The role of heterochrony in Schizanthus flower evolution- a quantitative analysis

Abstract

Changes in the timing of development within a lineage (heterochrony) have an enormous impact on floral diversification. However, comparative quantitative studies in a broad range of ontogenetic stages are still needed to identify the role of different heterochronic processes shaping diversification patterns. In this study, changes in petal development were compared across species in the genus Schizanthus showing floral diversification mediated by pollinators. In this genus, transitions from bee- to hummingbird- and moth- pollination are associated with lower lip diversification. Variation in size and shape of petals was quantified at key ontogenetic stages by using a traditional and a geometric morphometric approach. Buds from petal initiation until shortly before anthesis were considered. Lower lip reduction can arise by different combinations of ontogenetic processes such as a delay in organ initiation, a decrease in growth rate, and/or by terminal deletion of the final shape. These processes may interact having an additive effect along ontogeny. Compensatory effects can also occur, as an increase in growth rates can offset the effect of a delay in organ initiation. Heterochronic changes underlying independent reductions of the lower lip varied between lower lip structures: the wing and the keel. While wing reduction was achieved by similar ontogenetic process in both clades, keel reduction was achieved mostly by terminal deletion of the final shape in the moth-pollinated clade and by a decrease in growth rates in the hummingbird-pollinated clade. Therefore, the lower lip structures (keel and wings) originating from the same primordia are governed by different processes indicating that they are discrete developmental units. In conclusion, comparative quantitative studies along a broad range of ontogenetic stages are key to understand how different combinations of ontogenetic processes shape morphological variation and promote adaptive evolution.