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

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