Temperature-dependent pollinator-mediated selection on floral thermoregulation.

The thermal environment is one of the most pervasive agents of selection. Most plants cannot choose their microclimate, so understanding how they cope with thermal variability is of critical concern. Several floral traits can modify the floral thermal microenvironment, which may alleviate negative impacts of thermal extremes on gametophytes and plant-pollinator interactions. While estimates of selection on traits associated with thermoregulation exist, selection on floral thermoregulation itself (i.e. the deviance of floral temperature from air) has not been quantified. We quantified pollinator-mediated and viability selection on floral thermoregulation in an alpine and lower elevation population of a widespread plant, Argentina anserina (Rosaceae), over two seasons using three fitness components. At high elevation, pollinators favored floral warming-selection via seed number and pollen export was more positive than at low elevation. At low elevation however, selection favored cooling via pollen viability in one season. Across populations and years, selection favored floral warming under cooler conditions but cooling under warmer conditions, and this pattern was driven by pollinator-mediated selection. These results provide the first direct evidence of temperature-dependent selection on floral temperature modulation. Consistent geographic differences in selection should drive local adaptation of thermoregulatory mechanisms.