Book Review - Practical Genetics for Aquaculture
Eric M. Hallerman
Department of Fisheries and Wildlife Sciences
Virginia Polytechnic Institute and State University
Blacksburg, VA 24061 USA
Volume 3, June 2002
Genetics is an important and fast-developing discipline within the science and practice of aquaculture. Development and use of a high-performance stock is key to achieving production efficiency and profitability. Although aquaculturists grasp the importance of genetics, most struggle to understand how it might be applied to improve their own production stocks. The struggle is made all the more difficult by the pace and technical complexity of recent developments in aquaculture biotechnology. Hence, many students and practitioners would appreciate a concise and current explanation of genetics as applied to aquaculture. To meet this need, Greg Lutz of Louisiana State University and Fishing News Books have recently published Practical Genetics for Aquaculture.
The intended audience for this book is non-geneticists with an interest in aquaculture, and Lutz strove to achieve a balance between practical application and more technical issues. Strengths of the volume include a straightforward prose style that will prove accessible to a readership with widely varying degrees of familiarity with genetics as a science. The text and case studies offer clear explanations of principles and applications of aquaculture genetics. The text is supported with black and white pictures and diagrams. Each major chapter includes case studies illustrating application of the principles or techniques described in that chapter to particular species or species groups. Each chapter has its own listing of literature cited, easing the readeršs access to further reading. Weaknesses of the volume stem from limited depth on any one topic. Entry into the technical literature will be somewhat more difficult because the literature cited sections for each chapter are not extensive.
A brief description of Practical Genetics for Aquaculture will illustrate its strengths and weaknesses. There are 11 chapters. Chapter 1 provides a brief overview explaining the importance of the field of genetics. After a brief review of basic principles of Mendelian genetics, Chapter 2 addresses qualitative genetics. Coverage of theoretical bases is basic, explaining expression of traits encoded and one and two loci, but does not include explanation of pleiotropy, variable penetrance and expressivity, linkage, and sex linkage. The case studies build on the explanation of theory, and discuss expression of pearl and other colorations in tilapia, coloration patterns in ornamental fishes, and albinism.
The principles and application of quantitative genetics are covered in chapters 3 through 5. Chapter 3 lays out the theory pertaining to partitioning of phenotypic variance in quantitative traits and the estimation of heritability using nested family designs. There is a useful discussion of how the reproductive biology of the species determines which mating designs are most attractive. Case studies focus upon heritability estimation in channel catfish, freshwater prawn, crawfishes, gilthead sea bream, and salmonids. Overall, this is a useful and very accessible chapter. However, absence of key references may frustrate highly interested readers. The classical Introduction to Quantitative Genetics (Falconer and Mackay 1996) is not cited, nor after considerable discussion, are supporting citations for mixed models or software for executing mixed model analyses.
Chapter 4 discusses selection and realized heritability. After further explanation of partitioning phenotypic variance, estimation of heritability using regression or realized heritability designs is presented. More graphics would have helped the reader, for example, in understanding the experimental design for realized heritability. Procedures for multi-trait and family selection are presented clearly. Some subtleties of experimental designs are not mentioned, e.g., ignoring extreme phenotypes in regression-based analyses, and hence, citations to the more technical quantitative genetics literature would have been useful. A more explicit discussion of founder effects and genetic drift would have supported sections explaining lack of response to selection and conflicting results among selection programs. Useful case studies are presented in the context of brief sections on evaluating available strains, domestication selection, conflicting results, correlated responses, indirect selection, indirect measurement of a trait, environmental tolerances, differences between sexes, and genotype by environmental interactions. The chapter concludes with brief sections on selection for miscellaneous traits on finfishes, mollusks, and crustaceans. Although it would have provided an interesting case study illustrating many of the principles discussed in the chapter, there was no discussion of the prominent Norwegian selection program for Atlantic salmon.
The dominance-based components underlying expression of quantitative phenotypes - inbreeding, crossbreeding and hybridization - are discussed in Chapter 5. After a presentation of theory, there are descriptions of practice, including inbreeding impacts, exploitation of heterosis in production stocks, maternal effects, and breed formation. Case studies include carp, salmonids, monosex hybrids, and invertebrates. Rather cursory presentations are given on calculation of individual inbreeding coefficients and on random genetic drift and effective population size. The latter mentions only the effect of skewed sex ratio, omitting discussion of unequal family size and population bottlenecks, mechanisms of importance that are under the control of aquaculturists. The seminal work of Chevassus (1979) on hybrid salmonids is not mentioned. The seminal work of Kincaid (1976 a,b) demonstrating inbreeding depression in salmonids and estimating the degree of inbreeding in federal and state hatcheries are not mentioned, and his description of rotational line crossing is not referenced for the benefit of interested readers. The discussion of hybrid fish for stocking natural waters does not mention use of splake (brook trout x lake trout), that results are highly location-specific, and that some interspecific hybrids may backcross with parental species in the wild.
