Evolutionary and Ecological Considerations on Nectar-Mediated Tripartite Interactions in Angiosperms and Their Relevance in the Mediterranean Basin

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.

Eco-evolutionary processes shaping floral nectar sugar composition

“…In spite of the general warm and humid greenhouse conditions during the Cretaceous (mean annual temperature up to 33 °C), a steady shift towards significantly drier climates 25 gave rise to the radiation of angiosperms, particularly in mid-latitude ecosystems 26 . Such drier conditions can challenge a plant’s water balance and generate selection pressures against hexose-rich nectars 18 since they are more water-demanding than sucrose-rich nectars 24 , 27 . As a consequence, sucrose-rich nectars are likely to provide an evolutionary advantage under warmer and drier conditions, which is supported by the current dominance of sucrose-rich nectars under arid conditions 28 .…”

Secondary Metabolites in Nectar-Mediated Plant-Pollinator Relationships

“…It seems likely that multiple drivers, namely plant reproductive fitness, microorganism dispersal, and climatic and environmental parameters, were responsible for the evolution of the complex chemical profile of the modern floral nectar of angiosperms [ 176 ].…”

Nectar Secretion, Morphology, Anatomy and Ultrastructure of Floral Nectary in Selected Rubus idaeus L. Varieties

“…The distinctive traits of floral nectaries facilitate the identification of ecological and phylogenetic links between related taxa. The structure and functioning of nectaries in combination with the composition of secreted nectar provides comprehensive knowledge of the relationships between plants, pollinators, and the environment [17]. Nectary micromorphology, anatomy, and ultrastructure as well as nectary secretion are important elements of plant reproduction, as they ensure visits of insect pollinators foraging for the offered reward.…”

“…STY genes engaged in the auxin biosynthetic pathway play a key role in the formation of nectaries in Eudicots [18]. The nectary is independent of the floral organ specification homeotic genes ABC [17]. Nectaries in the Rosaceae family represent the receptacular type.…”