| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Book Review |
Department of Ecology and Evolutionary Biology, Natural History Museum and Biodiversity Research Center, University of Kansas, Lawrence, Kansas 66045 USA
Increasingly in biology, the words "model organisms" or "model systems" are bandied about, with Arabidopsis the poster child in plant molecular biology. A strong argument could be made that the tarweeds and silverswords (subtribe Madiinae, Asteraceae) are one of the best model systems for the study of plant evolution and diversification. This volume strengthens the case for "model organism" status for them. The tarweeds and silverswords have attracted the attention of some of the most prominent plant evolutionists and systematists for the past 75 yr, and this collection of papers demonstrates that this continues to be the case. Given its relatively small size (120 species) and restricted distribution in the California Floristic Province and the Hawaiian archipelago, one might wonder why this group has received so much attention. After all, there are much larger groups of flowering plants (including many genera) distributed over several continents that would seem to pose more interesting questions. The answer, provided by Sherwin Carlquist in the preface and Bruce Baldwin in the first chapter, is the remarkable diversity of the group in anatomy, morphology, cytology, ecology, etc., in both California and Hawaii. In both geographical areas, the diversity is telescoped in space, and thus it is even more apparent and spectacular. While the morphological diversity of the Madiinae would quite naturally attract the attention of plant systematists, it is the diversity of processes involved in the diversification of the group that has drawn anatomists, cytologists, ecologists, secondary chemists, and molecular and developmental biologists to it. As Carlquist states in the preface, "Each group (within the Madiinae) has its own story."
Hopefully, you are now convinced that the plants and the research done on them warrant book-length treatment. So, in this review I evaluate whether the book has done justice to the plants and those who have studied them. As will be evident by the end of the review, I came to a firm conclusion on this question after reading the book.
The book consists of 11 chapters written by a total of 10 contributors; the three editors wrote two or more chapters. The first two chapters were contributed by Bruce Baldwin; the first is on the natural history of the group, and the second is a lengthy discussion of "characteristics and diversity" of the Madiinae. The first chapter commences with a brief taxonomic history of the group, and then provides an overview of its distribution, ecology, and rarity. The second chapter, which is one of the two longest in the book, is basically a description of morphological variation in the Madiinae. Such subject matter, especially if one is not familiar with or particularly interested in the plants, can be tedious reading. Happily, that is not the case here, and I found the chapter an easy and enjoyable read. Why? First, it is not a comprehensive listing of characters and character states; instead, Baldwin emphasizes diagnostic characters for the Madiinae and for different lineages within the subtribe. Second, the drawings illustrate the diagnostic features so the reader can easily visualize them rather than becoming bogged down in long descriptions of taxa. In some instances, Baldwin skillfully blends the results of phylogenetic analyses (mostly from molecular data) into the discussion and the impact of the analyses on taxonomy. A wealth of data on distribution, breeding system, ecology, chromosome number, and even the etymology of the generic names, is woven into the discussions of taxa and lineages in such a way that a clear concept of the groups is provided, and the reader is not buried under a deluge of facts.
The third chapter, by Gerald Carr, is a comprehensive review of chromosome numbers and chromosome morphology in the Madiinae. An overview of chromosome evolution (structural and number changes) is presented, and the changes are viewed within an evolutionary-phylogenetic framework. I was a bit surprised to learn that over 97% of the species have been counted, clearly a testament to the efforts of Carr, Donald Kyhos, and earlier plant biosystematists (Clausen et al., 1934
; Kyhos et al., 1990
). An initial concern about this chapter was that, with the demise of courses in cytogenetics in institutions of higher learning, it might be rather tough going for many potential readers. However, Carr provides basic descriptions of chromosomal mutations and photographs of meiotic chromosomes illustrating the manifestations of the mutations in the heterozygous condition; these should be sufficient for understanding the mutations. A section describing chromosome differentiation within the Madiinae provides a perspective on differences in chromosome evolution in different groups. I particularly enjoyed Carr's discussion on determining the original base chromosome number in the Madiinae and inferring the evolution of chromosome numbers within a phylogenetic perspective. It reminded me that when chromosome counting was a popular activity among plant systematists and cytogeneticists, an evolutionary "spin" was often put on the results with scenarios on the evolution of numbers in a particular group, including hypothesizing the original base chromosome number. After viewing chromosome numbers within the framework of a phylogeny generated from molecular data, Carr (p. 70) concludes that "the impact of the molecular data on the interpretation of chromosome evolution and the original chromosome base number in the Madiinae is mind-boggling and serves to emphasize the importance of extreme caution in the evaluation and patterns of chromosome numbers, even in groups that are thought to be well understood."
