The compound eye of Drosophila melanogaster begins to differentiate during thelate third larval instar in the eye-antennal imaginal disc. A wave of morphogenesis crossesthe disc from posterior to anterior, leaving behind precisely patterned clusters ofphotoreceptor cells and accessory cells that will constitute the adult ommatidia of theretina. By the analysis of genetically mosaic eyes, it appears that any cell in the eye disccan adopt the characteristics of any one of the different cell types found in the mature eye,including photoreceptor cells and non-neuronal accessory cells such as cone cells.Therefore, cells within the prospective retinal epithelium assume different fatespresumably via information present in the environment. The sevenless^+ (sev^+) geneappears to play a role in the expression of one of the possible fates, since the mutantphenotype is the lack of one of the pattern elements, namely, photoreceptor cell R7. Thesev^+ gene product had been shown to be required during development of the eye, and hadalso been shown in genetic mosaics to be autonomous to presumptive R7. As a means ofbetter understanding the pathway instructing the differentiation R7, the gene and itsprotein product were characterized.
The sev+ gene was cloned by P-element transposon tagging, and was found toencode an 8.2 kb transcript expressed in developing eye discs and adult heads. By raisingmonoclonal antibodies (MAbs) against a sev^+- β-galactosidase fusion protein, theexpression of the protein in the eye disc was localized by immuno-electronmicroscopy.The protein localizes to the apical cell membranes and microvilli of cells in the eye discepithelium. It appears during development at a time coincident with the initial formation ofclusters, and in all the developing photoreceptors and accessory cone cells at a time priorto the overt differentiation of R7. This result is consistent with the pluripotency of cells in the eye disc. Its localization in the membranes suggests that it may receive informationdirecting the development of R7. Its localization in the apical membranes and microvilli isaway from the bulk of the cell contacts, which have been cited as a likely regions forinformation presentation and processing. Biochemical characterization of the sev^+ proteinwill be necessary to describe further its role in development.
Other mutations in Drosophila have eye phenotypes. These were analyzed to findwhich ones affected the initial patterning of cells in the eye disc, in order to identify other genes, like sev, whose gene products may be involved in generating the pattern. The adulteye phenotypes ranged from severe reduction of the eye, to variable numbers ofphotoreceptor cells per ommatidium, to sub de defects in the organization of thesupporting cells. Developing eye discs from the different strains were screened using apanel of MAbs, which highlight various developmental stages. Two identified matrixelements in and anterior to the furrow, while others identified the developing ommatidiathemselves, like the anti-sev MAb. Mutation phenotypes were shown to appear at manystages of development. Some mutations seem to affect the precursor cells, others, thesetting up of the pattern, and still others, the maintenance of the pattern. Thus, additionalgenes have now been identified that may function to support the development of acomplex pattern.
