Floral colour change in the eyes of pollinators: Testing possible constraints and correlated evolution

Abstract

Many of the 'adaptive' traits in angiosperm flowers occur only in some species within a clade. One good example of such traits is floral colour change, that is the retention of old, nonreproductive, rewardless, but fully turgid flowers in an altered colour. This trait has been viewed as a plant strategy to enhance distant pollinator attraction while minimizing visits to nonreproductive flowers. Considering that the same scenario appears true for most visually oriented flower visitors, however, it is unclear why this trait does not prevail in nature. To understand such evolutionary patterns, we conducted a screening search for floral colour change in 219 angiosperm species. We quantified spectral alterations in petals or petaloid organs and translated them into colour shifts through insect eyes. Using phylogeny-based comparative methods, we further explored possibilities accounting for the infrequent occurrences of floral colour change: tendency of related species to resemble each other more than those randomly drawn from the phylogenetic tree (i.e. phylogenetic signal), constraints imposed through pigment coloration and their functions and evolutionary correlates of floral colour change. Ultraviolet-sensitive insect eyes were estimated to perceive floral colour change more often than previously noticed. We detected significant phylogenetic signal in the degree of floral colour change. The evolution of floral colour change appeared constrained by pigment chemistry, because greater changes were more likely to occur in yellow or red floral parts. Postchange colours tended to retain distant visual detectability (i.e. green-receptor contrast), while reducing attractiveness in a close range (i.e. colour contrast), indicating another constraint imposed through the functions of pigment coloration. Data also suggest a possible evolutionary association between floral colour change and bee pollination. Results indicate that floral colour change is scarce in nature because it primarily evolved to accommodate bees or comparable pollinators and because its occurrence was constrained by phylogenetic relatedness, pigment chemistry and ecological functions. An understanding of floral evolution thus requires considering different causalities for trait diversity in an integrative manner.