Heteromorphic stamens are differentially attractive in Swartzia (Fabaceae)

The division of labour hypothesis between stamens has explained the evolution of divergent functions between dimorphic stamens in the same flower. However, little is known about whether the distinct type of stamens differs in attractiveness to pollinators. Therefore, we investigate whether the two types of stamens commonly found in Swartzia have different visual and olfactory attractants. We performed observations of anthesis dynamics, registration and collection of floral visitors, measurements of reflectance of floral parts and chemical analysis of the volatile organic compounds of the floral parts of two species, S. flaemingii and S. simplex. Both species have two distinct sets of stamens: one with smaller and abundant stamens in the centre of the flower and the other with fewer but larger abaxial stamens. The sets differ in UV reflectance (only S. simplex) and exhibit a distinct chromatic contrast. Concerning olfactory attractiveness, aliphatic compounds make up most of the odour of the two species, both whole flowers and most of their floral organs. On the other hand, only S. simplex presented apocarotenoids (as ionones) and benzenoids. Furthermore, there are differences in the proportion of volatiles emitted by the stamen in both cases, as the high proportion of sesquiterpenes among the smaller stamens compared to the larger ones. In conclusion, the two types of stamens found in S. flaemingii and S. simplex show a distinct attractiveness. In addition, our data have demonstrated diverse ways of differential attractiveness both between distinct stamens set per flower and between the two species from the same pollen flowers genus.

Connective modifications and origin of stamen diversity in Melastomataceae

The androecium of Melastomataceae presents notable modifications in its merosity, morphology between whorls and in prolonged connectives and appendages. We carried out a comparative study of six Melastomataceae species to shed light on the developmental processes that originate such stamen diversity. The development of stamens was studied using scanning electron microscopy and histological observations. The stamens of all species studied have a curved shape because they emerge on a plane displaced by the perigynous hypanthium. They are the last flower organs to initiate and therefore their growth is inwards and towards the floral center. Despite the temporal inversion between carpels and stamens in Melastomataceae, the androecium maintains the centripetal pattern of development, the antepetalous stamens emerging after antesepalous stamens. The isomerous androecium can be the result of abortion of the antepetalous stamens, whereas heterostemony seems to be caused by differences in position and the stamen development time. Pedoconnectives and ventral appendages originate from the basal expansion of the anther late in floral development. The delay in stamen development may be a consequence of their dependence on the formation of a previous space so that they can grow. Most of the stamen diversity is explained by the formation of the connectives and their appendages. The formation of a basal-ventral anther prolongation, which culminates in the development of the pedoconnective, does not differ from other types of sectorial growth of the connective, which form shorter structures.

Anther cones increase pollen release in buzz-pollinated Solanum flowers

The widespread evolution of tube-like anthers releasing pollen from apical pores is associated with buzz pollination, in which bees vibrate flowers to remove pollen. The mechanical connection among anthers in buzz-pollinated species varies from loosely held conformations, to anthers tightly held together with trichomes or bioadhesives forming a functionally joined conical structure (anther cone). Joined anther cones in buzz-pollinated species have evolved independently across plant families and via different genetic mechanisms, yet their functional significance remains mostly untested. We used experimental manipulations to compare vibrational and functional (pollen release) consequences of joined anther cones in three buzz-pollinated species of Solanum (Solanaceae). We applied bee-like vibrations to focal anthers in flowers with ("joined") and without ("free") experimentally created joined anther cones, and characterized vibrations transmitted to other anthers and the amount of pollen released. We found that joined anther architectures cause nonfocal anthers to vibrate at higher amplitudes than free architectures. Moreover, in the two species with naturally loosely held anthers, anther fusion increases pollen release, whereas in the species with a free but naturally compact architecture it does not. We discuss hypotheses for the adaptive significance of the convergent evolution of joined anther cones.

