Sexual dimorphism in a gynodioecious species, Aruncus aethusifolius (Rosaceae)

The sexual system of Aruncus aethusifolius (Amygdaloideae, Rosaceae), which is endemic to Korean, remains poorly understood. Here, we investigate the sexual system of A. aethusifolius based on a detailed morphological, micromorphological, and anatomical study. The natural population of this endemic species is restricted to Hallasan Mountain on Jeju Island. We found two types of plants in the natural population: hermaphrodite and female plants. Each sexual morph exhibited different floral traits, showing a modest form of sexual dimorphism. Significant quantitative differences were detected in the investigated organs (i.e., petal, sepal, ovary, stamen, nectary, and follicle). For example, hermaphroditic floral organs (i.e., petal, sepal, stamen and nectary) were larger than those of the female flowers (P < 0.001), but the ovary and follicle were relatively larger in female flowers than in hermaphrodites (P < 0.001). The fruit-setting rate (proportion of matured fruits/flowers) in female individuals (approximately 79.29%) was, on an average, more than twice that of hermaphroditic ones (approximately 34.41%). In addition, female flowers produced more seeds per follicle. Usually, 5–6 seeds per follicle developed in female flowers, whereas 3–4 seeds were produced in the hermaphroditic flowers. Accordingly, A. aethusifolius is another example of a gynodioecious species of flowering plants with clear sexual dimorphism.