CELL CLONES AND PATTERN FORMATION - STUDIES ON SEVENLESS, A MUTANT OF DROSOPHILA-MELANOGASTER

sevLY3,the only existing allele at the sev locus (1-33,2±0,2), behaves as strongly hypomorph or even as amorph. Ommatidia in a sev compound eye have only seven receptor cells, the position of the R7 pattern element being vacant. Various criteria showing that the missing cell is R7 have been verified. These include (i) anatomical characteristics of sev ommatidia; (ii) behaviour of central R cells in sev rdgB double mutants; (iii) medullary projection of central R cell axons; and (iv) mitotic pattern of sev imaginal discs. The analysis of morphogenetic sev-sev+ mosaics has shown that sev is expressed autonomously by R7 cells, indicating that the sev phenotype is not due to a sev genotype of ommatidial pattern elements other than R7. The study of third instar sev imaginal discs has not brought any direct evidence for death of clustered presumptive R7 cells; however, clonal analysis of the developing sev compound eye has given evidence of developmental parameters comparable to those of sev+, therefore favouring the hypothesis that R7 cells die in sev mutants. On the other hand, sev+ seems to be required for the determination of the R7 cells, since the sev phenotype cannot be uncovered during the last mitoses of heterozygous mutant cells. © 1979 Springer-Verlag.