�9 Indian Academy of Sciences

B O O K R E V I E W

Genetic structure and local adaptation in natural insect populations

Edited by Susan Mopper and Sharon Y. Strauss

Chapman & Hall, London; 1997; 449 pages.

Reviewed by AMITABH JOSHI*

There are essentially two options open to a reviewer of an edited compilation of articles on a common theme written by different authors. One is to briefly summarize the contents of each chapter sequentially, leaving the reader to do his or her own integration of the material covered. The other is to attempt to place the compilation in context of the current status of the field, and, perhaps, not go into too much detail of what each chapter contains. Both options have their pros and cons, and the choice of one over the other reflects not much more than the reviewer's subjective preference. Having said that, let me make my bias clear and state that I shall follow the latter approach in reviewing this book, which is a compilation of 16 chapters on various genetic aspects of local adaptation in insects, including both reviews and original results, written by many different authors, several of whom have made major contributions to the field.

The extremely high level of host specialization exhibited by phytophagous insects has interested ecologists and evolutionary biologists for well over three decades, in part because, all else being equal, it should intuitively be a more adaptive strategy to be a generalist, thus being freed from having one's evolutionary fate inextricably tied up with that of one's host. Thus the focus of much attention has been upon those factors, ecological and genetic, t h a t may facilitate the evolution of extreme specialization in phyto- phagous insects, leading potentially to host-race formation and sympatric speciation. A major paradigm in thinking on the evolution of specialization has been that of trade-offs in fitness on different hosts. In this view, insect genotypes that have relatively high fitness on a particular host species tend to have poor fitness on other hosts, a situation that may promote the formation of host races. This view, however, has been under criticism of late and it has become clear that in the absence of other ecological factors promoting host-

* Evolutionary and Organismal Biology Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, $akkur, Bangalore 560 064, India.

based assortative mating, the genetic prerequisites for such trade-offs to lead to host-race formation are extremely strin- gent and not likely to be very common in nature. Therefore interest has now begun to focus more on the role that host seeking and egg laying behaviour of females may play in the evolution of specialization.

At the same time, there is an increasing realization that the trade-offs paradigm may act at various levels of the insect-plant interaction (e.g. insect genotypes may adapt to individual plant genotypes, especially those of long-lived tree species), and that the genetic structure of phytophagous insect populations may be an active force in guiding the evolution of specialization, rather than being but the passive outcome of that evolutionary process. This is the conceptual framework in which this book is embedded, and although the focus of the book is consequently somewhat narrow, the treatment of the issues emanating from this view of the evolution of specialization is, nevertheless, satisfying in its depth. The individual chapters in the book are grouped into four major themes, starting with four chapters that review results from field studies, focusing on the evidence, or the lack thereof, for local deme formation. These are followed by a heterogeneous collection of four chapters dealing with various problems, both methodological and statistical, typically encountered in attempts to unequivocally demon- strate the existence of local adaptation, as well as the role that stochastic events may play in the evolution of locally adapted demes or host races. This is a particularly welcome compilation, as these are areas that have all too often been neglected by field ecologists in their work in this area. The remaining eight chapters of the book basically deal with a number Of issues relevant to understanding the roles of dispersal (migration) and geographical (spatial) structuring of populations as mediators of the evolution of plant-insect interactions.

Two of the papers in the first set of four review evidence for local adaptation in well-studied pine-leaf scale and pine interactions in western North America. Don Alstad (Uni- versity of Minnesota, St Paul, USA) reviews major results

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from about 20 years of field studies of the interaction between the black pine-leaf scale (Nuculaspis caliCbl, ica) and western yellow pine (Pinus ponderosa) and concludes that in this system the data do not support the hypothesis that insects adapt to characters of individual trees that are under strong genetic control. There is, however, some evidence that local adaptation may occur through density- dependent effects as well as plant-genotype-by-environment interactions. More importantly, the data suggest that drift may be playing a substantial role in maintaining genetic variation across demes, especially as the pine-leaf scale is relatively sedentary. Cobb and Witham (Northern Arizona University, Flagstaff, USA) similarly review the interaction between Matsucoccus acatyptus and Pinus edulis, and conclude that density-dependent migration probably plays a major role in preventing local adaptation to individual trees, although local adaptation at a regional level remains a possibility. In contrast to the studies on pine scales, Stilling and Rossi (University of South Florida, Tampa, USA) present evidence for local deme formation in a galling midge (Asphondylia borrichiae) on different islands off the Florida coast. It is not clear from their study, however, as to what the causative factors for deme formation are, although they suggest that the endophagous habit of the midges, which live in the tissues of their host, may play a role in promoting local adaptation at the island level. In some ways, these three chapters provide a good illustration of the problems facing the worker in this field. Fitness com- ponents and selection pressures are notoriously difficult to estimate in the field, and in the absence of such definitive evidence, all too often the case for local adaptation rests upon more equivocal results such as the outcome of reciprocal transplant experiments. A systematic review of major studies of local adaptation in terms of the kind of evidence that was generated, as well as of some of the conceptual issues involved in determining what constitutes rigorous evidence for local adaptation, can be found in the chapter by Boecklen and Mopper (Mopper: University of Southwestern Louisiana, Lafayette, USA), which brings the first section to a close.

