Abstract
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 Campanula species, of which only Campanula lactiflora has never been reported as a pollen source of the oligolectic bee Ch. rapunculi. We hypothesized that the flowers of Campanula species visited by Ch. rapunculi 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 Ch. rapunculi. Our results show that Campanula flowers reflect the light predominantly in the UV-blue/blue bee-color space and that Ch. rapunculi displays a strong innate preference for these two colors. Furthermore, we recorded spiroacetals in the floral scent of all Campanula species, but Ca. lactiflora. Spiroacetals, rarely found as floral scent constituents but quite common among Campanula species, were recently shown to play a key function for host-flower recognition by Ch. rapunculi. We conclude that Campanula species share some visual and olfactory floral cues, and that neurological adaptations (i.e. vision and olfaction) of Ch. rapunculi innately drive their foraging flights toward host flowers. The significance of our findings for the evolution of pollen diet breadth in bees is discussed.
Highlights
When navigating the landscape, pollinators are frequently confronted with a staggering diversity of flowers in the complex floral market and have to make choices
In corroboration to our hypothesis, we found that the flowers of most Campanula species were grouped into the UV-blue and blue bee-color space and that these colors are more attractive than others (e.g. UV-green and blue-green) to the Campanula pollen-specialist bee Ch. rapunculi
In terms of olfactory cues, we found that floral scent bouquets are species-specific and that highly specific volatiles, i.e. spiroacetals, which were shown to be the key compounds in Ca. trachelium for host recognition of C. rapunculi [34], are emitted by all species investigated with the exception of Ca. lactiflora
Summary
Pollinators are frequently confronted with a staggering diversity of flowers in the complex floral market and have to make choices. The decision may reflect either innate [2, 3] or learned preferences [4, 5] for some floral cues, which are possibly related to neurophysiological adaptations of pollinators (e.g. olfactory receptors) and their diet breadth [6,7,8]. Specialized (oligolectic) bees collect pollen only on plants of a given genus or family to rear their offspring [9, 10] and, must rely on host-specific floral cues to recognize host flowers unambiguously. Generalized (polylectic) bees, on the other hand, collect pollen on a vast array of plant species irrespective of phylogenetic relatedness, and might rely on more generalized floral cues [11]
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