Heterostyly, incompatibility system and pollinators in Varronia spinescens Borhidi (L.) (Cordiaceae)

Abstract

Heterostyly, the reciprocal position of sexual organs between different morphs within a population, is assumed to promote cross-pollination in hermaphroditic plants. Heterostyly is usually linked to an incompatibility system where only the crosses between the different morphs produce fruit. Although heterostyly has been frequently studied in plant species with specialized pollination systems, few studies have investigated this in species with a broad spectrum of floral visitors. Here, we describe the heterostyly of Varronia spinescens and conducted hand-pollination experiments in the field to determine the incompatibility system. We also analysed perianth morphology, morph-ratio, adaptive inaccuracy in reciprocity and pollinator visitation rate in relation to floral morph and number of open flowers. Our results indicate that V. spinescens is a distylous species with a high degree of reciprocity between the sexual organs, presenting a non isoplethic morph ratio with a bias towards the long style (L-) morph. The incompatibility system is heteromorphic and the studied population received visits by a very high number of visitor and pollinator species. Varronia spinescens is thus the heterostylous species of the genus with the second-highest number of registered pollinators. Our results suggest that in a generalist pollination system with little floral morphological restriction to pollinators, the maintenance of sexual polymorphism is likely associated with a high reciprocity between morphs.