The evolution of eyespot number and position across wings surfaces of nymphalid butterflies

Eyespot color patterns on nymphalid butterfly wings are prime examples of serial homologous traits, yet, the evolutionary origins of these repeated traits are still unresolved.  Ancestral state reconstructions show that eyespots originated once, close to the base of the Nymphalidae. The first eyespots originated in wing compartments Rs, M1, M2, M3, and Cu1 on the ventral surface of the posterior wing. We applied various evolutionary models in Pagel's 1994 test of correlated character evolution to a phylogeny of 397 nymphalid taxa to examine the pattern through which these original eyespots redeployed to novel wings (forewings), novel wing surfaces (dorsal surfaces), or novel wing cells (ScR, Cu2, Pc). In addition, we used likelihood ancestral state reconstructions to estimate when the original eyespots moved to these novel wings/surfaces. We show that serial homologous eyespots quickly appeared on the ventral surface of the anterior wing followed by a much later origin on dorsal surfaces. These latter eyespots show the strongest correlations with serial homologues on the ventral anterior wing, suggesting that they were co-opted from this surface. The M1, M2, and M3 eyespots were most likely to redeploy to novel wing cells within a wing surface. This means that eyespots "jumped" over adjacent wing cells - eyespots did not colonize the wing surface through stepwise redeployment into adjacent cells down the wing. Overall, our results suggest that eyespots originated as a small cluster of units on the ventral hindwing which later colonized homologous positions on a novel (anterior) wing, and finally colonized a novel (dorsal) surface. Colonization within a wing surface had little to do with adjacency. Serial homologues, thus, originate with specific positional information in a body, and move to novel body positions through the course of time.