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Imaginal Discs: The Genetic and Cellular Logic of Pattern Formation

Lewis I. Held, Jr. (Cambridge University Press, 2002).
ISBN 0521584450

The genetic analysis of the development of imaginal discs--communities of cells from which the external structures of the adult fly emerge during metamorphosis--has provided many key insights into the workings of evolutionarily-conserved developmental mechanisms. This book delves deeply into the intricacies of imaginal discs, with the aim of providing a comprehensive reference guide to the field. But this is no laboratory manual; rather, it is a fascinating cornucopia of historical and current theoretical models. Held's approach is to use deductive reasoning to interpret genetic data in a conceptual framework, and while the experimental data receive ample and clear coverage, the emphasis is squarely on models and how data can be used to either support or disprove them.

This is an unusual format for a book on developmental genetics. While everybody working in the field uses models (at least implicitly) to direct and order their thinking, overt discussion of the models is typically relegated to small schematics in the discussion section of papers. Here, the models take centre stage. The descriptions of the historical development of some of the more well established models are very illuminating and capture well both the stunning progress that has been made, and the twists and turns of the paths taken to achieve the current models. In highlighting these convoluted paths, the book illustrates clearly one of the key features of a good model--that it is, by necessity, "wrong", but that it is wrong in a useful way. The best models are always staging posts on the route to a deeper understanding (and improved model).

A particularly striking feature of the book is the gargantuan bibliography. Comprising 4900 entries, and taking up nearly a third of the book, it is more than comprehensive! What I particularly like about it is that it is so much broader in scope than one might imagine; in addition to the requisite references on imaginal discs, there are entries charting a way into related (but perhaps peripheral) areas such as noise in gene networks and patterning by reaction-diffusion models. This has a potential downside though, in that it is very easy to spend a great deal of time flicking backwards and forwards between the bibliography and the main text, losing the thread in the meantime... It is clear that much attention has been paid to the figures, and they convey the logic of the developmental systems well. However, I did find the absence of any micrographs of real imaginal discs disappointing. It is important to keep in mind what is actually being modelled here, and a few such figures would serve to "ground" the theoretical discussion.

So who (or what) is this book for? It is not an introduction; knowledge of fly development and the standard techniques of molecular and classical genetics are assumed. It can be dipped into as a reference on specific processes (e.g. bristle spacing), but is by no means exhaustive. It isn't always easy to locate a particular topic, either. There is no listing in the index for, say, "lateral inhibition", although it does appear in the list of models in an Appendix and is discussed in detail in the chapter on bristle patterns. Nor can I find any mention of "planar cell polarity" in the index, although it is discussed in the context of the eye disc in the main text. On the positive side, the glossaries of protein domains and key genes involved in imaginal disc development are useful resources that are instantly accessible. But the book is really aimed at those who already have a working knowledge of fly development and who want to see in detail the interplay between experimental data and theoretical models, and how this affects our way of looking at developing systems. There is (alas!) no discussion of mathematical formulations of the models, so the book is accessible to anybody with a background in fly development. Finally, all books age, and those dealing with developmental biology tend to age particularly quickly. The theoretical and historical emphasis of this book should help it to age gracefully.

Last Updated: 17 February 2004

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