Abstract
The Mediterranean basin hosts a high diversity of plants and bees, and it is considered one of the world’s biodiversity hotspots. Insect pollination, i.e., pollen transfer from male reproductive structures to conspecific female ones, was classically thought to be a mutualistic relationship that links these two groups of organisms, giving rise to an admirable and complex network of interactions. Although nectar is often involved in mediating these interactions, relatively little is known about modifications in its chemical traits during the evolution of plants. Here, we examine how the current sucrose-dominated floral nectar of most Mediterranean plants could have arisen in the course of evolution of angiosperms. The transition from hexose-rich to sucrose-rich nectar secretion was probably triggered by increasing temperature and aridity during the Cretaceous period, when most angiosperms were radiating. This transition may have opened new ecological niches for new groups of insects that were co-diversifying with angiosperms and for specific nectar-dwelling yeasts that originated later (i.e., Metschnikowiaceae). Our hypothesis embeds recent discoveries in nectar biology, such as the involvement of nectar microbiota and nectar secondary metabolites in shaping interactions with pollinators, and it suggests a complex, multifaceted ecological and evolutionary scenario that we are just beginning to discover.
Highlights
The Mediterranean basin hosts 25,000 angiosperm species and accounts for 7.8% of world plant diversity, while it covers only 1.6% of emerged land [1]
A nectar sugar profile rich in sucrose is a common trait of most angiosperms and a trait shared by most Mediterranean plants [22,26,34]
We postulate that this could have been an adaptation of angiosperm flowers to increasing aridity in the Cretaceous period, which influenced the evolution of flowers with a corolla tube
Summary
The Mediterranean basin hosts 25,000 angiosperm species and accounts for 7.8% of world plant diversity, while it covers only 1.6% of emerged land [1]. We discuss nectar traits in angiosperms in the light of recent advances in nectar biology, ecology, and evolution, taking the Mediterranean plants as a case study. As far as the evolution of nectar is concerned, there is recent evidence that the plant–arthropod relationships involved in pollination and mediated by nectar-like secretions were established well before the rise of angiosperms [27,28]. The ecological and evolutionary significance of this emerging tripartite relationship is still far from well understood [31] All these things suggest a complex ecological and evolutionary scenario concerning floral nectar. It is evident today that in its interactions with organisms, nectar exhibits an array of potential functions far exceeding that of a simple food reward
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