Butterfly wings display pattern elements of many types and colors. To identify the molecular processes underlying the generation of these patterns, several butterfly cognates of Drosophila appendage patterning genes have been cloned and their expression patterns have been analyzed. Butterfly wing patterns are organized by two spatial coordinate systems. One system specifies positional information with respect to the entire wing field and is conserved between fruit flies and butterflies. A second system, superimposed on the general system and involving several of the same genes, operates within each wing subdivision to elaborate discrete pattern elements. Eyespots, which form from discrete developmental organizers, are marked by Distal-less gene expression. These circular pattern elements appear to be generated by a process similar to, and perhaps evolved from, proximodistal pattern formation in insect appendages.
The appendages of arthropods and vertebrates possess a third, proximodistal patterning axis that is established after the primary anteroposterior and dorsoventral body axes by mechanisms that are largely unknown. The vestigial gene is required for formation of the entire Drosophila wing, and the dorsal/ventral boundary is shown to organize wing formation and vestigial gene expression. Interactions between dorsal and ventral cells in the growing imaginal disc induce vestigial gene expression through a discrete, extraordinarily conserved imaginal disc-specific enhancer. The link between dorsal/ventral compartmentalization and wing formation distinguishes the development of this sheet-like appendage from that of legs and antennae.
The initial steps of pattern formation in the developing Drosophila eye involve the coordination of cell cycles, changes in cell shape, and the specification of the R8 photoreceptor cell. These events begin several cell rows ahead of the morphogenetic furrow and are positively regulated by secreted signaling proteins and the proneural HLH transcription factor atonal (ato). Two HLH regulatory proteins that function to suppress neuronal development in other tissues, extra macrochaetae (emc) and hairy (h), are expressed ahead of the morphogenetic furrow. While neither h nor emc is required for photoreceptor cell determination, in emc-h-clones the morphogenetic furrow and differentiated eye field advance up to eight ommatidial rows ahead of adjacent wild-type tissue. This indicates that morphogenetic furrow progression and neuronal differentiation are negatively regulated by a combination of anteriorly expressed HLH regulatory proteins.
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