Molecular Evolutionary Genetics. by Masatoshi Nei. New York

Book reviews 74

Barry Polisky discusses replication control of recommendation, only mitigated by the high price, ColEl-type plasmids and Robert Knowlton considers which will limit its siting to library rather than copy number and stability of yeast plasmids, in the personal bookshelves. next two chapters; and in both systems the implica- ERIC REEVE tions for maximising expression of cloned genes are Institute of Animal Genetics examined. These form important current areas of University of Edinburgh applied research and the authors examine the problems which each system presents. Gary Stormo discusses translation initiation in both prokaryotes and eukaryotes. Some laboratories have Molecular Evolutionary Genetics. By MASATOSHI NEI. begun to collect data on the efficiencies of particular New York: Columbia University Press. 1987. 512 initiation sites, and Stormo concentrates on the effects pages. U.S. $50.00. ISBN 0 231 06320 2. of mRNA sequence on translation efficiency. This, of The rapid accumulation of data at the molecular course, brings up the very difficult problem of genetic level, from protein and DNA sequencing, structural features of the mRNA, which are difficult restriction enzyme analysis and electrophoretic vari- to predict from its sequence but can have an important ants have given us much information on the tempo effect on initiation rate. The E. coli data lead to the and structure of the evolutionary process and on following rules for maximizing the yield of a particular variability within populations. Major theoretical protein: (1) AUG is the best initiation codon; (2) A developments have ensued in order to interpret all the Shine-Dalgardo sequence of at least four nucleotides, new data. It is now about a quarter of a century since taken from the sequence AGGAGG, should be used; Zuckerkandl and Pauling first drew attention to the longer may help some messages; (3) The optimum steady molecular clock of protein evolution, over 20 spacing between the S-D sequence and the initiator years since Harris' and Lewontin and Hubby's codon is about nine bases. Some additional rules are demonstration of substantial polymorphism within also given. Stormo finds that the requirements for species inferred from gel electrophoresis, and 20 years efficient expression of eukaryotic proteins are quite since Kimura's proposal of the neutral theory to different from those for prokaryotes, but only some explain these data. There have been other fundamental less general proposals can be made for increasing gene discoveries, such as of different rates of evolution at expression. silent and coding sites and of concerted evolution of In by far the longest chapter in the book, de Boer multiple genes, and developments in theory such as in and Kastelein present data on biased codon usage, measures of genetic distance, phylogenetic tree struc- and show that genes coding for abundant proteins ture using molecular data and of models to describe have a marked bias towards major tRNA species evolution of gene families. As a consequence evolution compared with poorly expressed genes, which make and variation are now considered mainly by geneticists much more use of minor tRNA species. Data on at the molecular level. Although this may seem to translation time (amino acids per second) for strongly ignore the importance of adaptation and of an under- and poorly expressed genes are also given, and standing of the evolution and maintenance of variation evolutionary aspects of biased codon usage are of continuous traits, this reductive stance does reflect discussed. the amount of new data coming forward and the In the last three chapters Goldberg & Goff describe power of molecular analysis. the selective degradation of abnormal proteins in E. Masatoshi Nei and his group at Houston have been coli and the eight soluble proteases responsible; major contributors and stimulators of this work, for Dobner & Villa-Komaroff discuss the detection of example, on methods for distance measurement, for proteins produced by recombinant DNA techniques; tests of neutral and alternative theories, and for and Buell and Panayotatos consider how knowledge synthesising theory and observations. It is not, described in the previous chapters has been used for therefore, surprising that his Molecular Evolutionary the overproduction of proteins, specifically in E. coli Genetics is a most useful book which brings together which has been the major source of information. Each molecular data, methods for their analysis, and the chapter has a comprehensive list of references, and theoretical basis of their interpretation. It will serve as there is a detailed index. a reference book on both methodology and results, Many of the studies presented in this book were no and should be of value both to population geneticists doubt motivated by practical or commercial aims, but and to those molecular geneticists prepared to extend they have often made novel and valuable contributions their horizons to anything but the most superficial to basic molecular genetics, and this should make the evolutionary interpretation of their data. book of considerable interest to a wide readership. The greater part of the text deals with the kind of The book, in spite of its sixteen contributors (thirteen molecular data which has been collected: initially from USA, two from Geneva and one from France), protein and subsequently DNA sequences between reads as a unified whole, and the two editors are to be species, on electrophoretic, DNA sequence and restric- congratulated. I have to give the book a very warm tion fragment length variation within species and how

