Genes expressed during imaginal disc morphogenesis: IMP-E1, a gene associated with epithelial cell rearrangement.

A unique set of genes that encodes hormone-inducible transcripts found on membrane-bound polysomes is expressed during ecdysone-dependent morphogenesis of Drosophila imaginal discs cultured in vitro. The pattern of expression of one of these genes, IMP-E1, was analyzed in tissues from late third instar larvae and white prepupae by hybridization of asymmetric RNA probes to tissue sections. The IMP-E1 transcript was detected in all anterior and posterior imaginal discs except the ommatidial region of the imaginal eye disc. Within the imaginal leg disc, the IMP-E1 transcript is expressed more abundantly in the proximal than in the distal portions of the epithelium. The distribution of transcripts is consistent with the hypothesis that the IMP-E1 gene product is involved in the cell rearrangements associated with morphogenesis of the disc epithelium. The IMP-E1 transcript is also expressed at pupariation in glial cell layers that ensheath the brain. This glial cell transcript is the same size (7.5 kb) and polarity as the imaginal disc transcript, and is also transcribed in response to 20-hydroxyecdysone. Similarities between the morphogenetic changes in imaginal disc and glial cell layers during metamorphosis are discussed.