Molecular consequences of awdb3, a cell-autonomous lethal mutation of Drosophila induced by hybrid dysgenesis

The abnormal wing disc locus, which is at salivary gland chromosome position 100C-D of the Drosophila melanogaster genome, has been identified by a recessive lethal mutation, awdb3, induced by hybrid dysgenesis. When homozygous, this mutation causes abnormal development of the brain, the ovaries, and the larger imaginal discs as described in the preceding paper (C.R. Dearolf, E. Hersperger, and A. Shearn, 1988, Dev. Biol. 129, 159-168). The DNA corresponding to this locus was isolated from a genomic library prepared from awdb3 heterozygotes by screening with a P-element probe. The awdb3 allele resulted from the insertion of a P-element fragment into a gene that encodes an 0.8-kb poly(A)+ RNA. In mutant larvae, that 0.8-kb transcript is replaced by two chimeric transcripts that are 0.7 and 1.3 kb in length, both of which contain P-element and awd sequences. The wild-type awd+ gene transcript is most abundant during the second and third larval instars but is found at a lower level during every developmental stage as well as in continuous cell lines. Thus the awd+ gene transcript can be detected in normal larvae at a developmental stage long before defects are expressed in mutant larvae. Moreover, some tissues, for example the salivary gland of nonmutant, third-instar larvae, contain high levels of this transcript, even though these tissues appear to develop normally in mutant larvae.