Polymorphisms may be maintained within a species when selection pressures vary spatially. Thus, geographic variation in phenotypic traits may reflect differences in selection across a species range. Differences in selection pressures exerted by local pollinator groups can produce divergence in floral traits among plant populations, often according to predictable pollination syndromes. However, this process is poorly understood at range-wide scales, where both floral traits and pollinator assemblages may vary along a cline, or across a mosaic. Here, we investigated variation in floral traits across the range of Castilleja sessiliflora (Orobanchaceae), in relation to differences in local pollinator assemblages. We examined whether divergence in certain key traits may relate to visitation by syndrome-predicted pollinators. Across the species range, we measured floral and bract color and floral morphometric traits putatively related to pollinator efficiency (corolla tube length and width and stigma exsertion, among others). We performed pollinator observations at the eastern and western range extents, and examined whether variation in floral traits among populations corresponded to differences in visiting pollinator groups. In doing so, we aimed to quantify the degree and pattern of range-wide variation in floral traits, and to identify differences in local pollinator assemblages, which may contribute to floral divergence.
Results/Conclusions
Floral color and morphology varied dramatically across the range of C. sessiliflora. Floral color ranged from pink to white, varying clinally from pink in the southwestern range (Texas) to white in the northeast (Minnesota and Illinois), with intermediate morphs throughout. Corolla tube length was found to vary notably among populations in a mosaic-like pattern (rather than continuous gradient). At the eastern range extent, hawkmoths were observed to be the most common visitor to flowers, which were white in color, with long, narrow corolla tubes. To the west, both hawkmoths and large and medium-sized bees were common visitors, and plants displayed pale pink flowers with shorter, wider corolla tubes. Because white flowers and long floral tubes are associated with moth pollination and shorter, wider corollas may allow access to shorter-tongued pollinators such as bees, these results lend support to the role of differing pollinator assemblages in shaping floral traits across wide geographic scales. While more detailed pollination data are needed, these results demonstrate clear variation in floral traits, which may relate to variation in locally important pollinators. Such geographic variation in pollinator-mediated selection may contribute to the maintenance of floral polymorphisms in C. sessiliflora.