Ecogeographic rules explain spatial trends in biodiversity, species interactions and phenotypes(1). Gloger's rule and its corollaries state that pigmentation of endothermic animals will increase from more polar to equatorial regions due to changing selective pressures including heat, humidity, predation and UV irradiance(2-4). In plants, floral pigmentation varies within and among taxa, yet causes of wide-scale geographic variation are lacking. We show that Gloger's rule explains patterns of variation in UV-absorbing floral pigmentation in a widespread plant, Argentina anserina (Rosaceae). Specifically, the floral pigmentation pattern unique to the UV spectrum (UV 'bullseye') increases with proximity to the Equator in both hemispheres, and larger bullseyes are associated with higher UVB incidence. Experiments confirm UV as an agent of selection and bullseye size as a target. Results extend the generality of an ecogeographic rule-formulated for animals-to plants, implicating UV as a selective agent on a floral trait generally assumed to enhance plant-pollinator interactions. Global change is expected to alter UV irradiance in terrestrial systems(5), potentially intensifying the importance of UV-mediated selection to floral evolution. Because floral UV reflectance and pattern enhance pollinator attraction(6,7), altered selective regimes could disrupt coevolved plant-pollinator interactions, weakening an important ecosystem service(8).

Summary1. Colour patterns on flowers can increase pollinator visitation and enhance foraging behaviour. Flowers uniform in colour to humans, however, can appear patterned to insects due to spatial variation in UV reflectance on petals. A UV 'bullseye' pattern that is common among angiosperms -UV-absorbing petal bases and UV-reflective apices -purportedly functions as a nectar guide, enhancing pollinator orientation and experimental evidence suggests that UV reflectance increases floral apparency to pollinators. 2. We test the pollinator-attracting and pollinator-orienting functions of floral UV pattern and UV reflectance under natural conditions. Specifically, we address whether UV reflection alone, or UV pattern influences small bee and syrphid fly attraction rates (approaching, landing and foraging visits), foraging rates, and likelihood of foraging and orienting to the centre of flowers, using Argentina anserina, a species whose flowers exhibit variability in the size of the UV bullseye. We manipulated UV properties while maintaining uniformly yellow petals to create three phenotypes -uniformly UV-absorptive, uniformly UV-reflective, and inversed bullseye (reflective bases and absorptive apices) and compared insect visitation and behaviour to control flowers with the common UV bullseye phenotype. 3. The presence of UV pattern increased attraction rates by both bees and syrphid flies relative to either fully UV reflective or absorptive flowers. However, only in the inverse array did the bullseye phenotype elicit higher foraging rates than the test flower. Neither the presence of pattern, nor the reversal of the common pattern influenced the likelihood of pollinator foraging or orientating to the flowers' centre during a visit. 4. We provide some of the first evidence to suggest that flowers with spatial variation in UV reflectance may be more conspicuous to insects than those with petals that uniformly absorb or reflect UV, all of which are naturally occurring phenotypes. Further, we verify that the most common UV pattern in nature increases insect attraction and foraging rate relative to the inverse pattern. Results confirm a distance apparency function of the UV bullseye, but we argue for reconsideration of the notion that pollinators benefit from this ubiquitous floral motif through enhanced foraging efficiency.

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The reproductive assurance (RA) hypothesis predicts that the ability to autonomously self-fertilize should be favored in environments where a lack of mates or pollinators limits outcross reproduction. Because such limits to outcrossing are predicted to be most severe at range edges, elevated autonomy in peripheral populations is often attributed to RA. We test this hypothesis in 24 populations spanning the range of Campanula americana, including sampling at the range interior and three geographic range edges. We scored autonomous fruit set in a pollinator-free environment and detected clinal variation-autonomy increased linearly from the southern to the northern edge, and from the eastern to the western edge. We then address whether the cline reflects the contemporary pollination environment. We measured population size, plant density, pollinator visitation, outcross pollen limitation and RA in natural populations over two years. Most populations were pollen limited, and those that experienced higher visitation rates by bumblebees had reduced pollen limitation. Reproductive assurance, however, was generally low across populations and was unrelated to pollen limitation or autonomy. Neither pollen limitation nor RA displayed geographic clines. Finally, autonomy was not associated with pollinator visitation rates or mate availability. Thus, the data do not support the RA hypothesis; clinal variation in autonomy is unrelated to the current pollination environment. Therefore, geographic patterns of autonomy are likely the result of historical processes rather than contemporary natural selection for RA.

