The role of senseless in interommatidial bristle differentiation. K. Pepple ¹, B. Frankfort ¹, M. Rose ², M. Mamlouk ¹, G. Mardon ¹. 1) Dept Molec & Human Genetics, Baylor College Medicine, Houston, TX; 2) Program in Developmental Biology, Baylor College Medicine, Houston, TX.

   Senseless (Sens), a Zn finger transcription factor, is both necessary and sufficient for R8 photoreceptor differentiation. In sens mutant tissue, R8 fails to develop and instead adopts the R2/R5 fate. We have found that sens is also important in the differentiation of other cell types in the Drosophila eye, particularly those that develop during pupal stages. Specifically, there is a decreased number of cone and pigment cells in sens mutant clones. However, the most striking feature of the late sens eye phenotype is the near complete absence of interommatidial bristles. Examination of Sens expression in the pupal eye reveals that nearly all undifferentiated cells begin to express Sens at 6 hr APF (after pupal formation). At this time the proneural genes Ac/Sc are co-expressed in the Sens positive cells. By 15 hr APF, clusters of two Sens positive cells can be detected. These cells also express Cut. By 24 hr APF, four Sens/Cut positive cells are seen, but they are not equivalent. Two of the cells consistently show high levels of Sens expression and low levels of Cut while the converse is true for the second pair of cells. Additionally, in sens mutant tissue the number of Cut positive cell clusters is significantly decreased at 24 hr APF. We also note that when these Cut positive clusters are present, they consist of only two cells. Based on these data, we propose a model for bristle differentiation in which sens plays a role at two distinct developmental stages. Initially sens is important but not absolutely required for specification of the bristle sensory organ precursor (SOPI). But by 24 hr APF, after the division of the SOPII, high levels of sens are required for neuronal and glial cell differentiation. To test this model, we will further investigate the identities of the Cut and Sens positive cells in wild type and mutant tissue.