Expression pattern of a butterfly achaete-scute homolog reveals the homology of butterfly wing scales and insect sensory bristles

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.