Abstract
Petal color variation within species is common and may be molded by abiotic or biotic selection pressures, or neutral population structure. For example, darker flowers may be favored in cooler environments because they absorb more solar radiation, elevating the temperature of reproductive structures. Additionally, flower color may evolve to attract the dominant or most efficient pollinator type in a given population. Here, we evaluate geographic variation in petal coloration across the range of Campanula americana in Eastern North America and test whether color covaries with abiotic factors, the pollination community, and genetic structure established through post-glacial expansion. Consistent with other studies, flowers from cooler, higher latitude populations were less reflective across the UV-NIR spectrum than those from warmer populations. Local temperature explained variation in petal reflectance better than the pollinator community or colonization history. Petal color perceived by trichromatic bee pollinators displayed a strong longitudinal pattern but was unassociated with climatic factors and the pollinator community. Instead, pollinator-perceived color was tightly correlated with the geographic distance from C. americana's glacial refugium. In total, abiotic conditions appear to shape large-scale geographic variation in the intensity of petal reflectance while genetic structure is the strongest driver of pollinator-perceived petal coloration. This study highlights the importance of abiotic factors and historical processes associated with range expansion as major evolutionary forces shaping diversity of flower coloration on large geographic scales.
Highlights
Floral coloration can have strong effects on plant-pollinator interactions (e.g., Gigord et al, 2001; Guitián et al, 2017), and recent evidence highlights its impact on the microclimate experienced by pollen and ovules (Lacey et al, 2010; Koski and Ashman, 2015; van der Kooi et al, 2019)
We address the following questions: 1) Does petal coloration display geographic variation? and 2) If so, is it driven by pollinator communities, climatic variables, or historical range expansion? If selection is important in shaping color variation, we predict that a) petals will be less reflective in more northern populations where elevated absorbance is likely favored in cool environments, and b) petal color as perceived by pollinators will be correlated with the relative abundances of effective bumblebee pollinators and ineffective small solitary bees
Overall petal reflectance covaried with latitude, while petal color modeled using pollinator visual systems covaried with longitude
Summary
Floral coloration can have strong effects on plant-pollinator interactions (e.g., Gigord et al, 2001; Guitián et al, 2017), and recent evidence highlights its impact on the microclimate experienced by pollen and ovules (Lacey et al, 2010; Koski and Ashman, 2015; van der Kooi et al, 2019). Neutral population genetic structure has the potential to contribute to geographic variation in petal color as well (Rausher, 2008). Genetic structuring due to range expansion and contraction have been posited to shape geographic petal color variation in Gentiana lutea (Sobral et al, 2015). An understanding of both adaptive and non-adaptive processes can be crucial for explaining floral color evolution (Streisfeld and Kohn, 2005; Streisfeld and Kohn, 2007; Hopkins et al, 2012; Baranzelli et al, 2014; Berardi et al, 2016). Non-pollinator agents of selection are often invoked to shape variation in flower coloration (e.g., Strauss and Whittall, 2006)
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