Eur. J. Entomol. 92 (1): 169-187, 1995
Genetics of ecdysteroid-regulated central nervous system metamorphosis in Drosophila (Diptera: Drosophilidae)
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We are interested in identifying members of the generic pathway through which 20-hydroxyecdysone (20HE) mediates reorganization of the central nervous system (CNS) during metamorphosis. Our entry point is the Drosophila Broad-Complex (BR-C), an early 20HE-inducible locus with three genetic subfunctions, each represented by a lethal complementation group. Our previous analysis of mutants demonstrated that all three BR-C subfunctions are necessary for CNS morphogenesis and one is essential for visual system organization. We believe the mutant phenotypes result from faulty expression of genes normally regulated by the BRC family of zinc-finger proteins. BRC target genes are predicted to have expression patterns and/or mutant phenotypes that partially overlap with those of the BR-C. We have examined two candidate genes, IMP-E1 and Deformed (Dfd), to determine their positions relative to BR-C in the hormone-regulated pathway of CNS metamorphosis. Identified by Natzle and colleagues on the basis of 20HE-inducibility in imaginal discs, IMP-EI transcripts were also found in a subset of CNS glial cells. Our recent experiments show that BR-C expression is spatially and temporally poised to regulate IMP-E1 induction by 20HE. We examined IMP-E1 transcript accumulation in larval and prepupal CNS of BR-C lethal mutants representing each of the three complementation groups. In all three cases, IMP-E1 induction in the CNS of BR-C mutants was comparable to that of wildtype and of genetic controls. Thus, activity of any individual BR-C subfunction is not essential for IMP-E1 induction. Dfd is a homeotic selector gene in the Antennapedia complex whose larval CNS expression has been shown by others to be restricted to a subset of subesophageal ganglion cells. We have demonstrated that Dfd mutants manifest a defect in subesophageal ganglion metamorphosis, namely separation from the thoracic ganglion, indistinguishable from that of BR-C mutants. However, Dfd transcript accumulation in the CNS appears to be indifferent to 20HE levels in vivo or in vitro. Alternative models for the genetic pathways controlling CNS metamorphosis are discussed.
Keywords: Broad-complex, Deformed, IMP-E1, transcriptional regulation, steroid hormones, homeodomain, metamorphosis, neurogenetics, ecdysone, Drosophila, zinc-finger protein, central nervous system
Accepted: January 3, 1995; Published: February 24, 1995 Show citation
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