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Background: Lepidopteran wing scales are the individual units of wing color patterns and were a key innovation during Lepidopteran evolution. On the basis of developmental and morphological evidence, it has been proposed that the sensory bristles of the insect peripheral nervous system and the wing scales of Lepidoptera are homologous structures. In order to determine if the developmental pathways leading to Drosophila sensory bristle and butterfly scale formation use similar genetic circuitry, we cloned, from the butterfly Precis coenia, a homolog of the Drosophila achaete-scute (AS-C) genes--which encode transcription factors that promote neural precursor formation--and examined its expression pattern during development., Results: During embryonic and larval development, the expression pattern of the AS-C homolog, ASH1, forecasted neural precursor formation. ASH1 was expressed both in embryonic proneural clusters--within which an individual cell retained ASH1 expression, enlarged, segregated, and became a neural precursor--and in larval wing discs in putative sensory mother cells. ASH1 was also expressed in pupal wings, however, in evenly spaced rows of enlarged cells that had segregated from the underlying epidermis but, rather than give rise to neural structures, each cell contributed to an individual scale., Conclusions: ASH1 appears to perform multiple functions throughout butterfly development, apparently promoting the initial events of selection and formation of both neural and scale precursor cells. The similarity in the cellular and molecular processes of scale and neural precursor formation suggests that the spatial regulation of an AS-C gene was modified during Lepidopteran evolution to promote scale cell formation.

Initiation of Drosophila peripheral nervous system (PNS) development requires the achaete-scute complex (AS-C) and the atonal (ato) genes. The AS-C and ato encode basic helix-loop-helix (bHLH) transcription factors that dimerize in vitro with another bHLH protein, daughterless (da). da has many functions during Drosophila embryonic development, as it is required for proper sex determination, oogenesis, and neurogenesis. Here, we examine the expression and function of da within the developing Drosophila eye. The use of a monoclonal antibody to the Da protein revealed that Da levels are modulated across the developing eye disc. Within the morphogenetic furrow (MF) and photoreceptor cell R8, there is a cell-by-cell correspondence between high levels of Da protein expression and Ato protein expression. Mosaic analysis of adult tissue demonstrates that da function is cell autonomous and required within R2, R3, R4, R5, and R8. Examination of gene expression in da- imaginal disc clones reveals that da regulates Ato expression in the MF, affects the progression of the MF, and is necessary for the reestablishment of the G2 and M phases of the synchronized cell cycle posterior to the MF.

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.