When disparity arises: heterochronic development of spiral ginger flowers with contrasting pollination syndromes

Abstract The relationship between plants and their pollinators has played a crucial role in floral diversification throughout the evolution of angiosperms. The genus Costus L. presents floral morphologies associated with melittophilous and ornithophilous pollination syndromes, and species with intermediate morphology. To understand the origin of different morphologies within the genus, we used optical microscopy to analyse the floral ontogeny of seven Costus species with different floral morphologies. Our results demonstrate that the ontogeny of all species is similar in the early stages of development, and that disparity occurs later, mainly based on differences in growth of the staminodial labellum. Furthermore, we observed that in the early stages of development, the gynoecium of Costus presents bilateral symmetry, changing to an actinomorphic ovary and zygomorphic stigma at maturity. Thus, we suggest that the morphology of ornithophilous flowers arose from heterochronic events of the paedomorphy type. We also identified that this floral morphology, necessary for the transition between floral visitors, has evolved convergently through changes in the same developmental process. Our results also highlight similar patterns of gynoecium development and organization between Costaceae and Zingiberaceae, which may indicate a synapomorphy of these families.