Genetics and Evolutionary Mechanisms of a balanced flower color polymorphism in Whipple's Penstemon
Z. J. RADFORD , C. A. WESSINGER
Department of Biological Sciences, University of South Carolina, Coker Life Sciences 401, 700 Sumter St., Columbia, SC 29208, USA

Diversity in populations is shaped by the interplay of selective pressures and the genetic basis of phenotypes. Purifying or directional selection tends to favor a single optimal phenotype, leading to a reduction in genetic diversity over time. In contrast, balancing selection employs various mechanisms to maintain genetic diversity. The perennial wildflower Penstemon whippleanus exhibits within-population flower color polymorphism (FCP) across a wide geographic range in the Rocky Mountains, suggesting balancing selection is preserving genetic diversity. Here we examine the genetic basis and sources of selection acting on the flower color polymorphism in P. whippleanus . Using Illumina resequencing of 73 individuals across 10 populations, we identified the genetic basis for FCP as a MYB transcription factor regulating anthocyanin pigment biosynthesis. The alleles that underlie the FCP are shared across polymorphic populations, indicating the remarkable persistence of genetic variation across geographically widespread populations. This finding contrasts with scenarios where purifying or directional selection may result in multiple genetic origins for the same trait, acting on more recent timescales. Tests of molecular evolution suggest the nature of selection acting on the allelic variation. Finally, preliminary pollinator observations in natural polymorphic populations do not immediately identify the source of balancing selection.