THE SPATIAL EXPRESSION OF DROSOPHILA-ROTUND GENE REVEALS THAT THE IMAGINAL DISKS ARE ORGANIZED IN DOMAINS ALONG THE PROXIMAL-DISTAL AXIS

In Drosophila imaginal discs, pattern formation requires the activity of three positional information systems, antero-posterior (A/P), dorso-ventral (D/V) and proximo-distal (P/D). Three genes, Decapentaplegic, Distal-less and rotund (rn), involved in pattern formation along the P/D axis have been characterized. The rn gene is required in a sub-distal region, localized at a similar position along the P/D axis in all appendages; it encodes two major transcripts, m1.7 and m5.3, both expressed in the central region of all the major imaginal discs. The present study of these transcripts in several rn mutant favours m5.3 as encoding rn morphogenetic function in the imaginal discs. The fine characterization of its distribution partitions all major imaginal discs in domains along the P/D axis. The ventral and dorsal discs appear to be similarly but not identically organized: two P/D domains are evident in the wing and haltere discs whilst the leg and antenna discs appear to be composed of at least three. We also show that m5.3 is sex-regulated in the genital disc and that rn function is required for proper development of a sub-distal structure of the female genitalia. This suggests that the primordia of the female genitalia may be organized in a similar way to the other imaginal discs, and strongly supports the hypothesis that rn function is specific to pattern formation along the P/D axis and that it may be involved in the establishment or maintenance of this pattern.