Specification and Development of the Drosophila larval vision system

The eyes of the Drosophila larva, also termed as Bolwig’s organ (BO), are comparably simple and consist of a paired structure each eye containing only about 12 photoreceptors (PRs). There are two distinct subtypes: about four PRs express the blue - sensitive rh5 and about eight express the green – sensitive rh6 (Sprecher et al., 2007). Two transcription factors specify larval photoreceptor subtypes. Spalt (Sal) is exclusively expressed in Rh5 photoreceptors, where it is required for Rh5 expression. Sevenup (Svp) is restricted to Rh6 photoreceptors, where it represses sal and promotes Rh6 expression. A third transcription factor, Orthodenticle (Otd), expressed in all larval photoreceptors, acts only in the Rh5 subtype to promote Rh5 expression and to repress Rh6.These different PR subsets arise from neuronal precursors called R7 and R8 cells. Senseless (Sens), a Drosophila ortholog of the vertebrate Gfi1 transcription factor, plays a role by both negatively regulating R7-based features and positively enforcing R8-based features during terminal differentiation. It is demonstrated that Sens function together with the transcription factor Orthodenticle (Otd) to oppositely regulate R7 and R8 PR Rhodopsin gene expression in vitro (cook et al., 2007). It also mediates the very different cellular behaviours of Rh5 and Rh6 photoreceptors. Sens is restricted to Rh5 photoreceptors and must be excluded from Rh6 photoreceptors to allow them to die at metamorphosis.

My thesis deals with the investigation of Sens within sal/otd/svp network. Is it downstream of sal or parallel to it? Does it act in parallel with otd? Does it inhibit svp? Or is it just downstream of both sal and otd to negatively regulate expression of Rh6 in Rh5 cells? I will also be dealing with other transcription factors which could also be important for maintaining photoreceptor identity, Specification and Development.