Geographic Mosaics of Fly Pollinators With Divergent Color Preferences Drive Landscape-Scale Structuring of Flower Color in Daisy Communities

Abstract

The striking variation in flower color across and within Angiosperm species is often attributed to divergent selection resulting from geographic mosaics of pollinators with different color preferences. Despite the importance of pollinator mosaics in driving floral divergence, the distributions of pollinators and their color preferences are seldom quantified. The extensive mass-flowering displays of annual daisy species in Namaqualand, South Africa, are characterized by striking color convergence within communities, but also color turnover within species and genera across large geographic scales. We aimed to determine whether shifts between orange and white-flowered daisy communities are driven by the innate color preferences of different pollinators or by soil color, which can potentially affect the detectability of different colored flowers. Different bee-fly pollinators dominated in both community types so that largely non-overlapping pollinator distributions were strongly associated with different flower colors. Visual modeling demonstrated that orange and white-flowered species are distinguishable in fly vision, and choice experiments demonstrated strongly divergent color preferences. We found that the dominant pollinator in orange communities has a strong spontaneous preference for orange flowers, which was not altered by conditioning. Similarly, the dominant pollinator in white communities exhibited an innate preference for white flowers. Although detectability of white flowers varied across soil types, background contrast did not alter color preferences. These findings demonstrate that landscape-level flower color turnover across Namaqua daisy communities is likely shaped by a strong qualitative geographic mosaic of bee-fly pollinators with divergent color preferences. This is an unexpected result given the classically generalist pollination phenotype of daisies. However, because of the dominance of single fly pollinator species within communities, and the virtual absence of bees as pollinators, we suggest that Namaqua daisies function as pollination specialists despite their generalist phenotypes, thus facilitating differentiation of flower color by pollinator shifts across the fly pollinator mosaic.

Incomplete reproductive isolation and low genetic differentiation despite floral divergence across varying geographic scales inCastilleja

“…Such an interaction could result in the observed pattern in which inflorescence color is in contrast to the immediate local environment, whereby darker pigmentation of calyces (i.e., pink) is found in the southern range extent (characterized by bare ground of the Chihuahuan Desert and gypsum outcrops) and lighter calyx pigmentation (i.e., white) provides a strong contrast against the surrounding prairie vegetation in the north. Background substrate color has been shown to impact visibility of flowers of varying colors in bee‐ and fly‐pollinated systems (Bukovac et al, 2017; Ellis et al, 2021), and Crosswhite and Crosswhite (1970) previously hypothesized that the pigmentation of white flowers of C. sessiliflora may serve to increase their visibility to insects under low light conditions in the dry prairies of southern Wisconsin. Experimental assessments of the fitness impact of background contrast are needed to definitively determine the extent to which these factors may explain floral color variation in C. sessiliflora .…”

Spatial variation in bidirectional pollinator-mediated interactions between two co-flowering species in serpentine plant communities

“…Overall, our results thus suggest that the occurrence of competitive or facilitative interactions, even between the same plant species pair, is highly context-dependent. These differential responses might be explained by differences in plant richness among sites (BS: 62, RHA: 47, TP9: 39 plant species; Arceo-Gómez et al 2018 ), differences in pollinator assemblages ( Koski et al 2015 ) and/or due to changes in specific pollinator preferences among the study sites ( Ellis et al 2021 ). Geographic variation in the outcome of these interactions can also result from changes in the number and identity of the pollinator assemblage ( Gómez et al 2009a ) and plant community composition, as it is also exemplified in studies of diffuse selection (reviewed in Strauss et al 2005 ).…”

Achromatic Cues Are Important for Flower Visibility to Hawkmoths and Other Insects

“…Most of these backgrounds are different in colour from flowers. Although natural background structures (i.e., leaves, soil, and rocks) are roughly similarly achromatic (Menzel and Shmida, 1993;Ellis et al, 2021), recent studies showed that variation in background colour can determine the salience of flowers to pollinators. Bukovac et al (2017) modelled flower salience against more than 500 natural backgrounds and found that background colour has the potential to significantly change a flower's colour contrast.…”