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Evolutionary conservation of developmental mechanisms
TIBS 1 8 - OCTOBER 1 9 9 3
Clippingsfrom the phylogenetictree
As we learn more about the mechanisms that control development, it becomes possible to consider how they may have evolved. The title of this volume reflects current interest in the topic. The same is not always true of the articles it contains! Four papers in this compendium do make some serious attempt to consider evolutionary questions. Weisblat and his colleagues spell out the approach most explicitly: 'to understand which embryological phenomena constitute primary characteristics uniting a large group of species, and which are derived characteristics that set apart smaller groups of individual species'. 'What concerns us here', they say, 'is a third level of homology' (beyond the conservation of primary sequence, and the secondary level of conserved biochemical function). This third level is 'the phylogenetic conservation, not just of single genes, but rather of sets of genes interacting with one another in a reciprocal or hierarchical manner'. This approach subsumes a multitude of questions that are ripe for attack, from the specific to the general. Where on the phylogenetic tree do G-protein - coupled transduction mechanisms first appear? What was the primitive role of Hox genes? Is there a common set of genes that defines 'mesoderm' in all animal species? What is the inheritance that all eukaryotes derive from their common ancestor, and what parts of our developmental tool kit have been acquired along the way? It is too early to provide a definitive answer to any of these questions, but technical advances now make it possible to address them. Most obviously, the extensive availability of sequence data allows unambiguous definition of molecular homologies between widely divergent species. Less
obviously, sequence data are also providing the beginnings of a taxonomy for the most diverse eukaryotes, a taxonomy that can define phylogenetic relationships independently of morphology, and hence independently of the developmental processes that underlie it. At present, the most satisfactory subjects for comparative analysis are those that can be compared with some well-understood reference species. Thus it is no surprise that the papers on arthropod and annelid segmentation address the central theme of the book most directly. These three papers illustrate well the strengths and weaknesses of current approaches: the ease with which the expression of homologous genes can be compared between species; the difficulty of understanding what altered patterns of gene expression mean when manipulative experimental tools are not available. Hence the power of Dennell and Beeman's comparative study of homeotie genes in Tribolium and Drosophila, which combines descriptive molecular embryology with genetic analysis. Yet Patel illustrates that insight can come simply from describing gone expression in related species, particularly if, as in his case, some aspects of development are well conserved, while others are quite different. These few chapters apart, the remainder of the volume is less satisfactory. The individual chapters are well written; they describe good science. Yet a motley collection of good reviews does not make a good book. In some cases, even when a topic invites comparative discussion, none is provided. It is obvious, [or example, why Craig Thompson was asked to contribute. He works on the generation of antibody diversity in chickens. Unlike mammals, birds modify the sequence of their immunoglobin molecules principally by gone conversion. Here is a case where molecular mechanisms are understood in some detail, and the phylogenetic context is well known. But it is left entirely to the reader to draw evolutionary comparisons, to contemplate the functional significance - or historical contingency - of the difference between birds and mammals. Arthur Ransick has written a beautiful
article. He investigates the role of asymmetric cell division in Volvo): by a series of classic embryological manipulations. This is lovely stuff, and a model Ior teaching the essence of developmental biology. It's green, too. But apart from a short section on cell specification in other volvocines, no attempt is made to discuss the developmental significance of asymmetric cell division in other species. What a shame. Perhaps most depressing are the 'my organism does it too' papers. We learn that Dictyostelium uses kinases and phosphatases, that nematodes use epidermal growth factor and G proteins, that plants have homeoboxes and make Ca 2" currents when they are fondled. All good stuff, yet what these papers don't say is most revealing. Not one of the papers dealing with these diverse developmental systems so much as mentions the taxonomic position of their organism, or considers other species ti~at would be appropriate for outgroup comparisons. This is conservation without the evolution. It pays lipservice to the theme of the book, but we get the impression that the authors have no particular interest in, or knowledge of, the evolutionary questions prompted by their work. The actual history of life is irrelevant. This strictly comparative work is necessary. Perhaps the authors are wise to avoid speculative excess. Yet the excitement of 'this grand view of life' is so completely missing. It is only since the advent of molecular homologies that the different phyla of developmental biologists have had much to say to one another. '1"hat has been exciting enough. But the day may not be that far off when developmental biologists can talk to palaeontologists, and make testable guesses: Which transforming growth [actor-13-family gene has been overexpressed to make a particular lump on a bone grow longer and thinner? What change might have led to suppression of digits in the limbs of horses? That will be another new synthesis. There is some way to go yet.
