Opsin expression predicts male nuptial color in threespine stickleback

Abstract

Theoretical models of sexual selection suggest that male courtship signals can evolve through the build-up of genetic correlations between the male signal and female preference. When preference is mediated via increased sensitivity of the signal characteristics, correlations between male signal and perception/sensitivity are expected. When signal expression is limited to males, we would expect to find signal-sensitivity correlations in males. Here, we document such a correlation within a breeding population of threespine stickleback mediated by differences in opsin expression. Males with redder nuptial coloration express more long-wavelength-sensitive (LWS) opsin, making them more sensitive to orange and red. This correlation is not an artifact of shared tuning to the optical microhabitat. Such correlations are an essential feature of many models of sexual selection, and our results highlight the potential importance of opsin expression variation as a substrate for signal-preference evolution. Finally, these results suggest a potential sensory mechanism that could drive negative frequency-dependent selection via male-male competition and thus maintain variation in male nuptial color.

Keywords: gene expression; nuptial color; opsins; sexual selection.

Figure 1

Males that reflect more red display higher relative expression of the red‐sensitive LWS opsin. (a) Plot of the first canonical variate for color and opsin expression and the first canonical correlation between these variates for the abdomen. (b) Helioplot showing the canonical loadings along the first canonical variate axes for male reflectance and opsin expression. Solid bars indicate positive loadings, and clear bars indicate negative loadings

Figure 2

Redder males show greater absorbance (quantum catch) in the retina of orange‐red wavelengths due to their higher expression of the LWS opsin. Orange‐red (590–650 nm) absorbance is significantly correlated with the intensity of red coloration within males. The results displayed here were calculated using the median ${\mathrm{\lambda }}_{\mathit{max}}$ that averages across chromophores