Abstract
Eyespots are wing colour pattern elements repeatedly observed in many butterflies, whose developmental genetics has been extensively documented. Nevertheless, the evolutionary forces shaping their diversification across species still remain largely unknown. Here we investigate the evolution of eyespots in the 30 species of the neotropical genus Morpho. Morpho butterflies display a series of eyespots located on the ventral side of their wings, highly variable among species, ranging from large, conspicuous structures to relictual spots. Applying geometric morphometrics to a large sample (N = 910) spanning all Morpho species and both sexes, we assess eyespots number, position, size and shape. We detect a divergence in eyespots position between understory and canopy species that opposes an L-shaped configuration in canopy species to a line pattern in understory species, where the eyespots tend to fuse and form a stripe in white and pale species. This effect is stronger than expected based on a Brownian motion model of phylogenetic divergence, suggesting an adaptation to the microhabitat and an influence of the wing colour on the evolution of pattern elements. Remarkably, this shift in colour pattern is strongly correlated with a shift in wing shape. However, using a thin-plate spline interpolation, we show that the shape change is insufficient to explain the evolution of eyespots position as a developmental side-effect of wing shape evolution, also pointing at an adaptive effect. Finally, we find a significant negative correlation between eyespot relative size and the within-species variation in eyespot number, position, size and shape, suggesting a relaxed or apostatic selection on small eyespots. We hypothesize that contrasted ecology may explain the observed differences between species: large and phenotypically stable eyespots might act as attack deflectors; small, variable faded eyespots might rather enhance crypsis; and pale species stripe pattern might disrupt the outline of the wing.
Highlights
Eyespots are among the most studied wing color pattern elements in butterflies, and their developmental genetics is known in great detail [e.g., Nijhout, 1991; Brakefield et al, 1996; reviewed in Monteiro (2015)]: their development has been suggested to be controlled by reaction–diffusion processes involving the focal expression of morphogens typically located in between wing veins
A significant phylogenetic signal in eyespot number was detected in both sexes, with consistently stronger values in the hindwing close to the pattern expected under constant Brownian motion divergence K = 1 (Kmale forewing = 0.56 p = 0.014; Kmale hindwing = 0.95, p < 0.001; Kfemale forewing = 0.58, p = 0.015; Kfemale hindwing = 0.89, p < 0.001)
Overall our results suggest that the evolution of eyespots in Morpho butterflies results from a combination of neutral divergence and selection
Summary
Eyespots are among the most studied wing color pattern elements in butterflies, and their developmental genetics is known in great detail [e.g., Nijhout, 1991; Brakefield et al, 1996; reviewed in Monteiro (2015)]: their development has been suggested to be controlled by reaction–diffusion processes involving the focal expression of morphogens typically located in between wing veins. A prominent role for selection is often assumed (Kodandaramaiah, 2011), and the importance of neutral factors is seldom mentioned (but see Beldade et al, 2002; Allen, 2008). This is surprising as eyespot number and morphology can be highly variable within species (e.g., Maniola jurtina; Brakefield, 1984), suggesting a weak selection. Many species with a cryptic ventral color pattern harbor reduced, inconspicuous, and very variable eyespots, evocative of neutral variation, or apostatic selection, a form of selection where rare phenotypes are favored as they are less likely to be remembered by predators and detected (Clarke, 1969; Bond and Kamil, 2002)
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