Spectral sensitivities of the orchid bee Euglossa dilemma

Color vision is an essential aspect of most pollinators' lives. For several groups of diurnal pollinators (bees, butterflies, hummingbirds, etc.), color has proven to be a critical floral signal, conveying information about nectar quantity (Melendez-Ackerman, Campbell, and Waser 1997) and affecting pollinator decision-making (flower choice) (Chittka and Menzel 1992; Dyer et al 2012; Dyer et. al 2021). However, color itself is not a physical property of a flower but rather arises through the interaction between the light reflected from a surface and the perceptual abilities of an animal's sensory system. Therefore, color can be understood either as a cognitive construct (Skorupski and Chittka 2009), and therefore a perceptual feature (Garcia et al. 2020), or as a product of the co-evolution between animal’s sensory systems interacting with light in their environment (Thompson, Palacios and Varela, 1992).

Bees have long been studied for their color vision, and flower color is important for foraging and decision-making for multiple species of bees (Menzel et al., 1988, Chittka, 1992; Dyer and Spaethe 2008). Despite the high diversity of bees in the world, with thousands of species identified, the field of sensory ecology in bees has been dominated by studies of three species, namely the European honeybee (Apis mellifera), the buff-tailed bumblebee in Europe (Bombus terrestris), and the common eastern bumblebee in North America (Bombus impatiens). Nevertheless, recent efforts have been made to improve the understanding of the vision of other bee species, such as stingless bees in Brazil (Koethe et al. 2016) and in Australia (Dyer et al. 2016). In addition, other bee species have been tested for spectral sensitivity (Peitsch et al. 1992; Van Der Kooi et al. 2021), but many key bee taxa remain uninvestigated.

Although most bees measured thus far have exhibited a conserved pattern of trichromatic spectral sensitivity, with peak sensitivities corresponding to ultraviolet, blue, and green wavelengths, the precise location of these sensitivity peaks differs between species, potentially as an adaptation to distinctive visual environments or visually guided task requirements (Peitsch et al. 1992; Van Der Kooi et al. 2021). This can be true even for closely related species, such as Bombus terrestris and Bombus impatiens, where the peak sensitivity of the blue photoreceptor is significantly different between both species (Skorupski & Chittka 2010). Moreover, despite trichromacy being found in most species, at least one bee species appears to have evolved tetrachromatic vision: in addition to the “typical” hymenopteran complement of photoreceptor types, the bee Callonychium petuniae has a fourth photoreceptor with a peak sensitivity positioned at 593 nm (∼yellow) (Peitsch et al. 1992). This suggests that some bees may have evolved expanded color sensitivities, and it highlights the importance of measuring the spectral sensitivities across different species of bees, since there might be other non-trichromatic species. Finally, there remains a shortage of spectral sensitivity measurements from male bees, with published data from males available only for A. mellifera. For decades, male bees were not considered relevant in color experiments or other behavioral assays (Lichtenstein, Sommerlandt, and Spaethe 2015). However, recent studies that tested color learning in male bumblebees have shown that male bees perform as well as female worker bees in both laboratory (Lichtenstein et al., 2015, Wolf and Chittka, 2016) and field conditions (Muth et al. 2021). Thus, there is still much research needed to compare male and female bee color vision, particularly across the greater diversity of extant bee species.

Orchid bees are major pollinators in tropical forests and are closely related to honeybees, bumblebees, and stingless bees (Michener, 2007, Bossert et al., 2019). Orchid bees comprise approximately one-fourth of the total bee abundance in Neotropical forests (Roubik and Hanson 2004), acting as pollinators for multiple species of plants (Armbruster 2017) across distant areas (Roubik and Hanson, 2004, Gilbert, 1980). Male and female orchid bees also have well-described differences in their foraging and pollination behaviors: male orchid bees can forage over a few kilometers daily, while females have much more local ranges (Dressler, 1982, Roubik and Hanson, 2004). Both sexes forage for nectar (energy source), but males actively search for and collect scents (possibly to attract females) while females collect resin (used to build a nest) and pollen (used to feed larvae) (Dressler, 1982, Roubik and Hanson, 2004). Moreover, a recent study of a species of orchid bee brain (Brand et al. 2018) also suggests sexual dimorphism in brain anatomy related to color vision. Male orchid bees have larger eyes and medulla (the brain region associated with color vision), suggesting that they may invest more in their visual systems than females. It is possible that these differences in the brain relate either to differences in color sensitivities or color-based behaviors of male and female orchid bees. Despite their critical role in tropical pollination and the sex-specific differences in behavior noted above, the visual systems of male and female orchid bees have not been investigated to date. Thus, orchid bees present a compelling opportunity to test for potential ecological and sex differences in color perception.

Information about color perception is a critical piece in investigations of pollination biology. By considering the pollinator’s sensory abilities, one can better understand how pollination systems have evolved and how pollinators sense and interact with their environment. The objective of this study was to measure the spectral sensitivities and putative color vision of the orchid bee Euglossa dilemma and compare it to the known spectral sensitivities of other closely related bees. Because of neuroanatomical and behavioral differences, we hypothesized that there would be differences between male and female orchid bee visual systems. Finally, using previously published data, we compared E. dilemma’s spectral sensitivity and opsin protein structure to four closely related bees. Based on past observations of strong conservation in spectral sensitivities across bees, we predicted that variation in opsin sequences between these focal species would be low due to shared evolutionary history, and that opsin sequences would be the most similar between the two orchid bee species included in our analysis.

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