Pollinator, herbivore, and climatic selective pressures differ across a floral color transition zone

Spatial and temporal variations in climate and ecological interactions may underlie the origin and maintenance of floral color polymorphisms across a species range. Betalains are nitrogen-containing, phylogenetically restricted pigments that, like the widespread and well-studied anthocyanins and carotenoids, may attract pollinators, deter herbivores, and protect against abiotic stress.

We investigated which selection pressures underlie betalain pigmentation variation in Abronia fragrans, a wildflower polymorphic in floral color across its range.

While most populations produced white flowers, some in the south-central United States bore pink flowers with reduced floral display, suggesting a trade-off between pigment production and reproductive investment. A greenhouse experiment confirmed that floral pigmentation was heritable, though it was not correlated with other inflorescence traits across the species range, nor with pigmentation of most other flower or vegetative tissues. However, floral betalain concentration was positively correlated with hotter, wetter summers and milder winters across populations. Transplant experiments across this floral-color transition zone also revealed marked ecological differences: The pink-flowered site had higher seed set and leaf herbivory, while the white-flowered site experienced greater florivory. Leaf herbivore damage was highest on transplants that differed in floral color from the local morph at each site.

Although less-pigmented plants produced more flowers, this relationship did not translate to higher reproductive success, suggesting an ecological trade-off of pigmentation may drive the observed floral color pattern. Our findings underscore how complex ecological interactions may shape floral trait divergence and highlight the importance of integrating biotic and abiotic factors to understand the evolution of phenotypic variation.