Syntagms in development and evolution.

The genetic analysis of segmentation, neurogenesis, appendage formation and other developmental processes has revealed that the development of Drosophila can be broken down into discrete elementary operations. Thus development can be viewed as a stepwise process where each step is driven by a small group of genes working interactively. García-Bellido proposed that each of these groups be called a "syntagm". In this review, we will describe a series of developmental syntagms, and explore the consequences of this discontinuous organization of the developmental program on evolution.