Evolving diversity

Are there clades whose particular origin or evolutionary history are more adequately explained when considering the possibilities offered by changes at the level of developmental processes, instead of thinking in terms of the unceasing interplay between gene mutation and natural selection? Yes, this is exactly the field where evo-devo offers its best performances. These are stories rooted deep in time, where one must also consider the possibility that in due course even the 'rules of the game', such as the role of Hox genes, or more generally the genotype-phenotype relationships, have evolved along with their products. Or, they may be stories where adaptive explanations are unsatisfactory, and the course of evolution can appear to be driven more by the nature of variation that is produced at each generation than by adaptive necessities.

Jaume Baguna, Pere Martinez, Jordi Paps and Marta Riutort (Chapter 12) address the problem of early bilaterian evolution. Their work is based on the most recent molecular phylogenies and on new data on Hox/ParaHox and microRNA sets that identify acoelomorphs as the earliest branching extant bilaterians. Evidence for axial hom-ologies in gene expression between cnidarians and bilaterians and the evidence that cnidarians were at their origin bilaterally symmetric all point to an older last common ancestor for bilaterians. Thus, what under different phylogenetic hypotheses appear to be a number of phy-logenetically coincident character changes (the complex Urbilateria hypothesis) turn into a series of nodes connected by stem ancestors along which new characters were progressively acquired.

Staying with the problem of bilaterian origins and early evolution, but at the level of molecular genetics, Jean Deutsch and Philippe Lopez (Chapter 13) present a novel hypothesis that argues that the expansion of the Hox complex at the base of the bilaterian clade was produced by a series of transposition events, and that the Hox genes themselves originated from transposons. The authors support their hypothesis by a discussion of the similarity between the homeodomain and the DNA-binding domain of bacterial integrases and eukaryotic transposases, and through the investigation of some rearrangements of the Hox complex in the drosophilid lineage. This results in a scenario for the evolution of the Hox complex from the basic complement of Hox genes in the common ancestor of cnidarians and bilaterians that accounts for several properties of the extant Hox genes.

Diversity in embryogenesis of nematode worms is much higher than what would be expected on the basis of the degree of variation among the juvenile phenotypes. The abundance of early developmental variations appears somehow paradoxical, as these do not have any obvious impact on the structure or performance of the resulting worms. Einhard Schierenberg and Jens Schulze (Chapter 14) ask why there are so many different developmental pathways to reach essentially the same goal. Did the special body plan of nematodes prevent a degree of morphological diversification like in arthropods or vertebrates? The authors explore possible explanations, among which is the interplay between the genetic program and external conditions (inside or outside the organism) that determines the chance for deviations from an original developmental pattern to arise and to succeed.

A demonstration that evo-devo does not reduce to comparative developmental genetics is provided by Nigel Hughes, Joachim Haug and Dieter Waloszek (Chapter 15) who offer a palaeo-evo-devo perspective on the evolution of basal euarthropod development based on the fossil record. The chapter reviews the morphological development of early arthropods as reflected by the ontogenetic series of several species of trilobites and a basal crustacean lineage known as the 'Orsten'-type fauna. Particular attention is paid to the segmentation schedule. Patterns of segment generation shared by these primitive groups may provide insights into the developmental mode of basal Euar-thropoda, and thus into the evolution of arthropod ontogeny.

The origin and early evolution of land plants from aquatic ' algae' is discussed by Jane Langdale and Jill Harrison (Chapter 16). They review a selection of major steps in land plant evolution from an evo-devo perspective. These steps include the passage from haplontic to haplodi-plontic life cycle, the emergence of apical growth and branching development, the evolution of vascular and root systems, the advances in energy storage and reproduction strategies. Developmental data, both at molecular-genetic and morphological level, concur to reconstruct a scenario for these key evolutionary transitions.

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