Homeobox Genes DataBase

Drosophila Genes in Development: Notch signaling pathway

Scabrous

Function

The art of producing the right number of ommatidia begins with proteins involved in intercell communication. Two neurogenic genes, Scabrous, which is secreted, and Delta that's bound to the cell surface, help to insure that neighboring cells do not acquire the photoreceptor fate. Only 800 ommatidia should be formed per eye. Scabrous is first to be enlisted because Delta and its receptor Notch, acting without Scabrous, would give rise to a disorganised pattern of ommatidia. The targets of Scabrous are R8 cells, the first of the eight cells comprising the ommatidia. All furrow cells pass through an R8-competent stage. Scabrous activity, and subsequently action by Notch and Delta, restricts R8 differentiation to alternating groups of cells and confers a regular pattern on the R8 cells.

Scabrous is probably a second ligand for Notch, preparing the way for the primary ligand Delta to fulfill its role in lateral inhibition, a process restricting the number of ommatidia in the developing eye. Either an overexpression of Scabrous or its deficiency causes the same phenotype, a rough eye with oversized and fused ommatidia. This suggests that the concentration of Scabrous is a critical determining element in ommatidial differentiation (Mlodzik M., 1990 and Baker N. E., 1995).

Mutations at the scabrous locus affect cell-cell signaling during neural development.

Protein

Scabrous is a secreted protein with similarity to the beta and gamma chains of the fibinogens in the carboxy-terminal protions (Baker N. E., 1990 and Mlodzik M., 1990). Scabrous is not expected to polymerize, since it lacks crosslinking regions of fibrinogen (Baker N. E., 1990). The fibrinogen domain also has homology to a globular domain found in tenascin.

Subcellular location

Secreted

Expression Patterns

The highest levels of sca transcripts are found in embryos, pupae, and third instar imaginal discs. In embryos, sca transcripts are first detected at 3 hrs of development and by gastrulation, are prominent in the neurogenic ectoderm. Expression is higher in neuroblasts than in epidermal cells. There is expression in the PNS as well. During germ band extension, sca transcripts are seen in alternating domains of high and low expression along the anterior-posterior axis. (Mlodzik M., 1990)

During imaginal development, scabrous is expressed in the R8 photoreceptor precursor cells in the eye imaginal disc, and later in R2 and R5 cells. In the wing disc, scabrous expression is coextensive with the anlagen for anterior wing margin bristles and machrochaetae, and is controlled by genes of the achaete-scute complex (Mlodzik M., 1990).