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1 December 2008 The Birds And The Bees, From The Flower's Point Of View
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Understanding Flowers and Flowering has the ambitious goal of unifying molecular genetic and ecological viewpoints on floral production, development, and morphology—and it largely succeeds in that aim. Author Beverly Glover, of the University of Cambridge, is a specialist in floral color and epidermal cell morphology, and therefore has particular insight into aspects of floral biology that link ecology with genetics. The book covers topics ranging from angiosperm evolution to floral genetics to the ecological interactions of pollinators and floral form. As might be expected, some sections are stronger than others, but on the whole, this work fills a considerable gap among available texts by integrating evolutionary, ecological, and developmental genetic materials.

The 19 chapters are logically divided into three sections that can be described generally as evolution, developmental genetics, and ecology. The first section is perhaps the weakest (it is also the shortest), highlighting several questionable or out-of-date hypotheses and results. Among these, the most problematic is the assertion that the “mostly male” hypothesis is the “currently favored” model for the evolution of the hermaphroditic floral axis. This hypothesis holds that an important gene for promoting floral meristem identity, called LEAFY, was duplicated in the ancestor of all seed plants (angiosperms and gymosperms). The resulting two types of LEAFY evolved sex-specific functions—one primarily controlling male reproductive identity, and the other, female. These two flavors of LEAFY function are posited still to be present in extant gymnosperms, whereas in angiosperms, only the “male” LEAFY function persists. The explanation for this condition is that somehow the genes responsible for promoting ovule development came under the control of the male LEAFY in the ancestor of angiosperms, producing ectopic ovules on some male reproductive organs that ultimately evolved into carpels.

Although this hypothesis represented an important transition in models of angiosperm evolution by invoking simple changes in gene expression as potential mechanisms for evolutionary change, it cannot be considered viable at this point because substantial contradictory data also exist (i.e., the two gymnosperm LEAFY homologs are not, in fact, male and female specific). Of much greater importance are the homeotic models for flower evolution, which Glover references in subsequent chapters but incorrectly suggests are equivalent to “mostly male.” Aside from this point, I particularly missed the presence of any kind of angiosperm phylogeny figure, even a simplified one that glosses over the controversial points. This absence is also felt in later chapters that, to the author's credit, do an admirable job of including data from taxa other than the standard Arabidopsis. At least one angiosperm phylogeny would have been useful to help readers orient themselves in an evolutionary context when these different taxa are discussed.

The second section encompasses the developmental genetics of flowering time and floral development from a relatively broad perspective, with some consideration of the evolution of these genetic pathways. This section begins with a useful introduction to Arabidopsis biology and genetic manipulation as well as to the often confusing terminology used in flowering time control. Glover defines three major pathways—autonomous, photoperiodic, and vernalization—which feed into the floral pathway integrators that actually promote flowering. In this regard, Glover does not do herself any favors by using atypical terminology when discussing the flowering-repressor gene FLC (FLOWERING LOCUS C), included as part of the “autonomous inhibition” pathway. The odd point here is the use of the term “autonomous” in relation to FLC. There is also a well-characterized floral induction pathway that is more commonly referred to simply as the “autonomous” pathway. These autonomous loci act through several different genetic mechanisms to deactivate the floral repressor FLC and thereby allow flowering to occur. This is the first time I have seen FLC termed an “autonomous inhibitor,” and I can understand Glover's logic, but, as a rule, FLC is discussed in the literature as a component of the vernalization pathway.

One can always quibble about terminology, but I found the use of autonomous inhibition versus autonomous induction to be tricky in two regards. First, readers have to pay very close attention to catch which pathway is being discussed at any particular time (not necessarily a bad thing), and second—and more problematically—it may lead to confusion when readers go to the primary literature and do not see the same usage. This being said, the complexities of flowering time control in Arabidopsis make it one of the hardest subjects to explain clearly. Glover has achieved that quite well, and the data covered are very up to date, which will extend the shelf life of this text.

The latter part of the molecular genetic section focuses on the genetic control of floral meristem and floral organ identity. This subject is the one area that has been thoroughly covered in many other texts and literature reviews, so it is hard to add much that is new, but Glover does an excellent job of explaining the work from first principles. In particular, she gives full coverage to the fact that the A-class component of the ABC model of floral organ identity, which is often considered essential to the development of sepals, does not actually appear to be well conserved. This point often gets short shrift in other reviews, but it is very important for students to grasp as early as possible.

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The last section, covering variation in floral form, represents the most integrative material. What is the ecological value of zygomorphy (bilateral floral symmetry), and how is it genetically generated? What are the biochemical, genetic, and morphological bases for floral color, and how do the components interact with floral ecology? These questions are among the best-addressed points in this section and are very useful for students coming from both sides of the genetics/ecology divide. (The first half of the section could benefit from labeled illustrations of the genetic models, and the figures throughout the book are of variable quality.) The second half of the section focuses more on evolutionary- and ecological-minded questions concerning the role of pollination interactions in plant fitness. In particular, Glover takes on some of the controversy concerning “pollinator syndromes,” and she presents both sides evenhandedly.

Flowers and Flowering is a well-written text that would well serve undergraduates, early graduate students, or anyone with a solid biology background who is interested in floral biology. Although I found some points to criticize, I would still recommend the book, especially for those from an ecology perspective who want to learn more about floral genetics. I do think that further integration is needed among the disciplines of evolution, genetics, and ecology—both in this text and in others to follow—but this work will initiate that important process.

Elena M. Kramer "The Birds And The Bees, From The Flower's Point Of View," BioScience 58(11), 1089-1090, (1 December 2008). https://doi.org/10.1641/B581114
Published: 1 December 2008
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