Limb, tooth, beak: Three modes of development and evolutionary innovation of form.

The standard model of evolutionary change of form, deriving from Darwin’s theory via the Modern Synthesis, assumes a gradualistic reshaping of anatomical structures, with major changes only occurring by many cycles of natural selection for marginal adaptive advantage. This model, with its assertion that a single mechanism underlies both micro- and macroevolutionary change, contains an implicit notion of development which is only applicable in some cases. Here we compare the embryological processes that shape the vertebrate limb bud, the mammalian tooth and the avian beak. The implied notion of development in the standard evolutionary picture is met only in the case of the vertebrate limb, a single-primordium organ with morphostatic shaping, in which cells rearrange in response to signalling centres which are essentially unchanged by cell movement. In the case of the tooth, a single-primordium organ with morphodynamic shaping in which the strengths and relationships between signalling centres is influenced by the cell and tissue movements they induce, and the beak, in which the final form is influenced by the collision and rearrangement of multiple tissue primordia, abrupt appearance of qualitatively different forms (i.e. morphological novelties) can occur with small changes in system parameters induced by a genetic change, or by an environmental factor whose effects can be subsequently canalized genetically. Bringing developmental mechanisms and, specifically, the material properties of tissues as excitable media into the evolutionary picture, demonstrates that gradualistic change for incremental adaptive advantage is only one of the possible modes of morphological evolution.

Cell state switching factors and dynamical patterning modules: complementary mediators of plasticity in development and evolution.

Ancient metazoan organisms arose from unicellular eukaryotes that had billions of years of genetic evolution behind them. The transcription factor networks present in single-celled ancestors at the origin of the Metazoa (multicellular animals) were already capable of mediating the switching of the unicellular phenotype among alternative states of gene activity in response to environmental conditions. Cell differentiation, therefore, had its roots in phenotypic plasticity, with the ancient regulatory proteins acquiring new targets over time and evolving into the “developmental transcription factors” (DTFs) of the “developmental-genetic toolkit.” In contrast, the emergence of pattern formation and morphogenesis in the Metazoa had a different trajectory. Aggregation of unicellular metazoan ancestors changed the organisms’ spatial scale, leading to the fi rst “dynamical patterning module” (DPM): cell-cell adhesion. Following this, other DPMs (defi ned as physical forces and processes pertinent to the scale of the aggregates mobilized by a set of toolkit gene products distinct from the DTFs), transformed simple cell aggregates into hollow, multilayered, segmented, differentiated and additional complex structures, with minimal evolution of constituent genes. Like cell differentiation, therefore, metazoan morphologies also originated from plastic responses of cells and tissues. Here we describe examples of DTFs and most of the important DPMs, discussing their complementary roles in the evolution of developmental mechanisms. We also provide recently characterized examples of DTFs in cell type switching and DPMs in morphogenesis of avian limb bud mesenchyme, an embryo-derived tissue that retains a high degree of developmental plasticity.

Snakes and ladders: the ups and downs of animal segmentation.

Segmentation, the division of the body into repetitive modular subunits or metameres, is ubiquitous throughout the animal kingdom. This morphological motif appeared several times in widely divergent phyla, some without common segmented ancestors (Bateson 1894; Willmer 1990). Segmentation presents challenges to standard evolutionary narratives (Minelli and Fusco 2004), in part because segments are discrete structures, like rungs in a ladder, that are added or subtracted in an all-or-none fashion, and also because large changes in segment number can occur in evolutionary lineages with little sign of intermediate forms. Two recent papers, one on snakes (Gomez et al. 2008) and one on centipedes (Vedel et al. 2008), shed some light on these important questions. In vertebrates, segmentation takes the form of somitogenesis, in which paired blocks of tissue known as somites bud off at regular time-intervals from the presomitic mesoderm (PSM) that fl anks the notochord and proceed to give rise to vertebrae, ribs, muscle and dorsal dermis (Dequéant and Pourquié 2008). The numbers of segments in mammals, birds and fi sh are not very different, all falling well under 100, within a factor of 2 of each other. Some other groups, such as snakes, however, stand out by possessing an enormous number of vertebrae (130-500, compared to 65 in mouse, 55 in chicken, 33 in human and 31 in zebrafi sh; Vonk and Richardson 2008; Marx and Rabb 1972). While there has been much speculation as to how this atypical (for vertebrates) segmental phenotype may have conferred adaptive advantages to snakes and their ancestors (Willmer 1990), it seems remarkable, particularly in the context of the incrementalist scenarios favoured by the standard selectionist framework, that generation of such an extreme morphology was even attainable. The developmental dynamics disclosed in the snake and centipede studies show vividly how evolution of form can take abrupt turns. First, Gomez et al. (2008) showed that their experimental animal, the corn snake, makes its somites in a fashion similar to that of fi sh, birds and mammals. As previously predicted by Cooke and Zeeman (1976) and later shown experimentally by Olivier Pourquié and his colleagues (reviewed in Dequéant and Pourquié 2008), the molecular–genetic mechanism that underlies this process consists of a biochemical oscillator (known as the segmentation clock) and a gradient, or wavefront. The clock is now known to comprise the periodic expression of Notch pathway signalling components and, depending on the vertebrate class, Wnt and fi broblast growth factor (FGF) pathway components as well (Dequéant and Pourquié 2008). The wavefront, with its source at the tailbud, consists, at a minimum, of FGF8 (Dequéant and Pourquié 2008). The FGF gradient serves as gate for the formation of the somites in the following fashion: the PSM, which is locally synchronous with respect to the clock, reacts to attaining a specifi c clock-value (i.e. a critical concentration of one of the periodically changing components) by creating a fi ssure, but the tissue only does this when it is located at a point of the embryo’s axis where the FGF8 concentration is below a critical value. Because of the factor’s graded distribution, this position is substantially anterior to the tailbud. As the tailbud grows caudally, the shallow end of the gradient regresses in the same direction, progressively allowing new blocks of the PSM to bud off from the as-yet unsegmented region when the critical clockvalue next recurs in the newly disinhibited tissue. The snake embryo exhibited cyclic expression of Lunatic fringe (lfng), an enzyme of the Notch signalling pathway, as well as an FGF gradient (Gomez et al. 2008). The wavefront in snake embryos regressed caudally by one somite length every time a somite was formed, similar to what is observed in chicken, mouse and zebrafi sh models of somitogenesis (Dequéant and Pourquié 2008; Holley 2007). Thus.

