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Dryad

Selection maintains floral color polymorphism in the scarlet paintbrush, Castilleja coccinea, reflecting combined ecological factors

Fetterly, Emma1; Braum, Anna2; Wenzell, Katherine3; Kim, Chloe4; Ashley, Mary4; Steger, Laura5; Fant, Jeremie6

Research facility: Chicago Botanic Garden
Published Sep 08, 2025 on Dryad. https://doi.org/10.5061/dryad.0k6djhbb8

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Abstract

Premise: Evolutionary theory predicts polymorphism should be rare; however, variation in floral color is common, often attributed to drift, plasticity, or variable selection. Examining floral color polymorphism both within contact zones and across a species’ range can reveal mechanisms maintaining this variation. Here, we used a multistep approach to investigate spatially heterogeneous variation in floral bract color in Castilleja coccinea.

Methods: We compared frequencies of color morphs, floral morphology, fitness, and genetic structure in regional populations and a common garden. Next, we examined habitat differences, including plant communities and edaphic factors, as potential drivers of variation. Lastly, we leveraged herbarium and iNaturalist occurrence data to investigate whether patterns were consistent at the landscape scale.

Key results: Bract color in C. coccinea is genetically heritable, with yellow dominant over red, and is under selection. Populations are predominantly monomorphic, with color distance showing no correlation to genetic or geographic distance, despite significant genetic isolation-by-distance. Yellow morphs are associated with open wetlands, while red morphs occur at drier sites associated with nearby tree cover. Red morphs demonstrated lower fitness in a common garden, suggesting tradeoffs associated with pleiotropic effects of adaptation to drier soil conditions.

Conclusions: Differences in floral color between morphs are consistent with diversification associated with a shift in ecological niche. We identified variation in edaphic and habitat conditions as probable drivers of divergence in floral color. Additionally, variation in other floral traits suggests a combined role of pollinators and habitat differences acting in concert to maintain distinct floral color morphs.