Allelic interactions at the engrailed locus of Drosophila: engrailed protein expression in imaginal discs

Abstract

Many embryonic lethal engrailed ( en lethal ) mutations are known to partially complement the cuticular defects of the original engrailed mutation, en 1 . To explore the nature of this complementation, the adult phenotypes of several different en 1 en lethal transheterozygotes were compared with the corresponding patterns of engrailed protein expression in third larval instar imaginal discs (determined by immunofluorescence). Transheterozygotes of en 1 and deletions of the locus ( en Df ) typically show slight complementation in the adult cuticle. The pattern of engrailed protein expression in some en 1 en Df wing discs is indistinguishable from en 1 homozygotes, but in others the pattern is nearly normal. en 1 en Df leg discs appear to express engrailed protein normally. Transheterozygotes of en 1 and EMS-induced, cytological normal en lethal alleles have almost normal adult cuticle phenotypes and also exhibit normal patterns of engrailed protein expression in all of the thoracic imaginal discs. Surprisingly, the intensity of anti- engrailed staining in these discs is elevated relative to that in wild type. en 2 is an unusual lethal allele in that it does not complement either the en 1 adult cuticle phenotype or the protein expression pattern in imaginal discs. Moreover, the cytologically normal en lethal alleles also complement en 2 , at least partially. Both wing and leg imaginal discs from en 2 en lethal transheterozygotes show abnormal patterns of engrailed protein expression. These results are discussed in the context of an autoregulatory model for engrailed regulation.