Herkogamy, the spatial separation of sex organs in hermaphroditic plants, is thought to have evolved to reduce self-pollination and inbreeding within a population. Yet there has been little evidence to support these hypotheses. Much work in this field has focussed on heterostyly, in which two or three distinct and reciprocal herkogamy morphs are found in the same breeding population. The study of continuous variation in herkogamy, in which plants exhibit continuous variation in the separation of sex organs, has received far less attention. However, it warrants further attention, because the positions of plant sex organs can have profound implications for the mating biology of populations, and also because this form of herkogamy cannot be explained simply as a mechanism to prevent self-pollination. Instead, this variation could reflect contrasting selection pressures by dissimilar pollinators.
In a new study published in AoBP, Lázaro et al. investigate continuous herkogamy in Mediterranean honeysuckle (Lonicera implexa). This species is pollinated by long-tongued pollinators such as the hummingbird hawkmoth and butterflies, but also by other insects with shorter tongues, such as bees, flies and beetles. By using a herkogamy index that varied continuously from negative (reverse herkogamy; where the anthers are located above a recessed stigma) to positive (approach herkogamy; where the stigma is presented above the anthers) values, this study evaluates the effect of continuous variation in herkogamy on pollinator attraction, selfing capability, and plant fitness in L. implexa populations differing in the relative abundance of long vs short-tongued pollinators. The results show that different morphotypes might be favoured by different pollinator assemblages, and that continuous variation in herkogamy in this plant species might function as a reproductive strategy, optimizing seed quality when long-tongued pollinators are abundant, and increasing fruit set when they are rare. The authors acknowledge that their study has focussed on populations of L. implexa heavily pollinated by hawkmoths, but they hope that future work on other populations with more diverse pollinators will help to validate their results and offer more insights into the evolution of continuous herkogamy.