Fig. 8

Possible roles of Distal-less and spalt in pierid spot and nymphalid eyespot development. In late larval wing discs of B. anynana, both Dll (green) and sal (orange) are co-expressed at high levels in the centre of eyespots [32]. However, in late larval wing discs of P. canidia, Dll and sal are not expressed in spot centres. Both Dll and sal are expressed in mid-line fingers encroaching inwards from the wing margin. Eyespot centres in B. anynana are likely established through a reaction diffusion mechanism involving Wnt and BMP signalling [25]. The absence of Arm proteins and dpp expression in P. canidia spot centres suggests that spots may not develop through the same mechanism. In nymphalid eyespots, Dll and sal respond to signals emanating from the foci. However, in early pupal stages, both Arm and dpp are absent in spot centres in pierids. There may be central signalling cells that are present in spot patterns that are activating downstream genes (i.e. sal), but these central cells do not express Dll and Sal. An alternative model would be that sal is responding to a gradient of BMP ligands at specific thresholds (blue band in the early pupal stage). Inhibitory molecules secreted from the wing margin, as well as others expressed in specific wing sectors (not shown), would lead to sal expression and black spot markings of P. canidia in only specific wing sectors. In both butterfly lineages, both Dll and sal likely play an ancestral role in organising distal wing patterns as expression of both proteins have been observed along the marginal wing bands of B. anynana and J. coenia in early pupal wings [32].