Reconstructing an historical pollination syndrome: keel flowers

Abstract

Background Keel flowers are bilaterally symmetrical, pentamerous flowers with three different petal types and reproductive organs enclosed by keel petals; generally there is also connation of floral parts such as stamens and keel petals. In this study, the evolution of keel flowers within the order Fabales is explored to investigate whether the establishment of this flower type within one of the species-rich families, the Fabaceae (Leguminosae), preceded and could have influenced the evolution of keel flowers in the Polygalaceae. We conducted molecular dating, and ancestral area and ancestral state analyses for a phylogeny constructed for 678 taxa using published matK, rbcL and trnL plastid gene regions. Results We reveal the temporal and spatial origins of keel flowers and traits associated with pollinators, specifically floral symmetry, the presence or absence of a pentamerous corolla and three distinct petal types, the presence or absence of enclosed reproductive organs, androecium types, inflorescence types, inflorescence size, flower size, plant height and habit. Ancestral area reconstructions show that at the time keel flowers appeared in the Polygaleae, subfamily Papilionoideae of the Fabaceae was already distributed almost globally; at least eight clades of the Papilionoideae had keel flowers with a functional morphology broadly similar to the morphology of the first evolving Polygaleae flowers. Conclusions The multiple origins of keel flowers within angiosperms likely represent convergence due to bee specialization, and therefore pollinator pressure. In the case of the Fabales, the first evolving keel flowers of Polygaleae have a functional morphology that corresponds with keel flowers of species of the Papilionoideae already present in the environment. These findings are consistent with the keel-flowered Polygaleae exploiting pollinators of keel-flowered Papilionoideae. The current study is the first to use ancestral reconstructions of traits associated with pollination to demonstrate that the multiple evolutionary origins of the keel flower pollinator syndrome in Fabales are consistent with, though do not prove, mimicry.

Novelties in Securidaca (Polygalaceae, Polygaleae) for the Floras of Colombia and Venezuela

“…The keel flowers consist of five petals in Polygalaceae, only three of which are fully developed, and the abaxial one forms an asymmetric keel (Persson, 2001; Castro et al, 2008). According to Aygören Uluer et al (2022a,b), keel flowers in the two families represent a superficial functional and morphological convergence rather than a homologous similarity. Despite the fact that the flowers of these two Fabales lineages are not homologous, their similarity has led some authors to propose that this shared resemblance is more than convergance on a floral syndrome (Aygören Uluer et al 2022a,b).…”

“…According to Aygören Uluer et al (2022a,b), keel flowers in the two families represent a superficial functional and morphological convergence rather than a homologous similarity. Despite the fact that the flowers of these two Fabales lineages are not homologous, their similarity has led some authors to propose that this shared resemblance is more than convergance on a floral syndrome (Aygören Uluer et al 2022a,b). These authors also noted that by the time keel flowers appeared in the Polygaleae (46.98–45.16 Ma), the subfamily, Papilionoideae, was already distributed almost globally.…”