The Hierarchical Genome and Differentiation Waves

Richard Gordon

Reviewed by Andrew Yates

World Scientific/Imperial College Press 1999
Volume 1 (864 pp) and Volume 2 (971 pp)
ISBN 981-02-2268-8

The promise of unifying principles in biology is what draws most theoreticians to the subject. Richard Gordon of the University of Manitoba has strived towards this for many years, taking the processes at play in the early stages of embryonic development as his starting point. At its simplest level, in this book Gordon and his coworkers propose a new framework for understanding the control of differentiation and cell division in embryos. Bistable, biomechanical sensors or 'state splitters' that are components of the cytoskeleton respond to pressure waves initiated by groups of cells, and trigger differentiation according to whether the cell participates in either expansion or contraction. He devotes considerable effort to describing the implications of this idea and attempts to link the physical processes at play in development with genetics and evolution. However, he is only partially successful.

This book is potentially of interest to anyone in developmental or evolutionary biology, as well as theoreticians interested in the mechanisms of coordinated differentiation and structure formation in the embryo. The author has a multidisciplinary background and often this book suffers from a lack of focus - but this is compensated to some extent by the enthusiasm of his presentation. As theoretical biology it is almost entirely lacking in results grounded in mathematics or physics; but the idea of the cell state splitter, conserved over evolution, is a step towards 'hierarchical reductionism' in biology. This is surely an essential component of the understanding of complex systems.

Volume 1 is concerned with the historical background of embryology and developmental biology in general. It then details the principles of the state splitter and its role in responding to differentiation waves. From these concepts he develops his idea of a differentiation tree which maps the development of distinct tissue types in the embryo. He attempts to trace the evolution of the cell splitter mechanism and proposes that speciation occurs through topological changes in the tree structure. This connects developmental biology to phylogeny. Volume 2 presents the evidence for differentiation waves and a discussion. It concludes with a particularly exhaustive set of references and a selection of the author's key publications.

The author provides a remarkably detailed historical perspective in the first chapter and at many points throughout the rest of the text. This is valuable, and an underrated component of scientific literature. One occasionally feels in science that ideas or contributions are neglected in the forward stampede. However, in an area such as embryology, for decades rife with speculative and incomplete theories and often a hotbed for semi-mystical suppositions, it's questionable whether such a thorough review is relevant - particularly one which is frequently prone to over-reaching itself and leaping into metaphysics. I find it serves to obscure the central message, which itself points the way to many new areas of research.

It's a pity that so little attention is given to the initiation of mechanical differentiation waves that lead to asymmetries in the embryo and trigger the progress through the differentiation tree. The importance of initial conditions is still not understood, and here is sidestepped in a manner that would make a cosmologist proud.

The book is written in a proposition format, where at key points in the text new concepts or interpretations of data appear. These tend to inflate the potential significance of the ideas presented. However, this approach formulates the author's burgeoning thoughts more clearly. Whether a significant number of them are actually testable is another matter and often one has the impression that they are effectively post-it notes for the author who is developing his ideas as he writes. He generously admits "... every proposition is a speculation, and the whole edifice has the appearance of a tower of cards". However, despite this, the author has shown a way forward and his striving for a unified framework of understanding is a bold pursuit.

As a textbook, I think this fails; as a review or introduction it is too scattered; but as a magnum opus, or a chance for a scientist to expound ideas at leisure and set out a vision, it's a remarkable achievement. From the editing point of view, perhaps mightier use of the sword than the pen would have been in order, though.

UK Nonlinear News thanksWorld Scientific for providing a copy of this book for review.

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