Nectary size is a pollination syndrome trait in Penstemon

Evolution of complex phenotypes depends on the adaptive importance of individual traits, and the developmental changes required to modify traits. Floral syndromes are complex adaptations to pollinators that include color, nectar, and shape variation. Hummingbird-adapted flowers have evolved a remarkable number of times from bee-adapted ancestors in Penstemon, and previous work demonstrates that color over shape better distinguishes bee from hummingbird syndromes. Here we examined the relative importance of nectar volume and nectary development in defining Penstemon pollination syndromes.

We tested the evolutionary association of nectar volume and nectary area with pollination syndrome across nineteen Penstemon species. In selected species, we assessed cellular-level processes shaping nectary size. Within a segregating population from an inter-syndrome cross, we assessed trait correlations between nectar volume, nectary area, and the size of stamens on which nectaries develop.

Nectar volume and nectary area displayed an evolutionary association with pollination syndrome. These traits were correlated within a genetic cross, suggesting a mechanistic link. Nectary area evolution involves parallel processes of cell expansion and proliferation.

Our results demonstrate that changes to nectary patterning are an important contributor to pollination syndrome diversity, and provide further evidence that repeated origins of hummingbird adaptation involve parallel developmental processes in Penstemon.