Exploring the ontogenetic scaling hypothesis during the diversification of pollination syndromes in Caiophora (Loasaceae, subfam. Loasoideae)

Background and aims Phenotypic diversification of flowers is frequently attributed to selection by different functional groups of pollinators. During optimization of floral phenotype, developmental robustness to genetic and non-genetic perturbations is expected to limit the phenotypic space available for future evolutionary changes. Although adaptive divergence can occur without altering the basic developmental programme of the flower (ontogenetic scaling hypothesis), the rarity of reversion to ancestral states following adaptive radiations of pollination syndromes suggests that changes in the ancestral developmental programme of the flower are common during such evolutionary transitions. Evidence suggests that flower diversification into different pollination syndromes in the Loasoideae genus Caiophora took place during a recent adaptive radiation in the central Andes. This involved transitions from bee to hummingbird and small rodent pollination. The aim of this work was to examine if the adaptive radiation of pollination syndromes in Caiophora occurred through ontogenetic scaling or involved a departure from the ontogenetic pattern basal to this genus. Methods We used geometric morphometric variables to describe the shape and size of floral structures taking part in the pollination mechanism of Loasoideae. This approach was used to characterize the developmental trajectories of three species basal to the genus Caiophora through shape-size relationships (ontogenetic allometry). We then tested if the shape-size combinations of these structures in mature flowers of derived Caiophora species fall within the phenotypic space predicted by the development of basal species. Key Results Variation in the size and shape of Caiophora flowers does not overlap with the pattern of ontogenetic allometry of basal species. Derived bee-, hummingbird- and rodent-pollinated species had divergent ontogenetic patterns of floral development from that observed for basal bee-pollinated species. Conclusions The adaptive radiation of Caiophora involved significant changes in the developmental pattern of the flowers, rejecting the ontogenetic scaling hypothesis.