tsg is expressed in paired dorsal stripes reaching only partially to the lateral sides of the embryo. Expression is found in the dorsal ectoderm during germ band extention. There is no expression in the midline in amnioserosal cells [Images]. There is an additional domain of expression in the anterior most cells of the embryo (Mason, 1994).
A long-standing hypothesis posits that morphological changes may be more likely to result from changes in regulation of gene expression than from changes in the protein coding sequences of genes. The expression pattern of the twisted gastrulation (tsg) gene has been compared among five Drosophila species: D. melanogaster, D. simulans, D. subobscura, D. mojavensis, and D. virilis. The tsg gene encodes a secreted protein that is required for the specification of dorsal midline fates in the Drosophila early embryo. Tsg is unlike other secreted growth and differentiation factors in Drosophila in that its expression pattern can be experimentally varied and still result in normal development. Because of this, its regulatory region may be freer to diverge than that of other developmental genes whose misexpression may lead to lethal defects. Thus, the tsg gene may be a good indicator of the frequency and nature of evolutionary changes affecting patterns of gene expression. Over approximately 60 million years (Myr), the tsg gene has retained a dorsal-on/ventral-off pattern and a middorsal region of expression; but there have been marked changes in the middorsal domain of expression as well as the appearance/loss of other domains of expression along the anterior/posterior axis. Changes between closely related species (approximately 2-5 Myr since divergence) that are not reflected among more distantly related species suggest frequent changes in gene expression over evolutionary time. These changes in gene expression may serve as the raw material for eventual evolutionary changes in morphology (Mason, 1998).
tsg is one of seven known zygotic genes that specify the fate of dorsal cells in Drosophila embryos. Mutations in tsg cause at least some of the cells on the dorsal half of the embryo to adopt more ventral cell fates leading to the proposal that tsg participates in establishing, maintaining, or modulating a gradient of dpp. Mutations of tsg only affect the fate of a narrow strip of dorsal midline cells and do not affect dorsal ectoderm cells. However, the pattern of tsg expression is not coincident with the territories affected by tsg mutations. All tsg mutants lack the following midline-derived structures: labrum, labral sense organ, dorsal bridge and dorsal arm of the pharyngeal skeleton and the posterior tuft (Mason, 1994).
The Twisted Gastrulation (Tsg) protein is one of five secreted proteins required to pattern the dorsal part of the early Drosophila embryo. Unlike the Decapentaplegic (DPP) protein that is required to pattern the entire dorsal half of the embryo, Tsg is needed only to specify the fate of the dorsal midline cells. The tsg gene was expressed with different promoters to address its mechanism of action and relationship to Dpp. When expressed in a ventral stripe of cells, Tsg protein can diffuse to the dorsalmost cells and can rescue the dorsal midline cells in tsg mutant embryos. Despite elevated levels that exceed those needed for biological activity, there was no change in dorsal midline or lateral cell fates under any conditions tested. It is concluded that Tsg does not modulate an activity gradient of Dpp. Instead, it functions in a permissive rather than instructive role to elaborate cell fates along the dorsal midline after peak levels of Dpp activity have 'primed' cells to respond to Tsg. The interaction between Tsg and Dpp defines a novel type of combinatorial synergism (Mason, 1997).
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