Fifty years later: I. Michael Lerner's Genetic homeostasis (1954)--a valiant attempt to integrate genes, organisms and environment.
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Adaptation, Biological↗
Biomedical subjects
Brian K Hall
Publications and source records attributed to Brian K Hall.
Betrayed by Balanoglossus: William Bateson's rejection of evolutionary embryology as the basis for understanding evolution.
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Animals↗
Are breeding teeth in Atlantic salmon a component of the drastic alterations of the oral facial skeleton?
Upriver spawning migration of starving Atlantic salmon (Salmo salar) involves drastic skeletal alterations, among which a toothless stage followed by the appearance of a new set of so-called breeding teeth has been described. To investigate this phenomenon, we examined the patterns of tooth replacement on the lower jaws in different life stages of wild animals before and after spawning. Prior to spawning, every position held either a functional or a replacement tooth, both in first-time (grilse) and repetitive (salmon) spawners. Teeth were in a similar developmental stage every three positions along the tooth row. A functional tooth occurred in every third position and intermediate positions were taken by developing teeth. Within the process of replacement, teeth were resorbed and not shed. Our observations on an uninterrupted tooth replacement pattern provided no evidence of an intermediate toothless stage nor of a specialized breeding-teeth generation. Only animals that survived spawning (kelts) showed a highly variable tooth pattern, but with the initial "every third position" pattern still recognizable in some animals. We hypothesise that previous accounts describing a complete tooth loss/replacement relate to proliferation of the oral mucosa that conceals the teeth prior to the breeding period and to the use of maceration techniques that could have removed all teeth with an incompletely mineralised base.
Animals↗
In search of evolutionary developmental mechanisms: the 30-year gap between 1944 and 1974.
The approach I have elected in this retrospective of how I became a student of evo-devo is both biographical and historical, a case study along the lines of Waddington's The Evolution of an Evolutionist ('75), although in my case it is the Evolution of an Evo-devoist. What were the major events that brought me to developmental biology and from there to evo-devo? They were, of course, specific to my generation, to the state of knowledge at the time, and to my own particular circumstances. Although exposed to evolution and embryology as an undergraduate in the 1960s, my PhD and post-PhD research programme lay within developmental biology until the early 1970s. An important formative influence on my studies as an undergraduate was the work of Conrad Hal Waddington (1905-1975), whose writings made me aware of genetic assimilation and gave me an epigenetic approach to my developmental studies. The switch to evo-devo (and my discovery of the existence of the neural crest), I owe to an ASZ (now SICB) symposium held in 1973.
Adaptation, Biological↗
Perspectives on hyperphalangy: patterns and processes.
Hyperphalangy is a digit morphology in which increased numbers of phalanges are arranged linearly within a digit beyond the plesiomorphic condition. We analyse patterns and processes of hyperphalangy by considering previous definitions and occurrences of hyperphalangy among terrestrial and secondarily aquatic extant and fossil taxa (cetaceans, ichthyosaurs, plesiosaurs and mosasaurs), and recent studies that elucidate the factors involved in terrestrial autopod joint induction. Extreme hyperphalangy, defined as exceeding a threshold condition of 4/6/6/6/6, is shown only to be found among secondarily aquatic vertebrates with a flipper limb morphology. Based on this definition, hyperphalangy occurs exclusively in digits II and III among extant cetaceans. Previous reports of cetacean embryos having more phalanges than adults is clarified and shown to be based on cartilaginous elements not ossified phalanges. Developmental prerequisites for hyperphalangy include lack of cell death in interdigital mesoderm (producing a flipper limb) and maintenance of a secondary apical ectodermal ridge (AER), which initiates digit elongation and extra joint patterning. Factors of the limb-patterning pathways located in the interdigital mesoderm, including bone morphogenetic proteins (BMPs), BMP antagonists, fibroblast growth factors (FGFs), growth/differentiation factor-5 (GDF-5), Wnt-14 and ck-erg, are implicated in maintenance of the flipper limb, secondary AER formation, digit elongation and additional joint induction leading to hyperphalangy.
Animals↗
Francis Maitland Balfour (1851-1882): a founder of evolutionary embryology.
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Biological Evolution↗
The effects of estrogen, its antagonist ICI 182, 780, and interferon-tau on the expression of estrogen receptors and integrin alphaV beta 3 on cycle day 16 in bovine endometrium.
