Nymphalid eyespot serial homologues originate as a few individualized modules.

Serial homologues are repeated traits that share similar development but occur in different parts of the body. Variation in number of repeats accounts for substantial diversity in animal form and considerable work has focused on identifying the factors accounting for this variation. Little is known, however, about how serial homologues originally become repeated, or about the relative timing of repeat individuation relative to repeat origin. Here, we show that the serially repeated eyespots on nymphalid butterfly wings most likely arose as a small cluster of units on the ventral hindwing that were later co-opted to the dorsal and anterior wing surfaces. Based on comparative analyses of over 400 species, we found support for a model of eyespot origin followed by redeployment, rather than by the conventional model, where eyespots arose as a complete row of undifferentiated units that later gained individuation. In addition, eyespots most likely evolved from simpler pattern elements, single-coloured spots, which were already individuated among different wing sectors. Finally, the late appearance of eyespots on the dorsal, hidden wing surface further suggests that these novel complex traits originally evolved for one function (thwarting predator attacks) and acquired a second function (sexual signalling) when moved to a different body location. This broad comparative analysis illustrates how serial homologues may initially evolve as a few units serving a particular function and subsequently become repeated in novel body locations with new functions.