Pollinator-mediated selection on Krameria oil flowers: A flower-pollinator fit adaptation to an atypical oil-collecting behaviour?

BACKGROUND AND AIMS: Spatial variation in plant-pollinator interactions is a key driver of floral trait diversification. A so far overlooked qualitative aspect of this variation is the behavioural component on flowers that relates to the pollinator fit. We tested the hypothesis that variation in pollinator behaviour influences the geographical pattern of phenotypic selection across the distribution range of the oil-producing Krameria grandiflora (Krameriaceae). This variation mainly involves the presence or absence of flag petal grasping, which is only performed by representatives of Centris (Centridini, Apidae), an oil-collecting bee group highly associated with Krameriaceae pollination. METHODS: We quantified variation in floral traits and fitness and estimated pollinator-mediated selection in five populations at a large geographical scale comprising the entire species range. In each population, we sampled individual pollen arrival and germination as a fitness measure, indicating pollination success and pollination performance, which was then relativized and regressed on standardized flower-pollinator fit (flag-stigma distance), advertisement (sepal length) and reward (oil volume) traits. This generated mean-scaled selection gradients used to calculate geographical selection dispersion. KEY RESULTS: Unexpectedly, stronger selection was detected on the flower-pollinator fit trait in populations highly associated to the absence of the flag petal grasping. Geographical variation in selection was mainly attributed to differential selection on the flag-stigma distance generating a selection mosaic. This may involve influences of a spatial variation in pollinator behaviour as well as composition and morphology. CONCLUSIONS: Our results show the adaptive significance of the specialized "flag" petals of Krameria in the absence of the grasping behaviour and highlight the contribution of geographical variation in pollinator behaviour on flowers in driving selection mosaics, with implications for floral evolution, adaptation to pollinator fit and phenotypic diversity in specialized systems.

Fragility of nocturnal interactions: Pollination intensity increases with distance to light pollution sources but decreases with increasing environmental suitability.

Light pollution represents a widespread long-established human-made disturbance and an important threat to nocturnal pollination. Distance from the niche centroid where optimal environmental conditions join may be related to species sensitivity to habitat change. We estimated the environmental suitability of the plant species Erythrostemon gilliesii and of its guild of hawkmoth pollinators. We considered the overlap of suitability maps of both partners as the environmental suitability of the interaction. We used a three-year record of ten E. gilliesii populations to calculate pollination intensity as the number of individuals that received pollen per population. In addition, for each population, we measured the distance to the high light pollution source around a buffer of 15 km radius. Finally, we predicted pollination intensity values for environmental suitability ranging from 0 to 1, and distance to high light pollution sources ranging from 0 to 56 Km. Pollination intensity decreased along an axis of increasing environmental suitability and increased with distance to sources of light pollution. The highest values of pollination intensity were observed at greatest distances to sources of light pollution and where environmental suitability of the interaction was lowest. The prediction model evidenced that, when environmental suitability was lowest, pollination intensity increased with distance to sources of high light pollution. However, when environmental suitability was intermediate or high, pollination intensity decreased away and until 28 km from the sources of high light pollution. Beyond 28 km from the sources of high light pollution, pollination intensity remained low and constant. Populations under conditions of low environmental suitability might be more likely to respond to disturbances that affect pollinators than populations under conditions of high environmental suitability.

Daily fluctuations in pollination effectiveness explain higher efficiency of native over exotic bees in Lepechinia floribunda (Lamiaceae).

BACKGROUND: Despite Stebbins' principle of the most efficient pollinator being proposed decades ago, the most important pollinators are still mainly identified using the frequency of visits to flowers. This shortcoming results in a gap between the characterization of the flower visitors of a plant species and a reliable estimation of the plant fitness consequences of the mutualistic interaction. The performance of a mutualistic visitor depends on its abundance, behaviour, effectiveness (pollen removal and deposition per unit time) and efficiency (seed set per unit time) conditioned by the temporal matching between pollinator activity and temporal patterns of maturation of the sexual functions of flowers. Although there have been recent attempts to provide a conceptual and methodological framework to characterize pollinators' performance, few have combined all key elements of visitors and plants to provide an accurate estimation of pollinators' performance under natural conditions. METHODS: We complement information on the flower biology and mating system of the sub-shrub Lepechinia floribunda (Lamiaceae) to provide a daily quantitative estimation of performance (effectiveness and efficiency) of the more abundant pollinators, i.e. native bumble-bees (Bombus spp.) and leafcutter bees (Megachile sp.), and the exotic honey-bee (Apis mellifera). KEY RESULTS: Unlike honey-bees or leafcutter bees, native bumble-bees matched the daily pattern of nectar production and stigma receptivity, and showed higher effectiveness and efficiency. Despite the overabundance of honey-bees, visits occurred mainly when stigmas were not receptive, thus reducing the honey-bees' overall performance. CONCLUSIONS: Bumble-bees appear to be the most important pollinators and potential historical mediators of reproductive trait evolution in L. floribunda. Because the production of seeds by bumble-bees involved fewer pollen grains for plants and less investment in floral display than honey-bees, contemporary and expected changes in pollinator abundance may affect future L. floribunda floral evolution. If bumble-bees were to be further displaced by anthropogenic disturbance or by competition with honey-bees, their lower efficiency will select for a larger floral display increasing reproductive costs. This scenario may also impose selection to reduce dichogamy to match honey-bee foraging activity.

Phenotypic selection mosaic for flower length influenced by geographically varying hawkmoth pollinator proboscis length and abiotic environment.

Biotic and abiotic context may affect the intensity of interspecific interactions and subsequently drive locally particular phenotypic selection patterns on interacting traits. We evaluated the geographical variation of matching traits of the brush-type flowers of Caesalpinia gilliesii and of the proboscis length of its guild of hawkmoth pollinators, as well as their relationship with environmental variables. We assessed the geographical variation of interacting traits (style and filament vs mean proboscis length of the guild of hawkmoths) across seven populations and estimated phenotypic selection on the plant side. Interacting traits showed similar relationships with environmental variables. Phenotypic selection on the plant side was influenced by proboscis length and by environmental conditions. Mean proboscis length of the guild was shorter than previously recorded for the same study area, thus probably shifting the selective optima of flower length. We observed two presumptive coevolutionary cold spots where one-sided negative directional selection is acting on style length. The lack of selection on the pollinator side should be further confirmed. We provided joint evidence, mostly lacking, about the geographical variation of selective pressures on the plant side associated with both proboscis length and abiotic conditions. We suggest that recent environmental change may be shifting floral length optima.