Abstract

Many of the most successful animal groups are based on modular design. The body is made of a series of repeating units, or segments, that are modified to a greater or lesser extent to serve distinct functions. In vertebrates, modular organisation is most apparent early in development, but a reflection of this fundamental segmentation persists in the adult vertebral column and nervous system.

The gene networks that generate segments are particularly well understood in the fruit fly Drosophila. Here, segmentation is divorced from growth. All segments of the body are generated simultaneously, by a cascade of gene regulatory interactions operating within the egg while it is still a single, large, multinucleate cell.

Segmentation in vertebrates appears to use quite different mechanisms, involving temporal and spatial oscillations of gene expression within a growing population of cells. These oscillations are co-ordinated by cell-cell interactions. The oscillating pattern of gene expression becomes “frozen” at a moving wavefront in the embryo to generate a stable pattern of alternating cell states that underlies segmentation. Segments thus appear sequentially, from head to tail, as the embryo grows.

Recent work has made it clear that much of the Drosophila segmentation cascade is a relatively new invention that evolved within the arthropods, probably as an adaptation to speed up development. Other arthropods, for example centipedes, use mechanisms that may be more similar to those found in vertebrates than they are to the highly derived mechanism seen in Drosophila. Our own research seeks to understand what were the ancestral mechanisms of segmentation within the arthropods, and how these evolved into the gene regulatory cascade that we know from Drosophila.