We have shown previously that downregulation of intercaruncular stromal integrin alphavbeta3 in bovine endometrium on day 16 of the estrous cycle coincided with the antibody recognition of estrogen receptors (ER) in the luminal epithelium. In pregnancy, these changes were not observed. Our hypothesis was that on day 16 of the estrous cycle, estrogen from the dominant follicle causes a reduction in integrin alphavbeta3 and affects ERalpha in the luminal epithelium. The pregnancy recognition protein, interferon-tau (IFN-tau), may prevent downregulation of integrin alphavbeta3 and suppress ERalpha expression in the luminal epithelium. On days 14 to 16, heifers received uterine infusions of the anti-estrogen ICI 182, 780, estradiol 17beta, IFN-tau or the saline control. On day 16, reproductive tracts were collected for analysis of integrin alphavbeta3 and ERalpha. Estrogen receptor alpha immunoreactivity was largely restricted to the luminal epithelium in control animals. Using anti-ERalpha recognizing the amino terminus, estrogen-treated animals showed reactivity in the stroma, shallow and deep glands and myometrium as is typical of estrus, whereas ICI 182, 870 treated heifers showed little or no reactivity. In contrast, carboxyl terminus-directed antibodies showed a widespread distribution of ERalpha with reactivity detected in the uterine epithelium, stroma and myometrium of both estrogen and ICI 182, 780 treated animals. Heifers treated with IFN-tau had low ERalpha reactivity overall. Control and IFN-tau treated heifers had lower intercaruncular stromal expression of integrin alphavbeta3 in comparison to estrogen and ICI 182, 780 treatments. Overall, the results suggest that on day 16 of the estrous cycle, estrogen effects on integrin alphavbeta3 are indirect and do not directly involve ERalpha in the luminal epithelium. During pregnancy, interferon-tau may block ERalpha in the luminal epithelium but likely does not rescue integrin alphavbeta3 expression.
Animals↗
Embryological origins of developmental stability: size, shape and fluctuating asymmetry in prenatal random bred mice.
Ontogenetic patterns of fluctuating asymmetry (FA) can be used to test models for the mechanisms underlying stability during embryonic development (developmental stability). In this study, we ask whether developmental processes initially show high levels of instability that are subsequently dampened through active compensatory mechanisms or passive properties of developmental systems or whether the effects of instability accumulate during embryonic development causing random drift away from an earlier stable state. Previous work on this question has dealt with postnatal skeletal growth and thus been unable to effectively distinguish developmental instability from the effects of mechanically mediated variation in bone modeling and remodeling. Here, we report that FA variances of limb skeletal elements in CD1 mice decrease with gestational age from day 14 to birth (day 20.5). Thus, in mouse limbs, skeletal development is characterized by a high level of developmental instability initially that is reduced during subsequent prenatal development. These results are consistent with the existence of active mechanisms that compensate for the effects of minor perturbations or deviations during development. However, they are also consistent with Soule's model of allomeric variation in which the variance of structures is reduced as the number of independent developmental events that produce them increases. This study illustrates that predictions based on morphometric analyses can yield insights into general properties of developmental systems in cases where specific developmental mechanisms are not yet known.
Animals↗
Descent with modification: the unity underlying homology and homoplasy as seen through an analysis of development and evolution.
Homology is at the foundation of comparative studies in biology at all levels from genes to phenotypes. Homology is similarity because of common descent and ancestry, homoplasy is similarity arrived at via independent evolution. However, given that there is but one tree of life, all organisms, and therefore all features of organisms, share some degree of relationship and similarity one to another. That sharing may be similarity or even identity of structure and the sharing of a most recent common ancestor--as in the homology of the arms of humans and apes--or it may reflect some (often small) degree of similarity, such as that between the wings of insects and the wings of birds, groups whose shared ancestor lies deep within the evolutionary history of the Metazoa. It may reflect sharing of entire developmental pathways, partial sharing, or divergent pathways. This review compares features classified as homologous with the classes of features normally grouped as homoplastic, the latter being convergence, parallelism, reversals, rudiments, vestiges, and atavisms. On the one hand, developmental mechanisms may be conserved, even when a complete structure does not form (rudiments, vestiges), or when a structure appears only in some individuals (atavisms). On the other hand, different developmental mechanisms can produce similar (homologous) features. Joint examination of nearness of relationship and degree of shared development reveals a continuum within an expanded category of homology, extending from homology --> reversals --> rudiments --> vestiges --> atavisms --> parallelism, with convergence as the only class of homoplasy, an idea that turns out to be surprisingly old. This realignment provides a glimmer of a way to bridge phylogenetic and developmental approaches to homology and homoplasy, a bridge that should provide a key pillar for evolutionary developmental biology (evo-devo). It will not, and in a practical sense cannot, alter how homoplastic features are identified in phylogenetic analyses. But seeing rudiments, reversals, vestiges, atavisms and parallelism as closer to homology than to homoplasy should guide us toward searching for the common elements underlying the formation of the phenotype (what some have called the deep homology of genetic and/or cellular mechanisms), rather than discussing features in terms of shared or independent evolution.