All in all, the first set of tom" chapters are fairly repre- sentative of the book as a whole, in terms of the broad sweep of their outlook if not in the details. The study of local adaptation in phytophagous insects, and ecological specialization in general, has been characterized by the accumulation of vast amounts of information in field studies that remain open to several lines of interpretation and often can shed very little light on what the causal mechanisms for any observed or inferred effects may be. Part of this is due to the problems of studying the mechanisms of natural selection in the field. Chapters by Berenbaum and Zangerl (University of Illinois, Urbana, USA) and by Strauss and Karban (Strauss: University of California, Davis, USA) take up this issue in more detail, the former from the point of view of physiological adaptation of insects to plant allelo- chemicals, and the latter from more of a quantitative-genetic

perspective. These chapters, and especially the chapter by Rossiter (University of Georgia, Athens, USA), a full- fledged original contribution on the statistical analysis of genetic variation in the presence of environmentally induced maternal and paternal effects, should be carefully read by field ecologists interested in adaptation. The problems and issues discussed here are familiar enough to evolutionary geneticists who have the luxury of working with laboratory systems and elegant experimental designs, but have been, in my opinion, regarded with an almost cavalier disregard by people whose primary orientation is ecological rather than genetical, and who work exclusively with natural populations.

In the second haif of the book, McCauley and Goff (Vanderbilt University, Nashville, USA) provide a readable, though somewhat brief, overview of intrademic selection, thus highlighting the importance of geographical structuring not only among but within demes. There are several chapters dealing with dispersal and its evolution and quanti- fication in various insect systems ranging from galling insects to lepidopterans. Most of these chapters are some- what cursory, but nevertheless provide the reader with a good introduction to the relevant literature. (Chapters not individually mentioned here are from Costa [Western Carolina University, Cullowhee, USA], Hanks and Denno [Denno: University of Maryland, College Park, USA], Peterson and Denno, Gandon et al. [Olivieri: Universit6 Montpellier II, Montpellier, France], and Itami et al. [Itami: Arizona State University, Phoenix, USA].) Thomas and Singer (Singer: University of Texas, Austin, USA) summar- ize the extensive body of work on the ecology and evolution of diet preferences in the checkerspot butterfly (Euphydryas editha), one of the two lepidopteran genera--the other is PapiIio--for which we have a reasonably good under- standing of how the evolution of diet breadth is likely to have occurred. The book is brought to a close, very appro- priately, by Feder, Berlocher and Opp's (Feder: University of Notre Dame, Notre Dame, USA) review of over three decades of work on Rhagoletis flies, which have given evolutionary ecologists what is to date the clearest empirical evidence for host-plant-mediated race formation and possible sympatric speciation.

Overall, this is an excellent compilation of reviews and some original work that I would heartily recommend to anyone interested in host specialization in phytophagous insects. At s 65, it is certainly expensive, but the get-up of the book and its content do tend to justify the price, at least to a degree, especially for institutional purchases. The book does have, of course, a few shortcomings, which are inevitable in any compilation of this kind. Probably the most important of these is the absence of any overall introduction and conclusion, as well as lack of section introductions that could serve to string the chapters in each section together and place them in a broader context. Such introductions, I think, would have been especially helpful to readers not already very familiar with the development of

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this field, such as graduate students and beginning researchers. Also somewhat puzzling to me is the lack of any chapter devoted to studies on Papilio species, which, along with E. editha and Rhagoletis spp. (both represented very well in the book), are one of the best studied examples of the evolution of host preferences in phytophagous insects. More importantly, there are interesting differences in the way in which host preferences seem to evolve in these three systems, and although this issue is briefly dealt with

by Thomas and Singer, it would have been really nice to have a thorough comparative review of what is known about the ecology and evolution of specialization in all three systems. Nevertheless, the book has more than enough information on diverse insect-plant systems to make it very worthwhile reading, and, more importantly, also has a lot of discussion on methodological issues, such as the basis for drawing inferences about adaptation, that, unlike empirical data, cannot easily be found in the primary literature.

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