In the right place at the right time for pollination

Parnassia epunctulata, a herb with generalized pollination, which exhibits sequential stamen movement.

A key innovation in the evolution of plants was the origin of the hermaphroditic flower, where both male and female sexual functions occur in the same complex structure. However, this innovation created a significant problem: sexual conflict, in which the function of one sex is compromised by the proximity and function of the other. This led to a further fundamental challenge in the function of animal-pollinated, hermaphroditic flowers: minimizing such sexual conflict while still enabling the male and female fertile parts to contact pollinators in the same place. Two solutions to sexual conflict have been explored evolutionarily by plants: (1) spatial separation of fertile parts (herkogamy) and (2) temporal separation of sexual functions (dichogamy).

To evaluate the effect of partial dichogamy and movement herkogamy on pollination accuracy in ‘generalist’ flowers (flowers pollinated by a variety of animal species), a recent paper in Annals of Botany investigates Parnassia epunctulata, a plant with open, white flowers, from subalpine meadows. The stamens of this species show a remarkable pattern of repositioning, and dehisce one by one over several days before the female phase. This feature permitted the authors to examine whether the anthers and stigma are positioned accurately, facilitating pollen removal and receipt.

The open flowers were visited by a variety of pollinators, most of which were flies. Seed set was pollinator-dependent (bagged flowers set almost no seeds) and pollen-limited (manual pollination increased seed set over open pollination). Analyses of adaptive accuracy showed that coordinated stamen movements and style elongation (movement herkogamy) dramatically increased pollination accuracy. Specifically, dehiscing anthers and receptive stigmas were positioned accurately in the vertical and horizontal planes in relation to the opposite sexual structure and pollinator position. In contrast, the spatial correspondence between anthers and stigma was dramatically lower before the anthers dehisced and after stamens bent outwards, as well as before and after the period of stigmatic receptivity. This shows for the first time that a combination of movement herkogamy and dichogamy can maintain high pollination accuracy in flowers with generalized pollination. Staggered pollen and stigma presentation with spatial correspondence can both reduce sexual interference and improve pollination accuracy.

Armbruster, W.S., Corbet, S.A., Vey, A.J., Liu, S.J., & Huang, S.Q. (2013) In the right place at the right time: Parnassia resolves the herkogamy dilemma by accurate repositioning of stamens and stigmas. Annals of Botany, 113 (1): 97-103.