Animals↗
Seasonal changes in the lower jaw skeleton in male Atlantic salmon (Salmo salar L.): remodelling and regression of the kype after spawning.
The return of Atlantic salmon (Salmon salar) to their home river for spawning coincides with drastic skeletal alterations in both sexes. Most prominent is the development of a kype (hook) at the tip of the lower jaw in males. Salmon that survive spawning have to cope with the kype throughout their life, unless it disappears after spawning, as was suggested in the early literature. To understand the fate of the kype skeleton, we compared morphological and histological features of kypes from pre-spawned mature anadromous males (grilse) with post-spawned males (kelts). The kype of male grilse is supported by fast-growing skeletal needles that differ from regular dentary bone. In kelts, growth of the kype skeleton has stopped and skeletal needles are resorbed apically by osteoclasts. Simultaneously, and despite the critical physiological condition of the animals, proximal parts of the kype skeleton are remodelled and converted into regular dentary bone. Apical resorption of the skeleton explains reports of a decrease of the kype in kelts. The conversion of basal kype skeleton into regular dentary bone contributes to the elongation of the dentary and probably also to the development of a larger kype in repetitive spawning males.
Animals↗
Evo-Devo: evolutionary developmental mechanisms.
Evolutionary developmental biology (Evo-Devo) as a discipline is concerned, among other things, with discovering and understanding the role of changes in developmental mechanisms in the evolutionary origin of aspects of the phenotype. In a very real sense, Evo-Devo opens the black box between genotype and phenotype, or more properly, phenotypes as multiple life history stages arise in many organisms from a single genotype. Changes in the timing or positioning of an aspect of development in a descendant relative to an ancestor (heterochrony and heterotopy) were two evolutionary developmental mechanisms identified by Ernst Haeckel in the 1870s. Many more have since been identified, in large part because of our enhanced understanding of development and because new mechanisms emerge as development proceeds: the transfer from maternal to zygotic genomic control; cell-to-cell interactions; cell differentiation and cell migration; embryonic inductions; functional interactions at the tissue and organ levels; growth. Within these emergent processes, gene networks and gene cascades (genetic modules) link the genotype with morphogenetic units (cellular modules, namely germ layers, embryonic fields or cellular condensations), while epigenetic processes such as embryonic inductions, tissue interactions and functional integration, link morphogenetic units to the phenotype. Evolutionary developmental mechanisms also include interactions between individuals of the same species, individuals of different species, and species and their biotic and/or abiotic environment. Such interactions link ecological communities. Importantly, there is little to distinguish the causality that underlies these interactions from that which underlies inductive interactions within embryos.
Animals↗
Canalization, developmental stability, and morphological integration in primate limbs.
Canalization and developmental stability refer to the tendency of developmental processes to follow particular trajectories, despite external or internal perturbation. Canalization is the tendency for development of a specific genotype to follow the same trajectory under different conditions (different environments or different genetic backgrounds), while developmental stability is the tendency for the development of a specific genotype to follow the same trajectory under the same conditions. Morphological integration refers to the tendency for structures to show correlated variation because they develop in response to shared developmental processes or function in concert with other structures. All three phenomena are emergent properties of developmental systems that can affect the interaction of development and evolution. In this paper, we review the topics of canalization, developmental stability, and morphological integration and their relevance to primate and human evolution. We then test three developmentally motivated hypotheses about the patterning of variability components in the mammalian limb. We find that environmental variances and fluctuating asymmetries (FA) increase distally along the limb in adult macaques but not in fetal mice. We infer that the greater variability of more distal segments in macaques is due to postnatal mechanical effects. We also find that heritability and FA are significantly correlated when different limb measurements are compared in fetal mice. This supports the idea that the mechanisms underlying canalization and developmental stability are related. Finally, we report that the covariation structure of fore- and hindlimb skeletal elements shows evidence for morphological integration between serially homologous structures between the limbs. This is evidence for the existence of developmental modules that link structures between the limbs. Such modules would produce covariation that would need to be overcome by selection for divergence in hind- and forelimb morphology.
