The Dominance of Symmetry in the Evolution of Homo-oligomeric Proteins

The fact that aggregates of identical protein molecules are usually symmetric has remained an enigma. An idealized model of a soluble monomeric protein was constructed and accompanied through a simulated evolutionary process resulting in dimerization, in order to elucidate this peculiarity. The model showed that the probability of a symmetric association is by a factor of 100 or above higher than the probability of an asymmetric one. Unexpectedly, symmetry prevails in the dimer initiation phase much more than in the dimer improvement phase of evolution. The result is clear-cut and robust against a broad spectrum of model inadequacies. It rationalizes the predominance of symmetric homo-oligomers.     

Origin and Early Evolution of Plant Body Symmetry and Gravity Responses

Land plant bodies exhibit both apical–basal and radial symmetry, and they are able to detect and respond to gravitational forces. These attributes were, likely important factors in the success of earliest plants on land. This study focuses on features of charophycean green algae likely to have been pre‐adaptive to early establishment of plant symmetry and gravitational responses, though most modern charophyceans occupy aquatic habitats where the buoyancy of water counteracts the effects of gravity. Trait mapping suggests that even the earliest‐divergent modern members of the streptophyte clade have bodies whose symmetry departs significantly from the spherical condition, and that cellular mechanisms defining aspects of radial symmetry and polarized tip growth originated early. Genes, cell biological approaches, and taxa are identified for which further exploration is likely to illuminate early evolution of plant body symmetry and gravity responses.     

The Evolution of Human Brain Development

The human brain is a large and complex organ, setting us apart from other primates. It allows us to exhibit highly sophisticated cognitive and behavioral abilities. Therefore, our brain??s size and morphology are defining features of our species and our fossil ancestors and relatives. Endocasts, i.e., internal casts of the bony braincase, provide evidence about brain size and morphology in fossils. Based on endocasts, we know that our ancestors?? brains increased overall in size and underwent several reorganizational changes. However, it is difficult to relate evolutionary changes of size and shape of endocasts to evolutionary changes of cognition and behavior. We argue here that an understanding of the tempo and mode of brain development can help to interpret the evolution of our brain and the associated cognitive and behavioral changes. To do so, we review structural brain development, cognitive development, and ontogenetic changes of endocranial size and shape in living individuals on the one hand, and ontogenetic patterns (size increase and shape change) in fossil hominins and their evolutionary change on the other hand. Tightly integrating our knowledge on these different levels will be the key of future work on the evolution of human brain development.     

Heterochrony: the Evolution of Development

Heterochrony can be defined as change to the timing or rate of development relative to the ancestor. Because organisms generally change in shape as well as increase in size during their development, any variation to the duration of growth or to the rate of growth of different parts of the organism can cause morphological changes in the descendant form. Heterochrony takes the form of both increased and decreased degrees of development, known as “peramorphosis” and “paedomorphosis,” respectively. These are the morphological consequences of the operation of processes that change the duration of the period of an individual’s growth, either starting or stopping it earlier or later than in the ancestor, or by extending or contracting the period of growth. Heterochrony operates both intra- and interspecifically and is the source of much intraspecific variation. It is often also the cause of sexual dimorphism. Selection of a sequence of species with a specific heterochronic trait can produce evolutionary trends in the form of pera- or paedomorphoclines. Many different life history traits arise from the operation of heterochronic processes, and these may sometimes be the targets of selection rather than morphological features themselves. It has been suggested that some significant steps in evolution, such as the evolution of vertebrates, were engendered by heterochrony. Human evolution was fuelled by heterochrony, with some traits, such as a large brain, being peramorphic, whereas others, such as reduced jaw size, are paedomorphic.     

Perceptual Anthropology: The Cultural Salience of Symmetry

In this paper, I advance the position that knowledge about the universals of form perceived by the visual system is fundamental to a theory of how art communicates. I focus on how the perceptual system uses the universal property of symmetry to recognize and classify form. I propose that the symmetries that structure design parts in non-representational geometric patterns metaphorically encode a culture's fundamental relationships about the world. This metaphorical use of the property of symmetry is illustrated by showing how bifold symmetries in ceramic design embody Puebloan concepts of life. [symmetry of pattern, perceptual universals, worldview in design structure, metaphorical readings]     