The pre-Mendelian, pre-Darwinian world: shifting relations between genetic and epigenetic mechanisms in early multicellular evolution.

The reliable dependence of many features of contemporary organisms on changes in gene content and activity is tied to the processes of Mendelian inheritance and Darwinian evolution. With regard to morphological characters, however, Mendelian inheritance is the exception rather than the rule, and neo-Darwinian mechanisms in any case do not account for the origination (as opposed to the inherited variation) of such characters. It is proposed, therefore, that multicellular organisms passed through a pre-Mendelian, pre-Darwinian phase, whereby cells, genes and gene products constituted complex systems with context-dependent, self-organizing morphogenetic capabilities. An example is provided of a plausible 'core' mechanism for the development of the vertebrate limb that is both inherently pattern forming and morphogenetically plastic. It is suggested that most complex multicellular structures originated from such systems. The notion that genes are privileged determinants of biological characters can only be sustained by neglecting questions of evolutionary origination and the evolution of developmental mechanisms.

Generic physical mechanisms of morphogenesis and pattern formation as determinants in the evolution of multicellular organization.

Early embryos of metazoan species are subject to the same set of physical forces and interactions as any small parcels of semi-solid material, living or nonliving. It is proposed that such "generic" properties of embryonic tissues have played a major role in the evolution of biological form and pattern by providing an array of morphological templates, during the early stages of metazoan phylogeny, upon which natural selection could act. The generic physical mechanisms considered include sedimentation, diffusion, and reaction–diffusion coupling, all of which can give rise to chemical nonuniformities (including periodic patterns) in eggs and small multicellular aggregates, and differential adhesion, which can lead to the formation of boundaries of non-mixing between adjacent cell populations. Generic mechanisms that produce chemical patterns, acting in concern with the capacity of cells to modulate their adhesivity (presumed to be a primitive, defining property of metazoa), could lead to multilayered gastrulae of various types, segmental organization, and many of the other distinguishing characteristics of extant and extinct metazoan body plans. Similar generic mechanisms, acting on small tissue primordia during and subsequent to the establishment of the major body plans, could have given rise to the forms of organs, such as the vertebrate limbs. Generic physical processes acting on a single system of cells and cell products can often produce a widely divergent set of morphological phenotypes, and these are proposed to be the raw material of the evolution of form. The establishment of any ecologically successful form by these mechanisms will be followed, under this hypothesis, by a period of genetic evolution, in which the recruitment of gene products to produce the "generically templated" morphologies by redundant pathways would be favoured by intense selection, leading to extensive genetic change with little impact on the fossil record. In this view, the stabilizing and reinforcing functions of natural selection are more important than its ability to effect incremental change in morphology. Aspects of evolution which are problematic from the standard neo-Darwinian viewpoint, or not considered within that framework, but which follow in a straightforward fashion from the view presented here, include the beginnings of an understanding of why organisms have the structure and appearance they' do, why homoplasy (the recurrent evolution of certain forms) is so prevalent, why evolution has the tempo and mode it does ("punctuated equilibrium"), and why a "rapid" burst of morphological evolution occurred so soon after the origin of the metazoa.