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Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK
Institut de Génétique et de Biologie Moléculaire et Cellulaire, CNRS/INSERM/ULP, BP 163, 67404 Illkirch Cedex, CU de Strasbourg, France
* These authors contributed equally to this work
Author for correspondence at present address: Department of Zoology, Downing Street, Cambridge CB2 3EJ, UK (e-mail: pas49{at}cam.ac.uk)
Accepted 31 October 2001
In Drosophila the stereotyped arrangement of sensory bristles on the notum is determined by the tightly regulated control of transcription of the achaete-scute (ac-sc) genes which are expressed in small proneural clusters of cells at the sites of each future bristle. Expression relies on a series of discrete cis-regulatory elements present in the ac-sc gene complex that are the target of the transcriptional activators pannier (pnr) and the genes of the iroquois complex. Stereotyped bristle patterns are common among species of acalyptrate Schizophora such as Drosophila, and are thought to have derived from an ancestral pattern of four longitudinal rows extending the length of the scutum, through secondary loss of bristles. To investigate evolutionary changes in bristle patterns and ac-sc regulation by pnr, we have isolated homologues of these genes from Calliphora vicina, a species of calyptrate Schizophora separated from Drosophila by at least 100 million years. Calliphora vicina displays a pattern of four rows of bristles on the scutum resembling the postulated ancestral one. We find that sc in Calliphora is expressed in two longitudinal stripes on the medial scutum that prefigure the development of the rows of acrostichal and dorsocentral bristles. This result suggests that a stripe-like expression pattern of sc may be an ancestral feature and may have preceded the evolution of proneural clusters. The implications for the evolution of the cis-regulatory elements responsible for sc expression in the proneural clusters of Drosophila, and function of Pnr are discussed.
Key words: Diptera, Calliphora vicina, Sensory organ, achaete-scute, pannier
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