Pollen, anther, stamen, and androecium mimicry

Bees, flowers and UV

Ultraviolet light shining on flowers has various effects. In this review we assess functions of UV pigments and UV reflection patterns in flowers, including visual signalling by reflectance, fluorescence, and gloss, as well as protection against UV radiation. UV patterns originate from UV reflection and absorption in different floral parts and are visible to most pollinators, but invisible to humans. UV patterns can guide pollinators towards a floral reward, such as the centre-outward UV pattern, the so-called UV bull's eye. However, the diversity and complexity of floral colour patterns is much higher and may or may not include UV. For flower visitors, reflected UV light is merely a component of their colour vision rather than a UV signal processed separately. Yet, to humans it is a challenge to detect and represent UV reflectance in flowers. Advantages and limits of spectrophotometry, UV photography and false colour photography in bee view are discussed. Besides floral pigments causing absorption and fluorescence, flower signals can be produced by epidermal structures, i.e. smooth or conical epidermal cells, causing specular reflection (gloss) or refraction of light, and light-scattering structures causing reflection. Exposed nectar, pollen and stamens also display visual signals including UV. Finally, the absorption of UV light by pollen pigments protects the precious DNA inside the pollen grain from harmful UV radiation. UV-absorbing central parts on flowers also protect flower DNA by impeding the reflection of UV light from petals onto stamens and pollen. We briefly discuss how flower UV patterns may change in response to increasing global UV radiation, potentially influencing plant pollination.

Transcontinental patterns in floral pigment abundance among animal-pollinated species

Flower color arises primarily from pigments that serve dual functions: attracting pollinators and mitigating environmental stresses. Among major pigment types, anthocyanins and UV-absorbing phenylpropanoids (UAPs) fulfill one or both roles and should be widespread. Our review of the UV-vis absorption profiles of major floral pigments demonstrates that UAPs are the primary UV protectants. Next, we analyzed the floral pigment composition of 926 animal-pollinated species from California, Southern Spain, and Southeastern Brazil. UAPs were ubiquitous (the "dark matter" of the flower). Among the remaining pigment types, ~ 56% of species had anthocyanins, ~ 37% had carotenoids, and ~ 17% had chlorophylls (some species had > 1 pigment type). Pigment abundance varied in response to abiotic and biotic factors, particularly with pollinator type in California. Despite regional differences in environmental filtering, pollination guilds, and relatedness, UAPs are omnipresent and there is a transcontinental stable distribution of flower colors and their underlying floral pigments.

Pollinator response to yellow UV-patterned versus white UV-patternless flower dimorphism in Anemone palmata

Flower colour polymorphisms are uncommon but widespread among angiosperms and can be maintained by a variety of balancing selection mechanisms. Anemone palmata is mostly yellow-flowered, but white-flowered plants coexist in some populations. We analysed the distribution of colour morphs of A. palmata across its range. We also characterised their colours and compared their vegetative and sexual reproductive traits, pollinator attention and fitness. The range of A. palmata is limited to the Western Mediterranean, while white-flowered plants are restricted to Portugal and SW Spain, where they occur at low proportions. Yellow flowers have a characteristic UV pattern, with a UV-absorbing centre and UV-reflecting periphery, which is absent in the white morph. Colour features of both morphs were highly delineated, making it easy for pollinators to distinguish them. Both morphs were protogynous, with the same duration of sexual stages, and the main floral traits related to pollinator attraction, apart from flower colour, were similar. Hymenoptera and Diptera were the main pollinators, showing preference for the yellow morph, clear partitioning of pollinator groups between the two colour morphs and a marked constancy to flower colour during foraging. Both morphs combined clonal propagation with sexual reproduction, but sexual reproductive potential was lower in white-flowered plants. Finally, female fitness was higher in the yellow morph. Pollinator partitioning and colour constancy could maintain this polymorphism, despite the lower visitation rate and fitness of white-flowered plants, which could facilitate their clonal propagation.

Fields of flowers with few strikes: how oligolectic bees manage their foraging behavior on Calibrachoa elegans (Solanaceae)

In specialized plant-pollinator associations, partners may exhibit adaptive traits, which favor the maintenance of the interaction. The association between Calibrachoa elegans (Solanaceae) and its oligolectic bee pollinator, Hexantheda missionica (Colletidae), is mutualistic and forms a narrowly specialized pollination system. Flowers of C. elegans are pollinated exclusively by this bee species, and the bees restrict their pollen resources to this plant species. The pollen presentation schedules of C. elegans were evaluated at the population level to test the hypothesis that H. missionica females adjust their foraging behavior to the resource offering regime of C. elegans plants. For this, the number of new flowers and anthers opened per hour (as a proxy for pollen offering) was determined, and pollen advertisement was correlated with the frequency of flower visits during the day. Preferences of female bees for flowers of different stages were also investigated, and their efficiency as pollinators was evaluated. Pollen offering by C. elegans was found to be partitioned throughout the day through scattered flower openings. Females of H. missionica indeed adjusted their foraging activity to the most profitable periods of pollen availability. The females preferred new, pollen-rich flowers over old ones and gathered pollen and nectar selectively according to flower age. Such behaviors must optimize female bee foraging efficiency on flowers. Female bees set 93% of fruit after a single visit. These findings guarantee their importance as pollinators and the persistence of the specialized plant-pollinator association.