One of the side effects of the popularity of phylogenetic studies has been, particularly in plants, to minimize the significance of breeding relationships because the ability to cross is a plesiomorphic character. In a strict cladistic sense, this is true, but it is unfortunate that this strict dictum has resulted in a generation of plant systematists who lack a full appreciation of the value of data from breeding relationships in plant systematics and evolution. Chapter 4, by Carr, provides a broad and balanced discussion of the value of breeding relationships in studying plant evolution that has relevance well beyond the Madiinae and should be read by anyone who regards the ability to cross as nothing more than a plesiomorphic character. Among the topics Carr treats in this chapter, I found his discussion of the potential evolutionary importance of the ability to hybridize as compared to the occurrence of hybrids in nature particularly interesting. He argues that the low frequency of naturally occurring hybrids now seen in any group may not provide an accurate assessment of the past role of hybridization in the evolution of the group and it may underestimate its potential role in the future. That is, the ability to cross may be of greater evolutionary significance (either in the past or future) than would be suggested by the present occurrence of hybrids. For example, recent evidence shows that the Hawaiian silverswords are allotetraploids involving California tarweeds (Barrier et al., 1999
), yet the few naturally occurring tarweed hybrids in California offer no clues to the ancestry of the insular endemics. Another example cited by Carr of the potential importance of the ability to hybridize comes from the spontaneous hybrids among tarweeds in the transplant gardens of Clausen, Keck, and Hiesey. I think that Carr has a valid point in that the importance of hybridization in a group may be underestimated by the frequency of hybrids now seen. However, how important retention of the ability to hybridize will be among taxa now isolated ecologically or geographically is difficult or impossible to access, and may vary among groups of plants. For example, some widely disjunct species have retained the ability to cross through millions of years, yet it is doubtful that hybridization will ever be a factor under natural conditions.
Chapters 5, 6, and 7, consisting of some 40 pages, were contributed by Sherwin Carlquist and cover trichomes and glandular structures, leaf anatomy and adaptive radiation, and wood anatomy. Carlquist has carried out extensive investigations on the Madiinae for more than four decades, and much of the material presented in these chapters is a result of his efforts. Regrettably, comparative anatomy is not as fashionable as it once was; the three Carlquist chapters provide an elegant and compelling demonstration of just how regrettable it is that anatomical studies are rarely a part of systematic investigations. The emphasis in these chapters, rather than illustrating the systematic utility of anatomical data in the Madiinae (it is largely forgotten, but we are reminded in chapter eleven, that it was Carlquist's classic anatomical studies that established the close relationship between the tarweeds and silverswords), stresses the ecological correlates and possible adaptive significance of diversity in leaf and wood anatomy. By framing the description of diversity within the context of ecological-biological questions, Carlquist demonstrates the relevance of anatomical information in understanding evolution and diversification within lineages. His contrasts of the anatomy within and between the continental and Hawaiian lineages are especially interesting because they consider a fundamental question in evolutionary biology, the relative roles of the current ecology and ancestry in shaping the characters now present in the organisms. Carlquist's observations and data also call out for ecophysiological studies beyond the interesting results that have been obtained to date (Robichaux et al., 1986
). These three chapters show the value of anatomical data in understanding evolution and diversification within and among lineages and indicate where future research should be directed in the Madiinae.