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each can be analysed and interpreted. Thus there is - is one of the more mysterious. A Phycomyces extensive discussion on measures of distance, and sporangiophore, for example, will grow away from a their various pros and cons, on measurement of and physical barrier placed close to its growing zone, but inferences from polymorphism, and on tree construc- neither the mediator nor the receptor of this growth tion. Throughout, the underlying uniform rate molec- response has been identified. In geotropism too the ular clock theory gives a reference point for the receptor for the gravity stimulus is unknown. Phyco- measures and their comparisons, and the results are myces sporangiophores also exhibit a growth response related to the evolution by neutral substitution when exposed to a laminar wind. The extent of this interpretation. Subsequently there is a rather brief positive anemotropism depends on the humidity of review of population genetics theory. In these and the wind to which the sporangiophore is exposed. This previous chapters formulae are usually stated, perhaps observation suggests an effect of humidity on cell wall with verbal argument, and the reader is referred to the extension and this latter topic is the subject of another original papers for derivation. Sometimes this is chapter. It is generally envisaged that fungal hyphae rather frustrating, in that the reader can not readily grow at their tips by a process of 'softening' the rigid see what is the basis of the conclusions, but usually it enclosing wall, through the action of lytic enzymes, is more than sufficient and the book is therefore not followed by the incorporation of ' re-inforcing' cluttered up with formulae. The final chapter is material into the expanded cell wall. The kinetics and essentially an essay in which Nei discusses how the control of these two antagonistic reactions, either in molecular data square with models of evolution, and tropic growth response or in normal cell wall in which he puts forward his own synthesis. extension, has still to be satisfactorily quantified, but The book is clearly written and, as far as I detected, several models of cell wall growth are extensively and with few errors. (I think, however, that if Nei actually critically discussed. tries multiplying the matrices denned on page 355 he As one might expect, mutants which lack one or will find some transposition necessary.) more tropic growth responses have been isolated but This is a quite different book from Kimura's Neutral the contribution of genetics towards unravelling the Theory of Molecular Evolution. Nei's text deals more problems in sensory physiology has been disappoint- with methods of analysis rather than with either ing. Classical genetic analysis faces considerable developing a thesis or with mathematical population problems in Phycomyces. The multinucleate nature of genetics. Although Nei is clearly a confirmed neutral- the cell cycle, the uncertainty over the time of nuclear ist, he uses it rather as a framework for describing and fusion and meiosis, the complex tetrad analysis which interpreting data. As such it should become a standard follows zygospore germination, and the long dor- reference. mancy period are just a few of the complicating WILLIAM G. HILL factors which have hindered the genetic contribution. Institute of Animal Genetics Another surprising feature which is apparent from University of Edinburgh reviewing this book is the slight contribution which has been made up to now by genetic manipulation technology. I write 'surprising' because the possibility Phycomyces. Edited by E. CERDA-OLMEDO and E. D. of a major contribution certainly exists, especially in LIPSON. New York: Cold Spring Harbor Labora- the areas of differentiation and regeneration, and tory. 1987. 430 pages. $88.00. ISBN 0-87969-199- sexual morphogenesis. The book contains chapters 9. covering the physiology and cytology of these topics, This book is dedicated to the memory of Max but here I was struck by the potential for breakthrough Delbriick who was the source and inspiration behind using the techniques of molecular biology. For many projects, including the exploitation of Phyco- example, experiments on regeneration from segments myces as a system for studying several diverse of sporangiophores have shown the existence of a biological phenomena. The volume, consisting of 11 polarity of morphogenetic information which is chapters and 8 appendices, is a fitting testimony to his thought to be stored in the cell surface and cell influence. membrane in the form of a gradient. The isolation of The book's primary virtue is its comprehensiveness; membrane mutants should be feasible and the molecu- not only are a large number of unanswered questions lar analysis of these together with existing mutants discussed in depth, but the literature underpinning which are defective in sporangiophore development, each of them is thoroughly documented. In addition, should yield greater insight into differentiation and a comprehensive, 38 page bibliography is included at regeneration. Similarly in sexual morphogenesis the the end of the book to make it an indispensible analysis of differential gene activity associated with reference source of Phycomyces literature. pheromone-induced changes should allow the molecu- The most widely studied aspects of Phycomyces lar basis of hormone action to be described. biology are the tropic growth responses, of which In this review I have tried to convey some of the phototropism has been investigated most extensively, stimulation which I obtained from reading this book. but the avoidance of physical barriers - fugotropism I can recommend it as a valuable library purchase. It