Anatomy, Comparative↗
Differentiation and growth of kype skeletal tissues in anadromous male Atlantic salmon (Salmo salar).
The re-initiation of bone development in adult starving Atlantic salmon (Salmo salar) during their energetically expensive upstream migration is remarkable and deserves closer examination. Dramatic alterations of the skull bones and teeth, most prominently, the development of a kype in males, are widely known but little studied or understood. We describe the microstructure and the cellular processes involved in the formation of the skeletal tissues of the kype. Fresh bone material, obtained from animals migrating upstream was subjected to radiological, histological or histochemical analysis. We show that the kype is, in part, composed of rapidly growing skeletal needles arising at the tip of the dentary. Proximally, the needles anastomose into a spongiosa-like meshwork which retains connective tissue inside bone marrow spaces. Ventrally, the needles blend into Sharpey fiber bone. Skeletal needles and Sharpey fiber bone can be distinguished from the compact bone of the dentary by radiography. Rapid formation of the skeleton of the kype is demonstrated by the presence of numerous osteoblasts, a broad distal osteoid zone, and the appearance of proteoglycans at the growth zone. The mode of bone formation in anadromous males can be described as 'making bone as fast as possible and with as little material as possible'. Unlike the normal compact bone of the dentary, the new skeletal tissue contains chondrocytes and cartilaginous extracellular matrix. Formation of the skeleton of the kype resembles antler development in deer (a form of regeneration), or hyperostotic bone formation in other teleost fishes, rather than periosteal bone growth. The type of boneformation may be understandable in the light of the animals' starvation and the energetic costs of upstream migration. However, the structured and regulated mode of bone formation suggests that the skeleton of the kype has functional relevance and is not a by-product of hormonal alterations or change of habitat.
Animals↗
Limbs in whales and limblessness in other vertebrates: mechanisms of evolutionary and developmental transformation and loss.
We address the developmental and evolutionary mechanisms underlying fore- and hindlimb development and progressive hindlimb reduction and skeletal loss in whales and evaluate whether the genetic, developmental, and evolutionary mechanisms thought to be responsible for limb loss in snakes "explain" loss of the hindlimbs in whales. Limb loss and concurrent morphological and physiological changes associated with the transition from land to water are discussed within the context of the current whale phylogeny. Emphasis is placed on fore- and hindlimb development, how the forelimbs transformed into flippers, and how the hindlimbs regressed, leaving either no elements or vestigial skeletal elements. Hindlimbs likely began to regress only after the ancestors of whales entered the aquatic environment: Hindlimb function was co-opted by the undulatory vertical axial locomotion made possible by the newly evolved caudal flukes. Loss of the hindlimbs was associated with elongation of the body during the transition from land to water. Limblessness in most snakes is also associated with adoption of a new (burrowing) lifestyle and was driven by developmental changes associated with elongation of the body. Parallels between adaptation to burrowing or to the aquatic environment reflect structural and functional changes associated with the switch to axial locomotion. Because they are more fully studied and to determine whether hindlimb loss in lineages that are not closely related could result from similar genetically controlled developmental pathways, we discuss developmental (cellular and genetic) processes that may have driven limb loss in snakes and leg-less lizards and compare these processes to the loss of hindlimbs in whales. In neither group does ontogenetic or phylogenetic limb reduction result from failure to initiate limb development. In both groups limb loss results from arrested development at the limb bud stage, as a result of inability to maintain necessary inductive tissue interactions and enhanced cell death over that seen in limbed tetrapods. An evolutionary change in Hox gene expression--as occurs in snakes--or in Hox gene regulation--as occurs in some limbless mutants--is unlikely to have initiated loss of the hindlimbs in cetaceans. Selective pressures acting on a wide range of developmental processes and adult traits other than the limbs are likely to have driven the loss of hindlimbs in whales.
Animals↗
Latent homologues for the neural crest as an evolutionary novelty.
The neural crest is a craniate synapomorphy and a bona fide evolutionary novelty. Recently, researchers considering intriguingly similar patterns of gene expression, cell behaviors, and embryogenetic processes in noncraniate deuterostomes have suggested that cephalochordates, urochordates, and echinoderms or their ancestors might have possessed cells that were precursors to the neural crest or its constituent cells. To emphasize the caution with which similarities at genetic, cellular, or embryological levels should be interpreted as substantiations for cell, germ layer, or tissue homologies, we present and evaluate additional tantalizing evidence that could be considered as documenting neural crest precursors in precraniates. Furthermore, we propose an evolutionary context--latent homologue--within which these data should be interpreted.
Animals↗