In brush-footed butterflies (Nymphalidae), ancestral trichromatic vision is expanded to
tetrachromacy with red-sensitive basal photoreceptors R9 in red-shining ommatidia. A
red filtering pigment (rfp) shifts the absorption peak of the long-wavelength opsin (LW)
toward the red part of the spectrum. In ommatidia where LW opsin is expressed in cells
R1 and R2, these form an opponent pair with R9 (G+R–). In G+R– cells of Heliconius
melpomene, differences were observed in the sensitivity ratio between green and blue
parts of the spectrum (G/B). We asked whether spectral sensitivity systematically varies
with cell position along the dorsoventral (D–V) axis, and whether this variation can be
explained by the spatial structure of the light environment. We studied four species: H.
melpomene, H. cydno, Brenthis hecate and Morpho peleides. Spectral responses of cells
were measured electrophysiologically and their positions along the D–V axis recorded.
We captured eyeshine and verified rfp histologically. For H. melpomene, we calculated
photon catch of G+R– cells along the D–V axis relative to irradiance spectra measured
in the natural environment. A gradient of G/B sensitivity appeared in all species except
M. peleides. Green-sensitive cells dominated dorsally, blue-sensitive ones ventrally,
both inhibited in the red. This corresponds to the dorsal eye facing the sky, rich in
ultraviolet and blue light, and the ventral eye ground and vegetation, where green and
red predominate. Uneven eyeshine and more frequent red-shining ommatidia ventrally
confirm the role of filtering pigment in tuning vision. Comparison between species
shows the gradient of spectral sensitivity between green- and blue-sensitive cells is
species-specific, likely reflecting habitat light conditions.
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