The Interactive Fly

Evolutionarily conserved developmental pathways

Neural patterning and epidermal-mesenchymal interactions: conserved function of muscle segment homeobox.

In Drosophila muscle segment homeobox directed neural patterning is carried out cooperatively with ventral nervous system defective/NK2 and intermediate neuroblasts defective (ind) in a pattern similar to vertebrate msh, vnd and ind homologs. Interaction between Drosophila DPP and Short gastrulation (vertebrate homologs BMPs and chordin) restrict msh (vertebrate homologs the Msx genes) expression to the lateral most column of proneural clusters (in vertebrates the lateral-most portions of the neural plate). In a similar fashion vnd is restricted to the medial column of proneural clusters and ind is restricted to the intermediate column of proneural clusters. msh, vnd and ind are expressed in the neurectoderm, and are responsible for setting down the pattern of neuroblast formation prior to delamination. msh, ind and vnd serve a similar conserved function; regulation of expression of the achaete-scute complex to particular neuroblasts.

In vertebrates the vnd homolog Nkx-2 is restricted to the medial region of the neural plate, Ind homologs (Gsh proteins) are restricted to the intermediate region of the neural plate, between the lateral and medial regions, and Msx is restricted to the lateral neural plate. Msx and Nkx-2 are known to regulate the expression of vertebrate achaete-scute homolog (ash) in the developing neural column. It is suggested that the function of vertebrate Msx, Gsh and Nkx-2 in structuring the vertebrate neural tube are homologous to the function of Msh, Ind and Vnd in determining CNS structure in Drosophila, that is, all these proteins regulate the lateral to medial structuring of neuroblast diversity.

muscle segment homeobox is homologous to a growing number of divergent homeobox genes in vertebrates that appear to be involved in interactions between epithelial and mesenchymal cells (mesodermal cells that give rise to connective tissue). In vertebrates, msh is first expressed in ectoderm, followed by expression in mesoderm. A similar temporal and spatial pattern of gene activity occurs in flies; a switch from ectodermal to mesodermal expression. This suggests that vertebrate and Drosophila msh play similar developmental roles.

It is noteworthy that many homeodomain proteins implicated in rostrocaudal neural patterning of the vertebrate neural tube, including various Pax and Nkx proteins, and the Gsh, Msx, Gbx, and Tlx proteins — also possess an engrailed homology-1 like repression domain, a target of the Groucho (Gro) dedicated repressor. Indeed, in a sample of 165 vertebrate HD proteins, many expressed by neural cells, ~36% were found to possess an eh1 domain (see Supplemental table). Gro/TLE-dependent repression may, therefore, have a more pervasive role in establishing precise spatial patterns of neuronal generation along major axes of neural tube development. In addition, since homologs of the Nkx, Msx, and Gsh proteins control neuronal patterning along the dorsoventral axis of the Drosophila CNS, these results suggest that Gro/TLE-mediated corepression may be an evolutionarily conserved step in CNS patterning (Muhr, 2001).

date revised: 15 April 2001

Developmental Pathways conserved in Evolution

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