of seeds and mean seed mass was compared by hand-pollinated crosses in the subdioecious shrub Eurya japonica. The pollen was from male and hermaphrodite individuals, and the pollen recipients were females and hermaphrodites. Seed quality was also evaluated in terms of seed germination rate, seed germination day and seedling survival. Overall, pollen from males sired more fruits of larger size and more seeds than did pollen from hermaphrodites. The male advantage was observed when pollen recipients were females, whereas no effect was found in hermaphrodite recipients. Pollen from males also produced better quality seeds with higher germination rate and sooner germination day. Although these results could also be explained by a higher pollen load for crosses with male pollen donors, we took care to saturate the stigma regardless of the pollen donor. Therefore, these results suggest that male individuals of E. japonica have advantages in male fertility in terms of both quantity and quality. Our previous studies indicated that females exhibit higher female reproductive success compared with hermaphrodites. Thus, both the female and male functions of hermaphrodites are outperformed by females and males, respectively, raising the possibility that the subdioecious E. japonica at this study site is entering the transitional phase to dioecy along the gynodioecy–dioecy pathway.