Nemo is a core proneural target gene and a feedback inhibitor of Atonal in the Drosophila eye. Vilaiwan Fernandes¹, Lorena Braid^1,2, Esther Verheyen¹. 1) Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia, Canada; 2) Defence Research and Development Canada - Suffield, Biotechnology Section, Medicine Hat, Alberta, T1A 8K6, Canada.

   During neurogenesis a field of cells competent to take on neural fate is established by conserved tissue-specific proneural transcription factors. These factors trigger the expression of core target genes, which aid in refining the proneural domain by up-regulating proneural gene expression in the presumptive neuroblast while simultaneously down-regulating it in the surrounding cells via lateral inhibition. The eye is extensively used to study neurogenesis through the reiterative specification of R8 photoreceptor neurons. The proneural factor Atonal (Ato) is required for R8 specification; its expression is progressively refined from broad expression to intermediate groups (IGs, ~10 cells), to equivalence groups (~3 cells) and finally to single presumptive R8s. Nemo (Nmo) and Nemo-like kinases (Nlk) are conserved serine/threonine kinases, which regulate diverse developmental processes. We show that nmo is transcriptionally regulated by multiple proneural factors (Ato, Achaete and Scute) in multiple tissues. Furthermore, we describe a specific role for Nmo in R8 specification through inhibition and refinement of Ato. In nmo mutant eye discs Ato expression does not refine into IGs resulting in frequent presumptive R8 doublets, while over-expression of nmo in clones results in loss of IGs and greater spacing between individual R8s. These phenotypes suggest a role for Nmo during Notch-mediated lateral inhibition. Indeed, our genetic analyses reveal that Notch targets are reduced with loss of nmo. Hyper-activating Notch in nmo-deficient clones suppresses Ato refinement defects. Thus, Nmo is a novel component of the neuronal specification program in the eye downstream of Ato. Additionally, our data suggest that nmo may be a novel core proneural target, which may globally promote refinement of proneural domains through Notch-mediated lateral inhibition.