This is called the long-germband mode of segment determination. While most segmented animals-arthropods, annelids, and vertebrates-add segments to their body sequentially during growth, some higher insects have evolved a mode of segment determination in which segments form by simultaneous subdivision of the embryo. Second, gap genes played a crucial role during the evolution of segment determination.

Such gradient-based patterning occurs in most multi-cellular organisms studied so far (see for recent reviews). It solves a fundamental problem of embryonic patterning: how to establish discrete territories of gene expression based on regulatory input from a long-range protein gradient. The gap gene system implements the most upstream regulatory layer of the segmentation gene network, which determines both the position and the identities of body segments. Gap genes have attracted the interest of developmental, evolutionary, and systems biologists for three main reasons: First, they play a key role in patterning the early embryo. There are hundreds of publications in the literature describing genetic and molecular analyses of gap genes, their expression, regulation, and their regulatory effect on downstream targets, and yet, we are far from a complete understanding of its pattern-forming and regulatory capacities, not to mention its evolutionary history. The gap gene network of the fruit fly Drosophila melanogaster is one of the most thoroughly studied developmental gene regulatory networks.

My survey concludes that studies of the gap gene system continue to reveal interesting and important new insights into the role of gene regulatory networks in development and evolution. Finally, I discuss evidence on the evolution of gap gene expression from species other than Drosophila. This is followed by a brief discussion of molecular regulatory mechanisms for transcriptional regulation, as well as precision and size-regulation provided by the system. I discuss the regulatory mechanisms involved, and highlight the remaining ambiguities and gaps in the evidence. I focus on genetic and molecular evidence, which provides us with an almost-complete picture of the regulatory interactions responsible for trunk gap gene expression. This review attempts to synthesize the current knowledge of the gap gene network through a comprehensive survey of the experimental literature. Gap genes are involved in segment determination during the early development of the fruit fly Drosophila melanogaster as well as in other insects.

Shadow warrior 1997 levels