modularity, comparative embryology and evo-devo...

Review Modularity, comparative embryology and evo-devo: Developmental dissection of evolving body plans Shigeru Kuratani ⁎ Evolutionary Morphology Research Group, RIKEN Center for Developmental Biology, 2-2-3 Minatojima Minamimachi, Chuo-ku, Kobe 650-0047, Japan abstract article info Article history: Received for publication 18 March 2009 Revised 18 May 2009 Accepted 19 May 2009 Available online 23 May 2009 Keywords: Evolution Modularity Evolutionary novelty Morphological homology Body plan Developmental constraints Modules can be defined as quasi-autonomous units that are connected loosely with each other within a system. A need for the concept of modularity has emerged as we deal with evolving organisms in evolutionary developmental research, especially because it is unknown how genes are associated with anatomical patterns. One of the strategies to link genotypes with phenotypes could be to relate developmental modules with morphological ones. To do this, it is fundamental to grasp the context in which certain anatomical units and developmental processes are associated with each other specifically. By identifying morphological modularities as units recognized by some categories of general homology as established by comparative anatomy, it becomes possible to identify developmental modules whose genetic components exhibit coextensive expressions. This permits us to distinguish the evolutionary modification in which the identical morphological module simply alters its shape for adaptation, without being decoupled from the functioning gene network (‘coupled modularities’), from the evolution of novelty that involves a heterotopic shift between the anatomical and developmental modules. Using this formulation, it becomes possible, within the realm of Geoffroy's homologous networks, to reduce morphological homologies to developmental mechanistic terms by dissociating certain classes of modules that are often associated with actual shapes and functions. © 2009 Elsevier Inc. All rights reserved. The framework of bones being the same in the hand of a man, wing of a bat, fin of the porpoise and leg of the horse, - the same number of vertebrae forming the neck of the giraffe and of the elephant, - and innumerable other such facts, at once explain themselves on the theory of descent with slow and slight successive modifications. The Origin of Species by Means of Natural Selection (Darwin, 1859) …characters controlled by identical genes are not necessarily homologous… The converse is no less instructive… homologous structures need to be controlled by identical genese and homology of phenotypes does not imply similarity of genotypes. de Beer (1971) Introduction Understanding the evolutionary changes involved in development is not easy or simple. Changes can be seen at every hierarchical level of the developmental programs, anatomical patterns, or in the genes that function in morphogenetic development. To fill the gap between phenotypes and genes, the concept of modules has recently attracted the attention of researchers in evolutionary development (‘evo-devo’) as a possible link between the two. Modules are defined to represent semiautonomous units or elements that are connected loosely with others in a system (Raff, 1996; Carroll et al., 2001; Schlosser and Wagner, 2004; Klingenberg, 2008). There can be several different modules defined in different contexts, such as those defined as genetic, morphological, developmental and functional modules (for examples see Wagner et al. (2007), Klingenberg (2008)). Typically, the animal body is made up of anatomical units that can often evolve independently. For example, bats have obtained a wing based on the forelimb module and gnathostomes have acquired jaws differentiated from the mandibular arch. Embryonic units such as these are also the sites at which regulatory genes are specifically expressed. Importantly, a discrete and specific regulation of genes is often associated with an anatomical unit. In the present review, the concept of modularities is considered in the context of development and evolution, as a possible conceptual tool to connect morphological and developmental units. The central issue is: how can we grasp the interrelationships between different types of modules and how they have changed through evolutionary processes? Because the hierarchical organization of genes and any given anatomical structure do not correlate with each other, a conceptual framework has to be established to deal with the two variables simultaneously. To grasp the modular structures in evo-devo biology from a top-down perspective, the discussion below focuses mainly on the relevance of Developmental Biology 332 (2009) 61–69 ⁎ Fax: +81 78 306 3370. E-mail address: [email protected]. 0012-1606/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.ydbio.2009.05.564 Contents lists available at ScienceDirect Developmental Biology journal homepage: www.elsevier.com/developmentalbiology

modularity, comparative embryology and evo-devo...

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