Testing the Adaptive Significance of Personate Flowers in Penstemon (Plantaginaceae)

Floral diversity across plant lineages reflects evolutionary shifts in pollinator specificity and mating strategy. In outcrossing species, floral traits may govern whether plants are visited by a generalized or specialized set of pollinators, while selfing is associated with traits that improve the ability to autonomously reproduce. Penstemon, a genus endemic to North America, exhibits striking floral diversity shaped by pollinator-mediated selection. The genus contains three corolla morphologies-personate, tubular, and open-tubed. Personate flowers are displayed in a handful of Penstemon species and are a peculiar bee-pollinated floral type characterized by an upward bulge in the ventral portion of the corolla tube that obstructs the floral opening. Despite this flower morphology evolving independently a handful of times across the genus, little is known about the adaptive significance of personate flowers. To test the pollinator exclusion hypothesis, we observed bees visiting natural populations of Penstemon with open-tubed, personate, and tubular flowers. We found that both large and small bees visit all three flower types, but small bees alter their visitation behavior to enter personate flowers. To evaluate the autonomous selfing hypothesis, we compared the mating systems of P. hirsutus (personate) and P. smallii (open-tubed). Our mating system study revealed that P. hirsutus is more adapted to self-fertilization than P. smallii, exhibiting reduced inbreeding depression and increased autonomous selfing success. Whether differences in self-fertilization are due to flower shape differences is still unclear. Together, these findings attempt to contribute to understanding the evolution of floral diversity in Penstemon and demonstrate the adaptive significance of personate flowers.