Cellular Symmetry Breaking during Caenorhabditis elegans Development

The nematode worm Caenorhabditis elegans has produced a wellspring of insights into mechanisms that govern cellular symmetry breaking during animal development. Here we focus on two highly conserved systems that underlie many of the key symmetry-breaking events that occur during embryonic and larval development in the worm. One involves the interplay between Par proteins, Rho GTPases, and the actomyosin cytoskeleton and mediates asymmetric cell divisions that establish the germline. The other uses elements of the Wnt signaling pathway and a highly reiterative mechanism that distinguishes anterior from posterior daughter cell fates. Much of what we know about these systems comes from intensive study of a few key events—Par/Rho/actomyosin-mediated polarization of the zygote in response to a sperm-derived cue and the Wnt-mediated induction of endoderm at the four-cell stage. However, a growing body of work is revealing how C. elegans exploits elements/variants of these systems to accomplish a diversity of symmetry-breaking tasks throughout embryonic and larval development.Over the past few decades, the C. elegans embryo has become a premiere system for studying cellular symmetry breaking in a developmental context. During C. elegans development, nearly every division produces daughter cells with different developmental trajectories. In some cases, these differences are imposed on daughters before or after division through inductive signals, but many of these divisions are intrinsically asymmetric—an initial symmetry-breaking step creates polarized distributions or activities of factors that control developmental potential. Registration of the cleavage plane with the axis of polarity then ensures differential inheritance of these potentials. With respect to cell fates, the output of these asymmetric divisions is amazingly diverse, yet the embryo seems to accomplish this diversity through variants of a few conserved symmetry-breaking systems. Thus the C. elegans embryo provides an exceptional opportunity to explore not only the core mechanisms underlying cellular symmetry breaking, but also how evolution can reconfigure these mechanisms to do different but related jobs in multiple contexts.In this review, we focus most of our attention on two conserved systems that together account for much of the cellular asymmetry observed during C. elegans embryogenesis. The first, which is best known for its role in the early asymmetric cell divisions that segregate germline from the soma, involves a complex interplay between Par proteins, Rho-family GTPases, and the actomyosin cytoskeleton. Interestingly, the embryo exploits elements of this same system to break symmetry during cleavage furrow specification and to establish apicobasal polarity in early embryonic cells and in the first true embryonic epithelia. The second system we focus on involves an unusual application of WNT signaling pathway components and is used reiteratively throughout embryonic and larval development to distinguish anterior and posterior daughter cell fates. Rather than comprehensively review these systems, we highlight topics not extensively covered in other reviews.     

Evolution of Vertebrate Limbs: Robust Morphology and Flexible Development

SYNOPSIS. New information about molecular mechanisms of developmentcan be combined with existing knowledge about embryology, anatomyand paleontology to allow for an increased understanding ofevolutionary biology. The tetrapod limb is appropriate for suchan approach since much is known about both its structural variationand development. To this end we are investigating molecularregulatory mechanisms in urodele limb development and regeneration.Urodeles have unique patterns of limb development compared toother tetrapods. In addition they are able to regenerate theirlimbs as adults, thus providing the opportunity to conduct comparativestudies of the molecular mechanisms involved in developmentand regeneration in an identical genetic background. We haveinvestigated the role of several homeobox-containing genes inthe control of growth and pattern formation during limb developmentand regeneration, and have found that although there can beconsiderable variation in the ways in which expression of thesegenes is regulated in tune and space, their expression patternsrelative to morphological landmarks is conserved. These resultssuggest that the function of these genes is a deeply conservedfeature of all tetrapods, and may be the molecular manifestationof the homology between different limb types. These conservedsimilarities are overlaid with changes in the time at whichgenes are expressed and the sequence in which structures differentiate.It is these latter features that are most likely responsiblefor the wide variety of morphologies observed among tetrapodlimbs.     

Bridges between Development and Evolution

Adaptive evolution is usually assumed to be directed by selective processes, development by instructive processes; evolution involves random genetic changes, development involves induced epigenetic changes. However, these distinctions are no longer unequivocal. Selection of genetic changes is a normal part of development in some organisms, and through the epigenetic system external factors can induce selectable heritable variations. Incorporating the effects of instructive processes into evolutionary thinking alters ideas about the way environmental changes lead to evolutionary change, and about the interplay between genetic and epigenetic systems.     

The Future Vocation of Neural Stem Cells: Lineage Commitment in Brain Development and Evolution

Understanding the fate commitment of neural stem cells is critical to identify the regulatory mechanisms in developing brains. Genetic lineage-tracing has provided a powerful strategy to unveil the heterogeneous nature of stem cells and their descendants. However, recent studies have reported controversial data regarding the heterogeneity of neural stem cells in the developing mouse neocortex, which prevents a decisive conclusion on this issue. Here, we review the progress that has been made using lineage-tracing analyses of the developing neocortex and discuss stem cell heterogeneity from the viewpoint of comparative and evolutionary biology.     

The Evolution of Evolution: Reconciling the Problem of Stability

Evolutionary Science has, at least since the publication of Origin, been less concerned with the continuation of species in stable forms, than with the reconfiguration of forms into a host of varieties. So influential has this emphasis been that, over the years, “variation” has become a cardinal desideratum, even taking precedence over the macroevolutionary landscape. This orientation has made it much more difficult to objectively assess the meaning of non-change patterns such as periods of stasis, which appear to be widespread in most species. Yet, if stasis is an expectable outcome of evolutionary activity, this raises the possibility that there may be mechanisms and processes at many causal levels, acting on its behalf, without reference to the impetus toward persistent variation. Researchers have been reluctant to attribute stasis to a macroevolutionary tendency toward ‘stability’ despite the commonality of stasis in many species, and notwithstanding the many biological/behavioral processes that seem inclined to produce and maintain conformance, regulation and consistency. Speciation, paradoxically, is the best evidence for an overriding influence toward stability in that stability would seem to be a necessary condition prior to the development of isolating mechanisms. An alternative macroevolutionary model of biological activity is offered consisting of two tendencies, “variety” counterpoised with “stability” both acting in the service of the persistence of life.