Chapters 6 through 10 cover the methods encompassing the rapidly developing field of aquaculture biotechnology. Chapter 6 presents material on gynogenesis and androgensis, respectively, the production of individuals bearing only maternal or paternal genomes. Subtleties of different methodologies and how they yield different results are well presented. Case studies showing how gynogenesis and androgenesis have been applied to elucidate sex determination, linkage, and partitioning of phenotypic variance are presented. Of the many applications presented, the only omission I noted was the possible application of androgenesis for regeneration of whole fish from cryopreserved sperm. Chapter 7 presents methods and applications for induced polyploidy, the production of individuals with extra sets of chromosomes. Discussion of methods is straightforward, but not simplistic, and well supported with graphics. Case studies are presented on polyploidy in tilapia, cyprinids, salmonids, other finfishes, and bivalves. In what was otherwise a complete chapter, I noted omission of explanation of flow cytometry as a means of evaluating polyploidy, discussion of the pioneering work of Chourrout in inducing polyploidy in salmonids, and mention of commercialization of triploid Pacific oysters and rainbow trout.
Mechanisms of sex determination and control in major aquaculture species are discussed in Chapter 8. Case studies for tilapia, other finfishes, and crustaceans are presented. Flowcharts explaining the steps of sex reversal and progeny testing in indirect methods for sex reversal might have helped the unfamiliar reader. No mention of all-female salmonid stocks was surprising, because all-female production has come to dominate the rainbow trout sector.
Selective breeding and biotechnology both require controlling the reproduction of the species of interest. Chapter 9, covering control and induction of maturation and spawning is the best developed (40 pages) and most fully referenced in the volume. Many practical aquaculturists will appreciate inclusion of this chapter, but purists will question whether the chapter belongs in a genetics book.
The application of gene transfer techniques to aquatic organisms (Chapter 10) starts with a brief presentation on expression vectors and methods for achieving gene transfer. Development and field testing of transgenic lines is mentioned briefly. Case studies cover transgenic Indian catfish, tilapia, Atlantic salmon, and Chinese carp. A number of key works on transgenesis are not mentioned, including the prominent work of Fletcher and other groups on attempts to improve freeze resistance in Atlantic salmon and other species, and that of the Devlin group on growth enhancement and risk assessment for Pacific salmonids. The development, controversies, and possible commercialization of Atlantic salmon is not mentioned. The author asserts that it is difficult to assess or even speculate on the potential impact of genetically modified organisms on natural systems, although numerous reports have addressed risk assessment and risk management for transgenic aquatic organisms (e.g., ABRAC 1995, Muir and Howard 1999).
Much recent controversy concerns the possible ecological and genetic impacts of stocked or escaped cultured fish upon wild populations. Though it is well that Chapter 11 addresses genetic threats to wild stocks and ecosystems, the treatment is rather shallow and emphasizes the effects of hatchery fish stocked to support sport fisheries, only briefly mentioning the effects of escaped aquaculture stocks on wild populations and ecosystems. The chapter does not discuss the role of aquaculture in the establishment of non-native species, e.g., Atlantic salmon in the west coast of North America, Pacific oyster C. gigas in North America and Australia, and tilapia O. niloticus on several continents. The chapter lacks references to direct interested readers to landmark publications such as Hindar et al. (1991) or Ryman and Laikre (1991) or symposium volumes such as Schramm and Piper (1995) or Mustafa (1999).
There is a general index listing both topics and species discussed that will ease readeršs use of the book. There is, however, no glossary defining key terms. I note the absence of a chapter covering genomic mapping, quantitative trait locus detection, and genetic marker-assisted selection. Although these are not techniques that will be pursued by practicing aquaculturists, it would be well for them to be informed of these developments that will find future application in aquaculture genetics.
Of existing books on aquaculture genetics (Kirpichnikov 1980, Tave 1993, Purdom 1993), this book most resembles Tave's Genetics for Fish Hatchery Managers, now out of print. Lutz's Practical Genetics for Aquaculture will serve many aquaculturists, giving them an entry into principles and applications of genetics in aquaculture, although they will have to read the technical literature if the case studies did not address the species and trait of interest. More serious students or practitioners of aquaculture genetics will have to supplement this reading with publications from the technical literature and from Falconer and Mackay (1996) or Becker (1992). Teachers of aquaculture genetics will have to supplement this book with assigned readings from the technical literature to convey subtleties of genetic theory, experimental design, and data analysis. Established aquaculture geneticists will be familiar with the theory, but will find the case studies interesting. Overall, I find Practical Genetics for Aquaculture a useful book that I will put to good use in teaching my own aquaculture genetics course.