SYMMETRY SYSTEMS AND COMPARTMENTS IN LEPIDOPTERAN WINGS - THE EVOLUTION OF A PATTERNING MECHANISM

The wing patterns of butterflies are made up of an array of discrete p attern elements. Wing patterns evolve through changes in the size, sha pe and color of these pattern elements. The pattern elements are arran ged in several parallel symmetry systems that develop independently fr om one another. The wing is further compartmentalized for color patter n formation by the wing veins. Pattern development in these compartmen ts is largely independent from that in adjacent compartments. This two -fold compartmentalization of the color pattern (by symmetry systems a nd wing veins) has resulted in an extremely flexible developmental sys tem that allows each pattern element to vary and evolve independently, without the burden of correlated evolution in other elements. The lac k of developmental constraints on pattern evolution may explain why bu tterflies have diverged so dramatically in their color patterns, and w hy accurate mimicry has evolved so frequently. This flexible developme ntal system appears to have evolved from the convergence of two ancien t patterning systems that the butterflies inherited from their ancesto rs. Mapping of various pattern types onto a phylogeny of the Lepidopte ra indicates that symmetry systems evolved in several steps from simpl e spotting patterns. Initially all such patterns were developmentally identical but each became individuated in the immediate ancestors of t he butterflies. Compartmentalization by wing veins is found in all Lep idoptera and their sister group the Trichoptera, but affects primarily the ripple patterns that form the background upon which spotting patt erns and symmetry systems develop. These background pattern are determ ined earlier in ontogeny than are the symmetry systems, and the compar tmentalization mechanism is presumably no longer active when the latte r develop. It appears that both individuation of symmetry systems and compartmentalization by the wing veins began at or near the wing margi n. Only the butterflies and their immediate ancestors evolved a patter n formation mechanism that combines the development of a regular array of well-differentiated symmetry systems with the mechanism that compa rtmentalizes the wing with respect to color pattern formation. The res ult was an uncoupling of symmetry system development in each wing cell . This, together with the individuation of symmetry systems, yielded a n essentially mosaic developmental system of unprecedented permutation al flexibility that enabled the great radiation of butterfly wing patt erns.