Heterostyly is a system in which populations are composed of a set of different floral morphs that differ in the height of stigmas and anthers reciprocally, promoting pollen movement between the morphs. Although heterostylous species commonly have narrow tubular flowers pollinated by long-tongued insects, this system is sometimes evident in open-shaped flowers. However, its effectiveness has not been understood. We aimed to clarify how heterostyly functions in an open-shaped floral species; Nephrophyllidium crista-galli (Menyanthaceae), compared with a narrow tubular floral species; Primula cuneifolia (Primulaceae).
Field surveys were conducted in alpine snow-meadow in Hokkaido. We measured floral morphology and reproductive traits, and video-recorded the foraging behavior of insect visitors.
P. cuneifolia was commonly visited by long-tongued bumble bees. Despite low visitation frequency, seed-set rate was high in both floral morphs, indicating effective pollination between floral morphs. N. crista-galli was frequently visited by short-tongued dipteran insects. Contact frequencies of flies with stigmas and anthers were reciprocal between floral morphs, and seed-set rate was similar between floral morphs. Therefore, even in open-shaped flowers, heterostyly functioned as a system promoting outcrossing pollination by short-tongued generalists.