6) Constituents of Developmental Pathways (by Rey J. Rosa Morales)

        Hox genes are examples of homeotic selector genes, which control cascades of gene expression (i.e. transcription) during the patterning and development of particular tissues, organs, or regions of the body. If a selector gene such as Ultrabithorax is not expressed properly, specific tissues or organs may not develop at all, or may be transformed into inappropriate structures. Transcription factors, including most homeotic selector genes, control the expression of many other genes, including the structural genes that encode the proteins, such as enzymes and cell structural components that actually do the work of morphogenesis. The actions of transcription factors are often regulated in part by cell signaling pathways (Figure A). These pathways rely on receptor proteins in the cell membrane that respond to extracellular signals such as hormones and short range signaling proteins called Morphogens. Receptor proteins relay these signals to the genes encoding transcription factors. Seven such cell-signaling pathways (each named for a constituent protein, such as Hedgehog or Notch) have been found in animals; others are known in plants. All of these pathways are conserved between Drosophila and mammals, and all are involved in many aspects of morphogenesis and pattern formation throughout the developing body, having evolved many specific functions in particular animal lineages, often through gene duplication. Most cell Signaling pathways are used multiple times during development, suggesting that morphogenetic novelty may often evolve by re-deploying these pathways in different tissues and at different developmental stages. Cell Signaling pathways and transcription factors are linked into developmental pathways (also called developmental circuits).

Figure A. Transduction of intercellular signal through a cell signaling pathway. (Image from Evolution, Futuyma, 2005)

Evolutionary Development

References:

1. Adam S. Wilkins, Editor, (2002). The Evolution of Developmental Pathways. Sinauer Associates; 1st edition. 

2. Futuyma, D.J. (2005). Evolution. Sinauer Associates, Inc. Publishers Sunderland, Massachusetts U.S.A.

3. Gilbert S.F. (2000). Developmental Biology. 6th edition. Sunderland (MA): Sinauer Associates.