Drosophila Genes in Development: Proneural Genes
The proneural genes of Drosophila are required for the segregation of neural precursors from the ectoderm. One of the proneural genes, achaete (ac), is required for the formation of a subset of larval and adult sense organs. It is responsible for the allocation of specific cells (presumptive neuroblasts) to the neural fate.
ac is not only involved in neurogenesis but also in a number of other morphogenetic events that essentially resemble the actions of ac in neurogenesis. It is important in the development of Malphigian tubules, for example.Through cell proliferation and migration, lead by the tip cell, the Malpighian tubules elongate until they reach full size. In the absence of members of the AS-C, tip cells fail to segregate (Hoch M., 1994).
Achaete encodes protein with a basic central bHLH domain and a C-terminal acidic domain (Villares R., 1987).
Nuclear
Achaete is expressed in the presumptive neuroectoderm during embryonic development. Observations of wing imaginal discs reveal that achaete and scute are transcribed in a pattern that prefigures the future sites of neural elements, both ectopic and normal sensilla (Blair S.S., 1992).
AC protein is localized within nuclei of proneural clusters. Clusters grow in number and intensity of staining until the sensory organ mother cell (SMC) becomes discernable due to its more intensely stained nucleus. The AC protein disappears shortly before the SMC undergoes its first differential division (Cubas P., 1991).
The AC protein is expressed in a specified subset of neuroblasts in embryonic stages 8-11 (Doe C.Q., 1992). It first accumulates in late stage 8 embryos in a segmentally repeated pattern of two medial and two lateral clusters of 5-7 ectodermal cells per hemisegment. One cell per cluster, the future neuroblast, comes to express ac most intensely and delaminates toward the interior of the embryo. The remaining cells in the ectodermal cluster lose AC protein expression. Expression of AC protein in the neuroblast ceases after it has delaminated from the ectoderm and before it begins dividing (Skeath J.B., 1992). The ac and sc transcripts give a dynamic pattern in the blastoderm (sc) or gastrula (ac) through stage 11 embryos. They are present in clusters of cells on the ectoderm and internally near the mesoderm. They are expressed in most neurogenic regions. Their expression patterns are very similar except in stage 9 where sc is barely detectable and ac is expressed in four longitudinal rows of clusters (Romani S., 1987).
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Pair-rule
HOX-Pro