Molecular and histological characterizations of the Om(2D) mutants in Drosophila ananassae.

A series of transposon-induced optic morphology (Om) mutants found in a hypermutable marker stock of Drosophila ananassae provides a useful system for analyzing the molecular mechanism of eye morphogenesis. In the present study, one of the 25 Om loci so far reported, Om(2D), has been subjected to histological and molecular analyses as a first step toward understanding the role of Om genes in eye morphogenesis. Histological abnormalities observed during eye morphogenesis of the mutant, i.e. cell death within the eye-antennal discs of third instar larvae, and loss of the lamina, disorganized ommatidia and atrophied optic lobes in adults, were all comparable to those reported with various eye morphology mutants of D. melanogaster. Approximately 25 kb of genomic DNA including the Om(2D) locus was cloned by tom tagging. Southern blot and cloning analyses of two alleles of the Om(2D) locus revealed that insertions of the tom element occurred at three sites within 359 bp; two tandemly arrayed toms sharing one long terminal repeat at the junction and an internally deleted tom were present 359 by apart from each other in Om(2D)63, while a single tom in reverse orientation was present within the 359 by in Om (2D)10a. Host DNA sequences at the three insertion sites were TATAT or AATAT, and ATAT was duplicated upon the tom insertion. Three spontaneous revertants and one induced extreme derivative of Om(2D)63 were obtained and characterized. A complete revertant lost all the preexisting tom elements. Two partial revertants lost one or two of the preexisting tom elements. In the extreme derivative, an additional insertion sequence was found within the two tandem tom elements. Northern blot analysis showed two transcription units in the Om (2D) region: one was on the centromere side of the tom insertion site and expressed a 3.2 kb major RNA and several minor RNAs; the other resided on the telomere side of the tom insertion and expressed a 1.5 kb RNA. Both 3.2 kb and 1.5 kb transcripts were expressed throughout development, but the former was more abundant in mutant embryos and the latter more highly expressed in mutant third instar larvae than in the corresponding stages in wild type.