Herkogamy and dichogamy in style dimorphic flowers of Narcissus broussonetii (Amaryllidaceae)

Across the plant kingdom, there is an extraordinary diversity of flower forms, many of which are related to variation in the morphology and arrangement of reproductive organs. To prevent self-pollination, plants have evolved floral forms where sexual organs are separated spatially (herkogamy) or temporarily (dichogamy) within a single hermaphrodite flower. Both mechanisms are widespread among angiosperms and have been considered to restrict self-fertilization and promote outcrossing. Avoidance of self-interference is proposed to be the driving force for the evolution of these mechanisms, particularly in self-incompatible species. However, species with anthers and stigmas at different levels may increase the rate of imprecise pollen transfer. Non-reciprocal stylar dimorphism has been considered a transitional, unstable stage towards the evolution of reciprocal style dimorphism (distyly), to simultaneously avoid interference and lack of precision.

Ameguilla spp. in a population of Narcissus broussonetii (Morocco; Safi; 32º20’N, 9º16’W; 193 m). Image credit: Rocío Santos-Gally.

In a recent study published in AoBP, Barranco et al. investigate the spatial and temporal separation of sex organs in a population of the style dimorphic (long and short) and self-incompatible Narcissus broussonetti and their consequences in the reciprocity between the sex organs of morphs and their fecundity. The authors observed that long-styled plants and short-styled plants present different strategies to avoid sexual interference and both of them had negative consequences in the reciprocity between the sex organs of morphs. Long-styled plants exhibited a delay in stigma receptivity and a higher growth rate of the style after anthesis, while short-styled plants presented higher herkogamy and no delay in stigma receptivity. These findings suggest that the avoidance of self-interference, in stylar dimorphic Narcissus species, seems to be more critical than improvement of reciprocity between the sex organs of morphs.