Evolving pathways – Key Themes in Evolutionary Developmental Biology
A. Minelli, G. Fusco ( eds ) 2008 . Evolving pathways – Key Themes in Evolutionary Developmental Biology . Cambridge University Press , 426 pp., £ 60.00 . ISBN: 978-0-521-87500-4 hardback .
This is an informative and thought-provoking book for anyone interested in progress in evo-devo research but who cannot study daily the papers published in journals such as Development, Developmental Biology, Development, Genes and Evolution etc.
The book is based on a workshop which was held in Venice in May 2006, supplemented by some invited papers. It is subdivided into four parts and contains 21 contributions by 34 authors, mostly from European countries. While many of the authors come as no surprise, some names are unexpectedly absent.
It would be impossible to summarize all the papers here, so instead, I will mention a small selection which illustrates the wide scope of topics dealt with in the book.
G. B. Müller (Evo-devo as a discipline) provides an overview of the current status of evo-devo. He asks the question: Does evo-devo ‘solve biological problems that cannot be solved by other approaches?’. The author provides a short history of evolutionary and developmental biology, two separate endeavours which were for a long time disciplines in their own right and which had only brief periods of interfaces before they merged into one comprehensive discipline in the early 1980s.
Nevertheless, the distinct facets of the two approaches remain and only a few scientists and working groups are thinking hard about how to combine the different aims and methods.
The author recognizes four major ‘programs’: the comparative embryology program, the epigenetic and experimental program, the evolutionary genetics program and the theoretical biology program. Though the distinctness of these programs is slightly over-emphasized in the paper one can agree with the notion that they have developed their own scientific terms and languages and that the great challenge is to bring and to keep them together. Many meetings and congresses, including the one in Venice, have tried to fulfil this task.
C. R. Alonso expands on ‘The molecular biology underlying developmental evolution’. The author recurs to the model of transcriptional regulation proposed by Britten and Davidson which can be summarized as a ‘theory of how the eukaryotic cells may achieve multiple changes in gene activity from relatively simple signals’. Alonso argues that these ideas still have an enormous impact on our modern views in the evo-devo field. But whereas Britten and Davidson restricted their theory to ‘processes of cell regulation at the level of genomic transcription’ the present author tries to extend it to post-transcriptional regulation.
The author focuses on alternative regulatory levels to the mostly accepted enhancer regulation. He discusses the multiple regulatory mechanisms which determine the final output of a given gene, e.g. splicing, capping, polyadenylation and so on. The many biochemical steps involved in the information transfer from DNA to function puts enhancer sequences into a true minority. Some examples from Drosophila genes are presented (Dscam, Sxl and Ubx).
The author is convinced that, ‘as important as transcriptional networks might be, they represent one level and one level only within the multilayered architecture of gene regulation’. Alonso takes Francois Jacob’s view that evolution and natural selection cannot be compared to engineering but rather to ‘tinkering’. He calls for variation to be studied at all levels of gene regulation.
G. Scholtz (On comparisons and causes in evolutionary developmental biology) poses the question of whether causes can be narrowed down by physiological approaches only. It is generally held that this is only possible through experimental work. The author shows convincingly that variations in the developmental pathway in different species can be taken as a tool to distinguish between causal and non-causal influences. Non-causality can be demonstrated in several cases where causality seems to prevail. One can sort out modules which can be altered in development without affecting preceding or succeeding character states. Such modules are thus highly relevant in the distinction of homologous sub-units which are independent of one another and not causally related and which must be demonstrated to meet the requirement of complexity.
The author maintains that characters are four-dimensional, in space and time. Notions of development are therefore an integral part of a full ‘description’ of a given and compared character. This aspect has often been neglected.
The paper by R. Rutishauser, V. Grob and E. Pfeifer (Plants are used to having identity crises) serves as an example that research on plants is completely integrated in the promotion of ideas in evo-devo and provides new theoretical and practical impulses into the field. The authors take the vegetative (leaves, stems and roots) and reproductive organs (sepals, petals, stamens and carpels) of vascular plants which in many text-books are treated as clear-cut units, even categories or ‘crisp sets, perfectly excluding each other’. This contradicts the continuum model which demands fuzzy borderlines and intermediates. The different organs are then discussed and it is shown that in many cases one organ can be transformed into another and that many organs have a composite structure or, as the authors call it, a ‘compound identity’. The many examples of organ indistinction provided in the paper show that numerous genes which in principle act in one categorial structure can also be expressed in another one so that the phenotypic boundaries between the structures become blurred.
E. Schierenberg and J. Schulze (Many roads lead to Rome: different pathways to construct a nematode) take the classical exemplars of evo-devo research, the nematodes, and show that all the iron statements about nematode development can be subject to profound change and nevertheless produce a neat nematode. The model species Caenorhabditis elegans has a ‘determinative’ cleavage, the number of cells in every stage is invariant, the fate of the cells and their progeny can be defined from the first cleavages onwards. But C. elegans is not the only nematode on earth and it is by no means a primitive one. The early cleavages and cell fates of five other nematodes are described in brief. The most intriguing, but probably the most original (plesiomorphic) mode is found in the genus Tobrilus. It has a random cleavage, shows formation of a blastocoel and a gastrulation which confusingly resembles that in sea urchins. However, a perfect nematode hatches. ‘Why then are there different ways to reach essentially the same goal?’ Is it neutral evolution, or is it a hidden strategy to increase fitness? All this shows how much we are still groping in the dark, even in one of the best-studied groups.
The paper of N. C. Hughes, J. T. Haug and D. Waloszek (Basal euarthropodan development: a fossil-based perspective) demonstrates that even paleontologists are involved in evo-devo research and discussions. Some of the beautifully preserved ‘Orsten’-type specimens are clearly post-embryonic stages of various species of arthropods, mainly representatives of the crustacean lineage. Number of segments, articulation and morphology can be taken to reconstruct the course of development. Explanations for some forms of gradual anamorphic development are sought.
A. Stollewerk (Evolution of neurogenesis in arthropods) reports on recent advances in research into the development and genetics of ganglia, neuroblasts and specified neurons in various arthropods. The progress made in recent years in this field has been breath-taking. Apart from different insects, spiders, myriapods and crustaceans have been investigated. As in the nematodes, it has once more been shown that the model system of an arthropod, Drosophila, does not represent the original mode of development of the whole phylum. It rather appears that originally groups of neural precursors which form a distinct pattern in the ectoderm are specified by a conserved genetic network. A second step is the concentration and generation of individualised neuroblasts, as found in insects and several crustaceans. This may even have evolved twice independently. This assumption still has to be confirmed by the investigation of outgroup representatives such as the onychophorans.
As has been noted, many other contributions of equal weight and importance are assembled in this book. The book presents a well-balanced perspective of the different views, methods and results which make evo-devo such a thrilling experience. After a long period of separation the best methods and ideas from evolutionary and developmental biology have been combined. Scarcely unified, the different facets are once more in danger of drifting in different directions. It is not possible for the interested reader to follow all the aspects at the same time. Books like this are extremely helpful to keep up with the pace of progress. It presents the results of different authors and working groups as well as the divergent thinking and philosophies. The book can be warmly recommended, for discussions, for seminars, for thoughtful evenings.