Flower morphology and plant-bee pollinator interactions are related to stamen dimorphism in Melastomataceae

Approximately 20,000 species of flowering plant offer mainly pollen to their pollinators, generally bees. Stamen dimorphism, a floral trait commonly present in some pollen flowers, is thought to be associated with exclusive pollen provision for highly effective bee pollinators. Notwithstanding, little is known about how stamen dimorphism is related to other floral morphological traits and, consequently, plant-pollinator interactions at the community scale. Here we investigated the relationship between stamen dimorphism and other floral morphological traits, as well as the interactions with pollinators in plants of Melastomataceae. We characterized each plant species as stamen dimorphic or stamen isomorphic according to differences in size and shape between stamen sets. Data on interactions between the plants and their bee pollinators were analysed as quantitative bipartite networks. We found that petal and style size and shape were correlated to stamen dimorphism. Stamen dimorphic species present larger flowers and less variable style shapes than stamen isomorphic species. Furthermore, stamen dimorphism is associated with higher richness of visiting bees, i.e. higher ecological generalization. During the evolutionary history of Melastomataceae, the dependence on pollinators for fruit set has possibly favoured the evolution of larger flowers with dimorphic stamens, which in turn are able to make use of a larger spectrum of pollen-collecting bees, leading to ecological generalization.

Low bee visitation rates explain pollinator shifts to vertebrates in tropical mountains

Evolutionary shifts from bee to vertebrate pollination are common in tropical mountains. Reduction in bee pollination efficiency under adverse montane weather conditions was proposed to drive these shifts. Although pollinator shifts are central to the evolution and diversification of angiosperms, we lack experimental evidence of the ecological processes underlying such shifts. Here, we combine phylogenetic and distributional data for 138 species of the Neotropical plant tribe Merianieae (Melastomataceae) with pollinator observations of 11 and field pollination experiments of six species to test whether the mountain environment may indeed drive such shifts. We demonstrate that shifts from bee to vertebrate pollination coincided with occurrence at high elevations. We show that vertebrates were highly efficient pollinators even under the harsh environmental conditions of tropical mountains, whereas bee pollination efficiency was lowered significantly through reductions in flower visitation rates. Furthermore, we show that pollinator shifts in Merianieae coincided with the final phases of the Andean uplift and were contingent on adaptive floral trait changes to alternative rewards and mechanisms facilitating pollen dispersal. Our results provide evidence that abiotic environmental conditions (i.e. mountain climate) may indeed reduce the efficiency of a plant clade's ancestral pollinator group and correlate with shifts to more efficient new pollinators.

Intrafloral Color Modularity in a Bee-Pollinated Orchid

Flower color has been studied in different ecological levels of organization, from individuals to communities. However, it is unclear how color is structured at the intrafloral level. In bee-pollinated flowers, the unidirectional gradient in color purity and pollen mimicry are two common processes to explain intrafloral color patterns. Considering that floral traits are often integrated, usually reflecting evolutionary modules under pollinator-mediated selection, we hypothesize that such intrafloral color patterns are structured by intrafloral color modules as perceived by bee color vision system. Here, we studied the tropical bee-pollinated orchid Cattleya walkeriana, given its intrafloral color complexity and variation among individuals. Considering bee color vision, we investigated if intrafloral color modules arose among intrafloral patches (tip or base of the sepals, petals, and labellum). We expected a separate color module between the labellum patches (the main attractive structure in orchids) and petals and sepals. We measured the color reflectance and calculated the photoreceptor excitation, spectral purity, hue, and the chromatic contrast of the floral structures in the hexagon color model. Spectral purity (saturation) was higher in the labellum tip in comparison to petals and sepals, generating a unidirectional gradient. Labellum base presented a less saturated yellow UV-absorbing color, which may reflect a pollen mimicry strategy. C. walkeriana presented three intrafloral color modules corresponding to the color of petals and sepals, the color of the labellum tip, and the color of labellum base. These color modules were unrelated to the development of floral structures. Given the importance of intrafloral color patterns in bee attraction and guidance, our results suggest that intrafloral patterns could be the outcome of evolutionary color modularization under pollinator-mediated selection.