In chapter 8, Bruce Bohm and Ji Yong Yang provide a broad overview of the secondary metabolites sequestered by the Madiinae. The authors present a comprehensive and scholarly discussion, which is not surprising given the senior author's knowledge of secondary compounds and their distribution in the Asteraceae (Bohm and Stuessy, 2001
). In addition to enumerating the compounds that have been isolated from the Madiinae, the authors discuss the systematic utility of flavonoids at various levels in the subtribe. They conclude that their value for assessing relationships is rather limited for a variety of reasons, including the probable parallel losses (and perhaps gains) of compounds in different lineages. An interesting situation in which the array of flavonoids found in silversword hybrids is contrasted to compounds present in the parents is presented, with a possible biosynthetic explanation given. The topic of this chapter was perhaps the most challenging to discuss of any in the book because the systematic value of secondary metabolites within the Madiinae is limited, and there does not appear to be any information on the possible ecological significance of the compounds. Because flavonoids occur both as exudates and in vacuoles, it could be of interest to compare the array of compounds found in each. The authors rose to the challenge and did a nice job of bringing together a great deal of information.
Chapter 9, by Michael Purugganan, David Remington, and Robert Robichaux, discusses the evolution of floral regulatory genes (ASAP3/TM6 and ASAP1) in the Madiinae. A number of studies have demonstrated low diversity within and divergence among species in selectively neutral or near-neutral makers such as allozymes in insular groups that have undergone explosive morphological and ecological radiations. Clearly, these markers are not the "stuff" of diversification. The authors suggest that evolution in regulatory and not structural genes facilitates the type of rapid diversification seen in the silverswords. Purugganan and co-workers have studied floral regulatory gene evolution in the silverswords and tarweeds, the former group being of particular interest because of the mind-boggling variation in the capitulescences and capitula in the island lineage. Relating changes at the sequence level to different morphologies is an important goal in studies of regulatory genes. While this goal of elucidating the phenotypic effects of changes at the sequence level has not been achieved for the silverswords, phylogenetic analyses of the sequences have provided two important insights into the origin and radiation of the lineage. First, the silverswords are of allotetraploid origin, with two species of tarweeds as their ancestors. Second, sequence evolution in the floral regulatory genes is accelerated in the silverswords compared to the California tarweeds. The elevated rates at nonsynonymous sites are not a manifestation of overall increase at synonymous sites. The accelerated rates at nonsynonymous sites could result either from relaxed selection or selection for different phenotypes resulting from the sequence divergence. While studies elucidating the basis of developmental modifications resulting in phenotypic change are time-consuming and difficult, the results presented in this chapter give a small peek at the potential for bridging the enormous gap between reconstructing the pattern of evolutionary diversification, which has become commonplace using molecular data, and understanding the processes that generated the diversification.
Chapter 10, by Elizabeth Friar and Robert Robichaux, discusses the conservation genetics of the silverswords. The authors begin my emphasizing that, as with most island endemics, alien species and habitat destruction are the greatest immediate threat to loss of rare species. Nearly 75% of the silversword species are of conservation concern; preservation of the spectacular diversity within the lineage will require concerted and continuing efforts. The importance of knowing how alien species have affected the historical distribution of silversword species for shaping conservation plans is discussed; that is, determining whether species have always been rare or their numbers are declining is critical in formulating conservation strategies. The emphasis is on using single-locus molecular markers such as RAPDs and microsatellites to assess genetic variation within and between populations and to relate the data to biological-conservation issues. The chapter contains a very good discussion of general issues in conservation genetics using examples from the silversword alliance.
