An eye imaginal disc-specific transcriptional enhancer in the long terminal repeat of the tom retrotransposon is responsible for eye morphology mutations of Drosophila ananassae.

Optic morphology (Om) mutations of Drosophila ananassae are semidominant, neomorphic and nonpleiotropic, map to at least 22 loci scattered throughout the genome, and are associated with the insertion of the tom retrotransposon. Molecular and genetic analyses have revealed that eye morphology defects of Om mutants are caused by the ectopic or excessive expression of Om genes in the eye imaginal discs of third instar larvae. It is therefore assumed that the tom element carries tissue-specific gene regulatory sequences which enhance expression of the Om genes. In the present study, we examined whether or not the long terminal repeats (LTR) of the tom element contain such an eye imaginal disc-specific enhancer, using D. melanogaster transformants containing a lacZ gene ligated to the tom LTR. Analyses of lacZ gene expression in the eye imaginal discs of third instar larvae of 18 independently established transformant lines showed that the tom LTR was capable of enhancing lacZ expression in all the transformant lines, but the degree of enhancement varied between lines. In addition, the effect of the tom LTR lacZ gene evidently changed when the tom LTR construct was relocated to different chromosomal positions. On the basis of these findings, it is hypothesized that ectopic and excessive expression of the Om genes in the eye imaginal discs is induced by an eye imaginal disc-specific enhancer present in the tom LTR, the effect of which may be subject to chromosomal position effects.