B. H. Bowmen Hepatic Plasma Proteins.
N. DLizgi.ines Methods in Enzymology Vol.
Mechanisms of Function and Regulation
220 Membrane Fusion Techniques Part B
J. M. Walker and E. B. Gingold (ed3) Molecular Biology and Biotechnology (3rd
Academic Press, 1993. £44.00 (xii + 285 pages) ISBN 0 12 121070 7
Academic Press, 1993. £58.00 (xxviii + 462 pages) ISBN 0 12 1821212 8
edition) Royal Society of Chemistry, 1993. £39.00 (xviii + 428 pages) ISBN 0 85186 794 4
N. Di.izgtines Methods in Enzymology VoL
S. Neidle and M. Waring (eds) Molecular Aspects of Anticancer Drug-DNA Interactions Vol. 1 Macmillan Press, 1993. £65.00 (xi +
I. S. Zagon and P. J. McLaughlin (eds) Rec-
364 pages) ISBN 0 333 55115 X
£50.00 (xv + 255 pages) ISBN 0 412 49400 0
EvolutionaryConservationof Developmental Mechanisms edited by Allan Spradling, Wiley-Liss, 1993. $98.00 (xii + 219 pages) ISBN 0 471 58843 1
220 Membrane Fusion Techniques Part A Academic Press, 1993. £58.00 (xxvi + 433 pages) ISBN 0 12 182121 8
MICHAEL AKAM Wetlcome/CRC Institute, Tennis Court Road, Cambridge,UK CB1 1QR.
eptors in the DevelopingNervous System Vol. 2: Neurotransmitters Chapman & Hall, 1993.
401
Clippingsfrom the phylogenetictree
As we learn more about the mechanisms that control development, it becomes possible to consider how they may have evolved. The title of this volume reflects current interest in the topic. The same is not always true of the articles it contains! Four papers in this compendium do make some serious attempt to consider evolutionary questions. Weisblat and his colleagues spell out the approach most explicitly: 'to understand which embryological phenomena constitute primary characteristics uniting a large group of species, and which are derived characteristics that set apart smaller groups of individual species'. 'What concerns us here', they say, 'is a third level of homology' (beyond the conservation of primary sequence, and the secondary level of conserved biochemical function). This third level is 'the phylogenetic conservation, not just of single genes, but rather of sets of genes interacting with one another in a reciprocal or hierarchical manner'. This approach subsumes a multitude of questions that are ripe for attack, from the specific to the general. Where on the phylogenetic tree do G-protein - coupled transduction mechanisms first appear? What was the primitive role of Hox genes? Is there a common set of genes that defines 'mesoderm' in all animal species? What is the inheritance that all eukaryotes derive from their common ancestor, and what parts of our developmental tool kit have been acquired along the way? It is too early to provide a definitive answer to any of these questions, but technical advances now make it possible to address them. Most obviously, the extensive availability of sequence data allows unambiguous definition of molecular homologies between widely divergent species. Less
obviously, sequence data are also providing the beginnings of a taxonomy for the most diverse eukaryotes, a taxonomy that can define phylogenetic relationships independently of morphology, and hence independently of the developmental processes that underlie it. At present, the most satisfactory subjects for comparative analysis are those that can be compared with some well-understood reference species. Thus it is no surprise that the papers on arthropod and annelid segmentation address the central theme of the book most directly. These three papers illustrate well the strengths and weaknesses of current approaches: the ease with which the expression of homologous genes can be compared between species; the difficulty of understanding what altered patterns of gene expression mean when manipulative experimental tools are not available. Hence the power of Dennell and Beeman's comparative study of homeotie genes in Tribolium and Drosophila, which combines descriptive molecular embryology with genetic analysis. Yet Patel illustrates that insight can come simply from describing gone expression in related species, particularly if, as in his case, some aspects