Chapter 11, by Bruce Baldwin, is a marvelous overview of the phylogenetic history of the Madiinae and the processes involved in its evolution and diversification. Information from earlier chapters is placed within a phylogenetic context, and the result is fascinating reading. As with other chapters, but especially with this one because it is an overall synthesis, the general significance of the chapter transcends the taxonomic group. This chapter would indeed gratify Lincoln Constance because, for my money, it is the best evidence I have seen that the synthesis has not ended (Constance, 1964
). Baldwin shows how a treasure trove of information (generated largely by authors of earlier chapters) can be mapped onto phylogenies to formulate refined hypotheses on the pattern and process of evolution, whether it is the origin of the entire Madiinae, the origin of the silverswords, habitat shifts in the silverswords, or chromosomal evolution in groups of tarweeds. While much more could be said about this significant contribution to plant systematics and evolution, I suggest that you purchase the book and enjoy it firsthand. Baldwin, like his co-editors Carlquist and Carr, has a firm grasp on research in the Madiinae for the past 70–80 yr, and he weaves much of this earlier work into this chapter; his appreciation of earlier contributors comes through clearly. For example, he gives Carlquist (1959)
due credit for documenting from anatomical data that the silverswords and tarweeds are closely related long before Baldwin's own molecular studies provided elegant confirmation of Carlquist's conclusion.
This volume avoids the common pitfall of many edited works on a taxonomic group—namely, the varying quality of the chapters. The chapters in this book are relatively uniform because the Madiinae is a small taxonomic group, the contributors have done extensive research on the topics covered in their chapters, and there obviously had been considerable thought and discussion before settling on topics and organization. Naturally, certain chapters will be of greater interest than others, depending on the individual reader.
It is obvious from my remarks that I like this volume. What will be (or should be) the audience for it? Anyone interested in plant systematics and evolution, from advanced undergraduate to seasoned faculty, should have a copy on his or her bookshelf. The book would be ideal for a one-semester seminar at the upper undergraduate and graduate levels. The papers in this volume portray very effectively the diverse and dynamic nature of systematic botany. The contributors show that the discipline, in its best sense, involves more than generating a phylogeny and then enumerating groups that are or are not monophyletic according to various prior taxonomic treatments. This volume, particularly the last chapter, provides both elegant documentation of the value of a phylogeny for a group while at the same time showing that the value of the phylogeny increases in proportion to the other biological data available for the plants.
I will refrain from the almost obligatory comments on typos and other minutiae because I read the book for its content. As with any book, a few errors crept in here and there, but they are minimal. The Missouri Botanical Garden is to be commended for publishing this scholarly volume. Hopefully, the book will enjoy the popularity it deserves in the systematic community so that publishing it will turn out to be a sound business decision as well as the "right" thing to do from a scientific perspective.
 |
FOOTNOTES |
|---|
|
LITERATURE CITED |
|---|
 TOP LITERATURE CITED |
|---|
Bohm B. A. T. F. Stuessy 2001 Flavonoids of the sunflower family (Asteraceae). Springer-Verlag, Vienna, Austria
Carlquist S. 1959 Studies on Madiinae: anatomy, cytology, and evolutionary relationships. Aliso 4: 171-236
Clausen J. D. D. Keck W. M. Heisey 1934 Experimental taxonomy. Carnegie Institution of Washington Yearbook 33: 173-177
Constance L. 1964 Systematic botany—an unending synthesis. Taxon 13: 257-273[CrossRef]
Kyhos D. W. G. D. Carr B. G. Baldwin 1990 Biodiversity and cytogenetics of the tarweeds (Asteraceae: Heliantheae–Madiinae). Annals of the Missouri Botanical Garden 77: 84-95[CrossRef][ISI]
Robichaux R. H. K. E. Holsinger S. R. Morse 1986 Turgor maintenance in Hawaiian Dubautia species: the role of variation in tissue osmotic and elastic properties. In T. J. Givnish [ed.], On the economy of plant form and function, 353–380. Cambridge University Press, Cambridge, UK
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
