Floral evolution in Polemonium brandegeei (Polemoniaceae)

Abstract

Floral traits are typically associated with specific groups of pollinators. Yet, many flowering plants are pollinated by more than one group of pollinators. To explore the influence of multiple effective pollinators on floral traits, I examined how the pollinator assemblage of Polemonium brandegeei affects pollen movement and selection on floral traits. I documented phenotypic and genetic variation in floral traits, and quantified the effectiveness of floral visitors. In natural populations, I quantified pollen removal and deposition over two consecutive flowering seasons. I then performed a pair of novel array experiments to estimate selection through female (seeds set) and male (seeds sired) function on floral traits by two important pollinators, hawkmoths and hummingbirds. My analysis of field populations demonstrated that height and relative positioning of sex organs were important for the removal and deposition of pollen. Individuals of P. brandegeei displayed a large degree of continuous and heritable variation, particularly in the relative positioning of sex organs. Plants in the same population displayed stigmas recessed below (reverse herkogamy), or exserted above anthers (approach herkogamy). My array experiments determined that variation in herkogamy is likely maintained through contrasting selection, because hawkmoths selected for recessed stigmas but hummingbirds selected for exserted stigmas. While my results were the first to detect selection for reverse herkogamy by hawkmoths, I also identified selection for traits that are typically associated with both pollinators. For example, hawkmoths selected for narrow corolla tubes, and hummingbirds selected for longer corolla tubes. The selection I detected on floral traits through female function was generally stronger than through male function, which runs counter to traditional theory of gender-biased selection. My findings indicate that floral traits can conform to intermediate dimensions between the optima of two pollinators (herkogamy), or appear specialized to one pollinator (tube length). Therefore, cumulative arrangements of floral traits (floral design) can effectively function under pollination by two pollinators that exert different selection pressures on traits.