Fig. 3. Defective development in spl mutants. (A) Wild-type eye disc labeled for Senseless protein (magenta) and apoptotic cells with activated caspases (green). Posterior to column 0 in the morphogenetic furrow, Senseless labels one R8 cell in each ommatidium. Anterior to column 0, Senseless is expressed in a single column of 'intermediate groups', clusters of six to ten cells from which each R8 cell emerges. (B) spl mutant eye disc labeled as in A. Anterior to column 0, the intermediate groups contain fewer cells often expressing Senseless at lower levels than wild type. Posterior to column 0 many R8 cells are missing and those remaining express Senseless at varying levels. In the posterior of the eye disc cell death is elevated compared with wild type. (C) Detail from spl; GMRp35 eye disc. Cell death is prevented but caspase activation still occurs. Arrowhead indicates caspase activation in an R8 cell. R8 expression of Senseless is variable despite the blockade to cell death. (D) Wild-type eye disc labeled for the photoreceptor marker ELAV (green; left and middle) and R8-specific marker Boss (magenta; left and right). Note the consistent level of Boss expression. (E) spl; GMRp35 labeled as in D. Note the inconsistent Boss expression levels and variable numbers of photoreceptor cells. Arrow indicates a one-cell ommatidium lacking any Boss-expressing R8 cell.