Pollinator directionality as a response to nectar gradient: promoting outcrossing while avoiding geitonogamy

Abstract

Plants with multiple flowers could be prone to autonomous self‐pollination and insect‐mediated geitonogamy, but physiological and ecological features have evolved preventing costs related to autogamy. We studied the rare perennial herb Dictamnus albus as a model plant, with the aim of describing the plant–pollinator system from both plant and pollinator perspectives and analysing features that promote outcrossing in an entomophilous species. The breeding system and reproductive success of D. albus were investigated in experimental and natural conditions, showing that it is potentially self‐compatible, but only intra‐inflorescence insect‐mediated selfing is possible. Nectar analysis showed gender‐biased production towards the female phase, which follows the male phase, and during flowering, full blooming is found in flowers at the bottom of the raceme. Among a wide spectrum of insect visitors, three genera (Bombus, Apis, Megachile) were found to be principal pollinators. A study of insect behaviour showed a tendency towards bottom‐to‐top flights for the most important pollinators Bombus spp. and Apis mellifera: upward movements on the racemes could be explained by foraging behaviour, from more to less rewarding flowers. In accordance with the ‘declining reward hypothesis’, bumblebees and honeybees leave the plant when gain of reward is low, after which few flowers are visited, reducing the chance of self‐pollen transfer among flowers. Intra‐flower self‐pollination is prevented in D. albus by protandry and herkogamy, while the nectar‐induced sequential pattern of pollinator visits avoids geitonogamy and tends to maximise pollen export, promoting outcrossing. All these features for preventing selfing benefit plant fitness and population genetic structure.

Strategies of the honeybee Apis mellifera during visual search for vertical targets presented at various heights: a role for spatial attention?

“…However, when bees are foraging in a meadow with flowers at various heights, flowers can also appear in the upper part of the visual field and information received from the dorsal part of the eyes become important. For example, bees which visit raceme inflorescences tend to begin collecting nectar at the bottom of the inflorescence and ascend vertically step by step ( Fisogni et al , 2011; Ishii et al , 2008; Pyke, 1978; Valtueña et al , 2013; Waddington & Heinrich, 1979). In this situation the flowers below the just probed ones, which are perceived by the ventral visual field, typically contain no nectar, because the bee had just visited them ( Heinrich, 1975; Ishii et al , 2008).…”

Breeding system variability, pollination biology, and reproductive success of rare Polemonium caeruleum L. in NE Poland

“…On the other hand, a higher diversity of visitors implies diverse scenarios of foraging behavior and, in our opinion, it is a key factor determining the same reproductive success in both studied populations, KO and ZE. The self-compatible ZE population pollinated by A. mellifera reveals high fruiting as bee behavior commonly promotes mixed-mating, which raises reproductive success [42,43]. If the KO population is self-incompatible, geitonogamous pollen transfer causes ovules and cross-pollen discounting, which decreases reproductive success in many SI species [11,32,44,45].…”

Reproductive biology and nectar production of the Mexican endemic Psittacanthus auriculatus (Loranthaceae), a hummingbird‐pollinated mistletoe

“…The observed reproductive system in P. auriculatus flowers allows self‐ and cross‐pollination; however, protandry, male‐biased nectar production and variation in nectar offered in P. auriculatus flowers are traits that promote cross‐pollination and reduce geitonogamy (see also Fisogni et al . ).…”