The evolution of plant form

Abstract

‘The evolution of plant form’ is volume 45 of Wiley-Blackwell's ‘Annual Plant Reviews’, and it provides an interdisciplinary analysis of this field, ranging from paleobotany to genomics. The first half of the book addresses morphological innovations in plant evolution by clade in an up-to-date phylogenetic context. An introductory chapter presents an overview of the characteristics of each major land plant group mapped onto a recent phylogenetic framework. It includes a detailed discussion of a selection of fossils in relation to characteristics not shown by any living groups, although the affinities allocated differ from treatments of other authors (e.g. Boyce, 2008; Harrison and Langdale 2008). The following five chapters present accounts of the morphology of each major land plant group, including bryophytes, lycophytes, ferns, gymnosperms and angiosperms. The chapter on bryophytes provides a good level of detail about liverworts, mosses and hornworts in the context of recent group-specific phylogenies. This is accompanied by some interesting speculation about character transitions within the bryophytes, and between the bryophytes and the vascular plants, with hypotheses about how they could be tested. The chapter on lycophytes presents an informative overview of morphological distinctions between the three major extant lycophyte lineages, whilst the one on ferns discusses the labile modularity of fern architecture in an explicit homology framework. Particular attention is paid to the evolution of embryos, shoots, leaves, and sporangium bearing structures. The chapter on gymnosperms provides an overview of the main organ systems in gymnosperms and discusses divergences in leaf and seed form between groups. The clonal analyses of Korn (2001, 2002) would have added to the discussion of shoot and leaf development here. The chapter on angiosperms addresses hypotheses relating to the origin of flowers from reproductive shoot systems, the innovation of the carpel, and the elaboration of floral symmetry in an integrated morphological and molecular context. The second half of the book moves into a consideration of genetic mechanisms which could have underpinned the radiation of land plant phyla. Chapter 7 presents an accessible and up-to-date explanation of QTL approaches which assesses the genetic basis of morphological diversity. It goes on to discuss genetic signatures of selection and how they can be statistically evaluated. A set of minimum requirements for determining a causal link between genes and character evolution is clearly identified, and some case studies from flowering plants are discussed in more detail. Chapter 8 gives an overview of plant genome evolution in relation to the generation of genetic diversity. It considers genome size within the land plants in relation to gene density and factors that influence genome evolution. The attributes of genomes sampled across the plant tree of life are discussed in relation to the amplification of gene families that may have been important in plant evolution. Chapter 9 presents three specific areas where evo-devo studies using reverse genetics have recently made an impact, namely the evolution of reproductive shoots, floral whorls and leaves. The discussion is informative and accessible. Chapter 10 considers the interplay between environment and development and its significance in plant evolution, and a final chapter pulls together the main themes addressed in the book, pointing out some areas for further future research. Strengths of this book are that the progression of topics covered by chapter is clear and logical, and the information covered is up-to-date and well integrated. I particularly liked the way that a discussion of fossil taxa is intercalated with considerations of living taxa throughout the book. I found some of the hypotheses of evolution interesting and innovative. In my view the book is more firmly rooted in a morphological and phylogenetic context than a developmental and genetic context, although the latter contributions were also clear and accessible. I thought that recent advances in developmental genetics could have allowed the authors to postulate specific testable hypotheses about some of the particular character changes distinguishing groups. In particular I felt that a consideration of mechanisms underpinning the innovation of the embryo, a vegetative sporophyte shoot, and branching would have added something, and these areas have been covered by recent reviews in the developmental literature (e.g. Harrison et al., 2010; Bosca et al., 2011; Domagalska and Leyser, 2011). Overall I felt that this book will be a great first port of call for those interested in morphological evolution and it will be useful at all levels from undergraduate onwards. It will be particularly useful for molecular and developmental biologists wishing to move into a comparative approach.