Secondary pollen presentation: More than to increase pollen transfer precision

Abstract

Pollination precision and efficiency have been deemed to be important driving forces in floral evolution. Herkogamy reduction is a main mechanism to increase pollination precision. Secondary pollen presentation (SPP), by which pollen is presented on other floral organs especially pistils, has been widely accepted as a special mechanism to increase pollen transfer precision through spatial reduction of the anther-stigma distance, that is, minimized herkogamy. This overlooks a potential driving force, that is expanding the pollination niche through converting pollen thieves and nectar robbers into effective pollinators. We selected two species as study models with typical pistillate SPP, Pavetta hongkongensis Bremek. (Rubiaceae) and Scaevola taccada (Gaertn.) Roxb. (Goodeniaceae). In both species, two distinct pollinator functional groups were recognized. Short-tongued bees and flies fed on pollen on stigmas but also stole pollen from anthers and robbed nectar, whereas long-tongued hawkmoths and butterflies only collected nectar. Emasculation had no influence on long-tongued pollinators, but significantly decreased the visitation frequency of short-tongued visitors and fruit set, compared to intact flowers, demonstrating short-tongued visitors did not effectively pollinate and acted merely as pollen thieves or nectar robbers when SPP was absent. Data from the two plant species clearly indicated pistillate SPP has additional adaptive advantages of converting ineffective visitors into pollinators and consequently widening the pollination niche, which could help plants overcome environmental stochasticity. Our results suggest that multiple selective forces drive the evolution of SPP and the minimization of herkogamy.

Potential role of damselflies in the pollination of Callitriche rimosa Fasset

“…Since the damselflies depend on freshwater bodies for oviposition and larvae development (Corbet, 1999) and they use the leaves of C. rimosa as a landing surface when are waiting for a mate, this occasional interaction favours both plants and insects' reproduction. In this sense, the accessory pollen presentation in this plant species, despite being less specialized as the secondary pollen presentation, could generate adaptive advantages of converting occasional visitors/perchers into pollinators and, consequently, increase the pollination niche of Callitriche (see Xu et al, 2021). It would be important for future studies to conduct a damselfly-exclusion experiment under natural conditions to compare plant reproductive success with and without damselflies.…”

Pseudopollen in Camellia oleifera and its implications for pollination ecology and taxonomy

“…This is possibly because the framework of their cell walls is composed of unbranched edges. On this basis, we hypothesize that the existence of pseudopollen is a pollen presentation strategy for C. oleifera to protect pollen from waste ( Xu et al, 2021b ). Additionally, with the development and maturity of pseudopollen, intracellular polysaccharide content gradually decreased.…”