The formation of neuroblasts requires the HMG box protein Soxneuro. M. Buescher ¹, FS. Hing ², W. Chia ^{1, 2}. 1) Dev. Neurobiology, King's College, London, UK; 2) Institute of Molecular and Cell Biology, 30 Kent Ridge Crescent, Singapore 117609.

   The Drosophila embryonic central nervous system develops from a segmentally reiterated array of 30 neural progenitor cells called neuroblasts. We have isolated several alleles of soxneuro, a gene encoding a HMG box protein with strong homology to the Sox1/2/3 proteins. Loss of soxneuro results in a severe hypotrophy with strong spatial and temporal aspects: I. Neuroblast formation is severely affected in the lateral and intermediate regions of the CNS whereas neuroblasts that derive from the ventral region form quite normally. II. Late arising neuroblasts (S3-S5) are more strongly affected than early forming neuroblasts.
    In soxneuro mutant embryos the establishment of achaete and lethal of scute expressing proneural clusters in the neuroectoderm remains unaltered. However, subsequent upregulation of achaete and lethal of scute expression and initiation of asense expression which normally accompany the segragation of neuroblasts does not take place. Therefore in the absence of soxneuro function, achaete and lethal of scute expression are insufficient to confer neural potential to ectodermal cells. Soxneuro does not act to antagonize the anti-neurogenic effect of productive Notch signalling: in a genetic background of ineffective Notch signalling, neuroblast segregation remains dependent on soxneuro.
    A second HMG domain protein fish hook- is expressed in the ventral and intermediate region of the neuroectoderm at the time of neuroblast segregation. While the removal of only fish hook does not result in the loss of neuroblasts simultaneous removal of soxneuro and fish hook leads to an enhancement of the soxneuro phenotype. Therefore soxneuro and fish hook function is partially redundant with respect to neuroblast formation.