The Evolution of Flower Morphology in Gentianaceae‐Swertiinae and the Roles of Key Innovations and Niche Width for the Diversification of Gentianella and Halenia in South America

Abstract

The circumscription of genera in Swertiinae (Gentianaceae‐Gentianeae) was traditionally based on the presence of ovary or petal nectaries, variation of corolla tube length, level of stamen insertion, and presence and position of fimbriate appendages. Using a molecular phylogenetic framework, we found high homoplasy of these characters among and within the 14 genera of Swertiinae, and three of them (Gentianella, Lomatogonium, Swertia) were resolved as highly polyphyletic. We suggest that simple changes of elongation zones during the ontogeny of the corolla tube can explain most of the variation found. This hypothesis also allows us to homologize the different fimbriate flower appendages of different taxa. Halenia is the only genus of Swertiinae with spurred flowers and has the highest speciation rate known to us. We found that its nectar spurs did not function directly as a key innovation promoting rapid speciation but that changes in speciation rate in Halenia resulted from the colonization of Central and South America and the availability of new habitats and/or the presence of more potential pollinators of spurred flowers there. Only in a very young lineage of Halenia in South America does the presence of nectar spurs appear to have accelerated speciation directly. Speciation rates, flower morphological diversity, and other characters were compared between Halenia and Gentianella in South America. Gentianella proved to be much more diverse in flower morphology and probably pollination syndromes than both a younger South American lineage and an equally old Central American lineage of Halenia. We hypothesize that after a short initial phase of rapid speciation, the nectar spurs may constrain further diversification. In comparison, the flexibility in flower morphology as found in South American Gentianella has opened wider niches for diversification.