Background: Hox proteins specify segment identity during embryogenesis and have typical associated expression patterns. Changes in embryonic expression and activity of Hox genes were crucial in the evolution of animal body plans, but their role in the post-embryonic development of lineage-specific traits remains largely unexplored. Here, we focus on the insect Hox genes Ultrabithorax (Ubx) and Antennapedia (Antp), and implicate the latter in the formation and diversification of novel, butterfly-specific wing patterns.
Results: First, we describe a conserved pattern of Ubx expression and a novel pattern of Antp expression in wing discs of Bicyclus anynana butterflies. The discrete, reiterated domains of Antp contrast with the typical expression of Hox genes in single continuous regions in arthropod embryos. Second, we show that this pattern is associated with the establishment of the organizing centres of eyespots. Antp upregulation is the earliest event in organizer development described to date, and in contrast to all genes implicated in eyespot formation, is exclusive to those centres. Third, our comparative analysis of gene expression across nymphalids reveals unexpected differences in organizer determination.
Conclusions: We show that the Antp's recruitment for the formation of novel traits in butterfly wing discs involved the evolution of new expression domains, and is restricted to a particular lineage. This study contributes novel insights into the evolution of Antp expression, as well as into the genetic mechanisms underlying morphological diversification. Our results also underscore how a wider representation of morphological and phylogenetic diversity is essential in evolutionary developmental biology.