Segmentation is a fundamental feature of the body organization of annelids and arthropods and, in a rather different form, of chordates. The early embryonic development of these different animals is extremely diverse, with great variability in cell number and size, in rates of development, in cell lineage patterns, and in the roles of prelocalized cytoplasmic determinants and of cell-cell interactions. Nevertheless, each embryo becomes subdivided into a precise number of similar (but not identical) morphological units arranged in a precise sequence, and a central problem of developmental biology is to discover the mechanism(s) by which this is achieved. The relationship between the processes of segmentation in different types of embryo is also a topic of great relevance to evolutionary biology.
Segmentation has long been studied in various invertebrate and vertebrate embryos with the techniques of traditional embryology: lineage analysis, ‘cut and paste’ surgical operations, cytoplasmic injections, irradiation and theoretical modelling. More recently, however, it has become possible to analyse transcripts and protein products from genes that are (or may be) involved in segmentation. These advances in the molecular analysis of segmentation have stemmed from elegant studies on the developmental genetics of Drosophila. This work has defined maternal genes which set up positional information within the oocyte and zygotic genes which progressively refine this into the segmented body pattern.
The integration of embryological, genetical and molecular results from Drosophila is rapidly deepening our understanding of the early development of this insect. Furthermore, since many of the Drosophila ‘segmentation genes’ have highly conserved regions (such as the famous ‘homeobox’), it has been possible to screen the genomes of other animals for homologous genes, which may have homologous or similar functions, and thus to begin to integrate embryological and molecular studies in other insects, other arthropods and in vertebrates.
The analysis of segmentation is an exciting and advancing field and the meeting on which this volume is based was stimulating and enjoyable. We hope the book reflects these qualities.
The Editors
