Visual and Olfactory Floral Cues of <i>Campanula</i> (Campanulaceae) and Their Significance for Host Recognition by an Oligolectic Bee Pollinator

<div><p>Oligolectic bees collect pollen from a few plants within a genus or family to rear their offspring, and are known to rely on visual and olfactory floral cues to recognize host plants. However, studies investigating whether oligolectic bees recognize distinct host plants by using shared floral cues are scarce. In the present study, we investigated in a comparative approach the visual and olfactory floral cues of six <i>Campanula</i> species, of which only <i>Campanula lactiflora</i> has never been reported as a pollen source of the oligolectic bee <i>Ch</i>. <i>rapunculi</i>. We hypothesized that the flowers of <i>Campanula</i> species visited by <i>Ch</i>. <i>rapunculi</i> share visual (i.e. color) and/or olfactory cues (scents) that give them a host-specific signature. To test this hypothesis, floral color and scent were studied by spectrophotometric and chemical analyses, respectively. Additionally, we performed bioassays within a flight cage to test the innate color preference of <i>Ch</i>. <i>rapunculi</i>. Our results show that <i>Campanula</i> flowers reflect the light predominantly in the UV-blue/blue bee-color space and that <i>Ch</i>. <i>rapunculi</i> displays a strong innate preference for these two colors. Furthermore, we recorded spiroacetals in the floral scent of all <i>Campanula</i> species, but <i>Ca</i>. <i>lactiflora</i>. Spiroacetals, rarely found as floral scent constituents but quite common among <i>Campanula</i> species, were recently shown to play a key function for host-flower recognition by <i>Ch</i>. <i>rapunculi</i>. We conclude that <i>Campanula</i> species share some visual and olfactory floral cues, and that neurological adaptations (i.e. vision and olfaction) of <i>Ch</i>. <i>rapunculi</i> innately drive their foraging flights toward host flowers. The significance of our findings for the evolution of pollen diet breadth in bees is discussed.</p>