Floral specialization increases pollination precision: the roles of floral orientation, symmetry, fusion and stamen number

Zygomorphic flowers last longer: the evolution of floral symmetry and floral longevity

Floral longevity, the length of time a flower remains open and functional, is a phylogenetically conserved trait that balances floral costs against the rate at which flowers are pollinated. Floral symmetry has long been considered a key trait in floral evolution. Although zygomorphic (bilaterally symmetric) flowers typically receive fewer floral visitors than actinomorphic (radially symmetric) flowers, it is yet to be determined whether this could be associated with longer floral longevity. Using newly collected field data combined with data from the literature on 1452 species in 168 families, we assess whether floral longevity covaries with floral symmetry in a phylogenetic framework. We find that zygomorphic flowers last on average 1.1 days longer than actinomorphic flowers, a 26.5% increase in longevity, with considerable variation across both groups. Our results provide a basis to discuss the ecological and evolutionary costs of zygomorphy for plants. Despite these costs, zygomorphy has evolved numerous times throughout angiosperm history, and we discuss which rewards may outweigh the costs of slower pollination in zygomorphic flowers.

Harvesting pollen with vibrations: towards an integrative understanding of the proximate and ultimate reasons for buzz pollination

Buzz pollination, a type of interaction in which bees use vibrations to extract pollen from certain kinds of flowers, captures a close relationship between thousands of bee and plant species. In the last 120 years, studies of buzz pollination have contributed to our understanding of the natural history of buzz pollination, and basic properties of the vibrations produced by bees and applied to flowers in model systems. Yet, much remains to be done to establish its adaptive significance and the ecological and evolutionary dynamics of buzz pollination across diverse plant and bee systems. Here, we review for bees and plants the proximate (mechanism and ontogeny) and ultimate (adaptive significance and evolution) explanations for buzz pollination, focusing especially on integrating across these levels to synthesize and identify prominent gaps in our knowledge. Throughout, we highlight new technical and modelling approaches and the importance of considering morphology, biomechanics and behaviour in shaping our understanding of the adaptive significance of buzz pollination. We end by discussing the ecological context of buzz pollination and how a multilevel perspective can contribute to explain the proximate and evolutionary reasons for this ancient bee-plant interaction.

Convergent evolutionary patterns of heterostyly across angiosperms support the pollination-precision hypothesis

Since the insights by Charles Darwin, heterostyly, a floral polymorphism with morphs bearing stigmas and anthers at reciprocal heights, has become a model system for the study of natural selection. Based on his archetypal heterostylous flower, including regular symmetry, few stamens and a tube, Darwin hypothesised that heterostyly evolved to promote outcrossing through efficient pollen transfer between morphs involving different areas of a pollinator's body, thus proposing his seminal pollination-precision hypothesis. Here we update the number of heterostylous and other style-length polymorphic taxa to 247 genera belonging to 34 families, notably expanding known cases by 20%. Using phylogenetic and comparative analyses across the angiosperms, we show numerous independent origins of style-length polymorphism associated with actinomorphic, tubular flowers with a low number of sex organs, stamens fused to the corolla, and pollination by long-tongued insects. These associations provide support for the Darwinian pollination-precision hypothesis as a basis for convergent evolution of heterostyly across angiosperms.