Multiple Mechanisms of Photoreceptor Spectral Tuning in Heliconius Butterflies

Area of Science:

• Evolutionary biology
• Vision science
• Genomics

Background:

• Color vision evolution is often studied by adding photoreceptors, but loss mechanisms are less understood.
• Heliconius butterflies provide a model to study the loss of UV2 photoreceptors and its impact on color vision.

Purpose of the Study:

• To investigate the molecular and cellular mechanisms underlying color vision evolution in Heliconius butterflies, focusing on the loss of the UV2 photoreceptor.
• To compare the visual systems of butterflies that have retained the UV2 photoreceptor with those that have lost it.

Main Methods:

• Intracellular recordings to measure photoreceptor sensitivity.
• ATAC-seq to analyze chromatin accessibility and gene regulation.
• Antibody staining to identify opsin expression and localization.

Main Results:

• UV2 receptor loss in Heliconius melpomene and Heliconius ismenius limits short-wavelength color vision.
• Mechanisms for UVRh2 gene modulation include chromatin reorganization (H. melpomene) and pseudogenization (H. ismenius).
• Loss of UV2 shifts the sensitivity of the remaining UV1 photoreceptor to longer wavelengths; filtering pigments and opsin co-expression are widespread.

Conclusions:

• Heliconius butterflies exhibit diverse strategies to modulate color vision, including genetic alterations and spectral tuning of photoreceptors.
• Species may compensate for UV receptor loss by shifting the sensitivity of remaining UV and blue photoreceptors.
• Visual system diversity suggests varying selection pressures for color discrimination in Heliconius.