Neurectodermal expression of msh begins in embryonic stage 5 as discontinuous patches in several segments that later extend and merge to form stripes the length of the embryo. The msh-expression domain corresponds roughly to the dorsal half of the neurectoderm. From this region, 4 S1 neuroblasts of the lateral column delaminate and strong msh expression is seen in only one of these. This expression is transient and disappears by stage 9. At stage 10, expression is seen in many dorsal S3-S5 neuroblasts and in their putative neurectodermal proneural clusters. Expression is also seen in some of their immediate progeny. All are from the dorsal half of the neurogenic ectoderm but not all neuroblasts in this region express msh.

msh transcripts are detected at all stages tested except 0-2hr embryos on northern blots. The highest levels of expression are seen in embryos. A detailed description of the spatial pattern of expression in the embryo is given. In summary, expression is first observed at the cellular blastoderm stage in the lateral ectoderm and evolves into a pattern of eight stripes. The lateral expression prefigures the appearance of lateral proneural clusters of the CNS. During the first wave of neuroblast segregation, expression is restricted to delaminating neuroblasts (1 in 4 lateral neuroblasts per hemisegment express msh) and is excluded from ectodermal cells. Subsequent waves of neuroblast segregation are also preceded by msh expression in cell clusters in the the lateral neurectoderm. msh is also expressed in sensory organ precursors of the PNS. Three additional sites of msh in the embryonic ectoderm include the precursors to the stomatogastric neurons, the region that will give rise to neurons of the brain and optic lobes, and the salivary gland placodes. msh expression is seen in dorsal mesodermal cells that will give rise to dorsal body wall muscles and ventral-lateral mesodermal cells that will give rise to fat body. In wing discs, msh is expressed in a highly restricted pattern. Highest expression occurs in three patches of cells in the prospective dorsal hinge region, two of which include proneural clusters. Areas of weaker expression include a row of cells that will develop into sensilla of the anterior wing margin.

msh transcripts are detected at all stages tested except third instar larvae on northern blots. A peak of expression occurs in 8-12hr embryos. By in situ hybridization, transcripts are first detected at stage 5 of embryogenesis in a bilateral series of segmentally repeated bands of dorsal lateral ectodermal cells. At around stage 8, this is replaced with a transient ventral lateral band of msh expression extending along most of the germ band. In stage 10, expression is seen in segmentally repeated clusters of neuroblasts in the thoracic and abdominal segments. By stage 12, in addition to strong expression in segregated neuroblasts, expression is seen in clusters of mesodermal cells. By stage 13, expression is primarily in the CNS and in the developing somatic musculature (corresponding mostly to the developing dorsal and lateral somatic body wall muscles). Expression is also seen in the foregut, hindgut, and fat body.

Dr protein is expressed in the male genital discs of third instar larvae. Expression is localized to a medial domain on the ventral surface of the A9 primordium encompassing the bnl-expressing cells but extending laterally and posteriorly; and (2) a subset of the btl-expressing cells that are adjacent to those expressing bnl.

At embryonic stage 15, Dr expression is seen in specific cell clusters in each brain neuromere. Protcerebral Dr expression begins at stage 12 and continues through embryogenesis. A domain of expression is observed in the deutocerebrum consisting of about 30 cells, which are mainly located near and within the protocerbral-deuterocerebral boundary. Another expression domain is located in the tritocerebrum distributed along the entire neuromere. Dr expression is also observed in cells closely associated with the preoral commissure and cervical as well as longitudinal axon tracts. Expression is also observed in specific cell clusters at the mediolateral margin of each individual neuromere in the posterior brain and ventral nerve cord. Expression is also observed in cell bodies associated with the paired longitudinal connectives, resembling longitudinal glial cells.

Expression in procephalic neuroblasts stage 9-11: tritocerebrum - d4-8; deuterocerebrum - d2-13