of development are well conserved, while others are quite different. These few chapters apart, the remainder of the volume is less satisfactory. The individual chapters are well written; they describe good science. Yet a motley collection of good reviews does not make a good book. In some cases, even when a topic invites comparative discussion, none is provided. It is obvious, [or example, why Craig Thompson was asked to contribute. He works on the generation of antibody diversity in chickens. Unlike mammals, birds modify the sequence of their immunoglobin molecules principally by gone conversion. Here is a case where molecular mechanisms are understood in some detail, and the phylogenetic context is well known. But it is left entirely to the reader to draw evolutionary comparisons, to contemplate the functional significance - or historical contingency - of the difference between birds and mammals. Arthur Ransick has written a beautiful
article. He investigates the role of asymmetric cell division in Volvo): by a series of classic embryological manipulations. This is lovely stuff, and a model Ior teaching the essence of developmental biology. It's green, too. But apart from a short section on cell specification in other volvocines, no attempt is made to discuss the developmental significance of asymmetric cell division in other species. What a shame. Perhaps most depressing are the 'my organism does it too' papers. We learn that Dictyostelium uses kinases and phosphatases, that nematodes use epidermal growth factor and G proteins, that plants have homeoboxes and make Ca 2" currents when they are fondled. All good stuff, yet what these papers don't say is most revealing. Not one of the papers dealing with these diverse developmental systems so much as mentions the taxonomic position of their organism, or considers other species ti~at would be appropriate for outgroup comparisons. This is conservation without the evolution. It pays lipservice to the theme of the book, but we get the impression that the authors have no particular interest in, or knowledge of, the evolutionary questions prompted by their work. The actual history of life is irrelevant. This strictly comparative work is necessary. Perhaps the authors are wise to avoid speculative excess. Yet the excitement of 'this grand view of life' is so completely missing. It is only since the advent of molecular homologies that the different phyla of developmental biologists have had much to say to one another. '1"hat has been exciting enough. But the day may not be that far off when developmental biologists can talk to palaeontologists, and make testable guesses: Which transforming growth [actor-13-family gene has been overexpressed to make a particular lump on a bone grow longer and thinner? What change might have led to suppression of digits in the limbs of horses? That will be another new synthesis. There is some way to go yet.
B. H. Bowmen Hepatic Plasma Proteins.
N. DLizgi.ines Methods in Enzymology Vol.
Mechanisms of Function and Regulation
220 Membrane Fusion Techniques Part B
J. M. Walker and E. B. Gingold (ed3) Molecular Biology and Biotechnology (3rd
Academic Press, 1993. £44.00 (xii + 285 pages) ISBN 0 12 121070 7
Academic Press, 1993. £58.00 (xxviii + 462 pages) ISBN 0 12 1821212 8
edition) Royal Society of Chemistry, 1993. £39.00 (xviii + 428 pages) ISBN 0 85186 794 4
N. Di.izgtines Methods in Enzymology VoL
S. Neidle and M. Waring (eds) Molecular Aspects of Anticancer Drug-DNA Interactions Vol. 1 Macmillan Press, 1993. £65.00 (xi +
I. S. Zagon and P. J. McLaughlin (eds) Rec-
364 pages) ISBN 0 333 55115 X
£50.00 (xv + 255 pages) ISBN 0 412 49400 0
EvolutionaryConservationof Developmental Mechanisms edited by Allan Spradling, Wiley-Liss, 1993. $98.00 (xii + 219 pages) ISBN 0 471 58843 1
220 Membrane Fusion Techniques Part A Academic Press, 1993. £58.00 (xxvi + 433 pages) ISBN 0 12 182121 8
MICHAEL AKAM Wetlcome/CRC Institute, Tennis Court Road, Cambridge,UK CB1 1QR.
eptors in the DevelopingNervous System Vol. 2: Neurotransmitters Chapman & Hall, 1993.
401