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.
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.
Barranco, D., Arroyo, J., & Santos-Gally, R. (2019). Avoiding sexual interference: herkogamy and dichogamy in style dimorphic flowers of Narcissus broussonetii (Amaryllidaceae). AoB PLANTS, 11(4). https://doi.org/10.1093/aobpla/plz038