Spatial variation in pollinator communities or behaviors can underlie floral diversification. Floral traits in the UV spectrum are common and mediate plant-pollinator interactions, but the role of pollinators in driving or maintaining their geographic variation has not been fully explored. We identify an altitudinal cline of increasing relative size of the UV bullseye pattern in Argentina anserina (Rosaceae) flowers, and assess whether pollination context contributes to clinal variation. At four sites that varied in altitude, we document the pollinator assemblage, and pollinator preference and visitation behavior. We then determine how pollinator visits affected pollen receipt and export. Finally, we describe how the functional relationship between UV floral phenotype and pollen receipt changed with altitude. Floral UV bullseye size increased with altitude, which corresponded with a change from a hymenopteran- to a dipteran-dominated pollinator assemblage. While dipteran and hymenopteran preferences for bullseye size were similar, flowers with large bullseyes received more foraging visits than those with small bullseyes at higher altitude. The reverse was observed at the lower altitudes; pollinators approached large-bullseye flowers often but rarely foraged. These differences are expected to affect fitness because foraging visits increased pollen export and receipt. Indeed, when natural variation in bullseye size was considered, it had a stronger effect on pollen receipt than other traits (flower size, display, or color). Plants with larger bullseyes tended to receive more pollen at the highest-altitude site, while those with smaller ones received more pollen at the lowest-altitude site. Results suggest that altitudinal changes in preference and behavior of the overall pollinator assemblage, but not differential preferences of pollinator taxonomic groups, could contribute to clinal variation in a UV floral trait for a generalist-pollinated plant.

Background Obtaining an optimal flower temperature can be crucial for plant reproduction because temperature mediates flower growth and development, pollen and ovule viability, and influences pollinator visitation. The thermal ecology of flowers is an exciting, yet understudied field of plant biology. Scope This review focuses on several attributes that modify exogenous heat absorption and retention in flowers. We discuss how flower shape, orientation, heliotropic movements, pubescence, coloration, opening–closing movements and endogenous heating contribute to the thermal balance of flowers. Whenever the data are available, we provide quantitative estimates of how these floral attributes contribute to heating of the flower, and ultimately plant fitness. Outlook Future research should establish form–function relationships between floral phenotypes and temperature, determine the fitness effects of the floral microclimate, and identify broad ecological correlates with heat capture mechanisms.

SummarySelection driven by biotic interactions can generate variation in floral traits. Abiotic selection, however, also contributes to floral diversity, especially with respect to patterns of pigmentation. Combining comparative studies of floral pigmentation and geography can reveal the bioclimatic factors that may drive macroevolutionary patterns of floral color.We create a molecular phylogeny and measure ultraviolet (UV) floral pattern for 177 species in the Potentilleae tribe (Rosaceae). Species are similar in flower shape and visible color but vary in UV floral pattern. We use comparative approaches to determine whether UV pigmentation variation is associated with geography and/or bioclimatic features (UV-B, precipitation, temperature).Floral UV pattern was present in half of the species, while others were uniformly UV-absorbing. Phylogenetic signal was detected for presence/absence of pattern, but among patterned species, quantitative variation in UV-absorbing area was evolutionarily labile. Uniformly UV-absorbing species tended to experience higher UV-B irradiance. Patterned species occurring at higher altitudes had larger UV-absorbing petal areas, corresponding with low temperature and high UV exposure.This analysis expands our understanding of the covariation of UV-B irradiance and UV floral pigmentation from within species to that among species, and supports the view that abiotic selection is associated with floral diversification among species.

Highlights d Globally, UV-absorbing pigmentation of flowers increased during the 20 th century d Pigmentation increased with ozone decline in taxa with pollen exposed to ambient UV d Pigmentation declined with temperature rise in taxa with pollen shielded by petals d Rapid floral pigmentation responses to global change may impact pollination