Beyond pollinators: evolution of floral architecture with environment across the wild sunflowers (Helianthus, Asteraceae)

Background – There is a growing appreciation that the evolution of floral traits is shaped by selection not only from pollinators, but also factors like abiotic stress, florivory, and disease. Many such pressures impose counter-selection for smaller, less attractive flowers. In self-incompatible taxa, floral investment in attraction is required to ensure cross-pollination, setting up a potential energetic trade-off with investment in fecundity through pollen and seeds.

Methods – This study assesses ecological and evolutionary causes of variation in floral morphology across the diverse genus Helianthus (Asteraceae), focusing on floral size, colour, water content, and relative investment in attractive but sterile ray florets versus non-showy but fertile disc florets.

Key results – All floral traits were found to be highly evolutionarily labile, and the trade-off in relative investment between ray and disc florets was found to evolve independently of floral size. Both floral size and disc water content were strongly correlated with source site climate and soil characteristics, with larger heads and higher water content repeatedly evolving in more fertile and drier habitats consistent with aspects of the resource-cost and enemy-escape hypotheses of floral trait evolution, respectively. The evolution of disc colour and relative ray-disc investment was not explained by life history, flowering period, or source site environmental characteristics, suggesting that the evolution of these traits may instead be driven by other selective pressures, including perhaps pollinators.

Conclusions – Together the results of this study suggest that the macroevolution of sunflower floral architecture is likely driven by selective pressures from multiple biotic and abiotic factors, with habitat environmental conditions influencing some but not all aspects of floral morphology.