Cryptic dioecy of Symplocos wikstroemiifolia Hayata (Symplocaceae) in Taiwan

Symplocos wikstroemiifolia Hayata is one of the few morphologically androdioecious species in Symplocaceae. Although this species has been proposed as cryptically dioecious, little is known about its patterns of sexual expression in the field. We studied the breeding system and reproductive biology of S. wikstroemiifolia in Taiwan. Field investigations showed that anthers of most morphologically hermaphroditic flowers did not produce pollen grains; thus, this flower type should be considered female. Nearly all flowering individuals produced only male or female flowers. The sex ratios were slightly male-biased, but did not significantly deviate from 1:1, congruent with the proposed cryptic dioecy status. However, two individuals produced flowers that may have been functionally hermaphroditic, suggesting a variation of sexual expression in S. wikstroemiifolia. There were about 12 stamens per male flower but only five staminodes per female flower. No locules and ovules existed in the male flowers, while two locules within the ovary of the female flowers contained four ovules each (two large, two small). Male individuals produced cymes and thyrses, whereas female individuals only produced cymes. Some of these morphological characteristics differed from those previously described. Anthesis of most male and female flowers began before dawn and lasted for one to two days. Nevertheless, anther dehiscence, which only occurred under dry conditions, was not restricted to a specific time period during the day. The mean natural fruit set was 6.7%, which is relatively low compared to those in other Symplocos or dioecious species. In conclusion, S. wikstroemiifolia exhibited a nearly complete gender dimorphism, but with certain flexibility in sexual expression, and should thus be considered cryptically dioecious.