Diego Rasskin-Gutman currently works at the Instituto Cavanilles de Biodiversidad y Biología Evolutiva, University of Valencia. Diego does research in Evolutionary Biology, Paleobiology and Developmental Biology. Their current project is 'Development of Skull Networks'.
Skills and Expertise
Research Items (69)
- Apr 2014
Morphological EvoDevo is a field of biological inquiry in which explicit relations between evolutionary patterns and growth or morphogenetic processes are made. Historically, morphological EvoDevo results from the coming together of several traditions, notably Naturphilosophie, embryology, the study of heterochrony, and developmental constraints. A special feature binding different approaches to morphological EvoDevo is the use of formalisms and mathematical models. Here we will introduce anatomical network analysis, a new approach centered on connectivity patterns formed by anatomical parts, with its own concepts and tools specifically designed for the study of morphological EvoDevo questions. Riedl’s concept of burden is tightly related to the use of anatomical networks, providing a nexus between the evolutionary patterns and the structural constraints that shape them.
Network models of the tetrapod skull in which nodes represent bones and links representsutures have recently offered new insights into the structural constraints underlying theevolutionary reduction of bone number in the tetrapod skull, known as Williston’s Law.Here, we have built null network model-derived generative morphospaces of the tetrapodskull using random, preferential attachment, and geometric proximity growth rules. Ourresults indicate that geometric proximity is the best null model to explain the disparityof skull structures under two structural constraints: bilateral symmetry and presence ofunpaired bones. The analysis of the temporal occupation of this morphospace, concomitantwith Williston’s Law, indicates that the tetrapod skull has followed an evolutionary pathtoward more constrained morphological organizations.
- Mar 2014
The tetrapod skull has undergone a reduction in number of bones in all major lineages since the origin of vertebrates, an evolutionary trend known as Williston’s Law. Using connectivity relations between bones as a proxy for morphological complexity we showed that this reduction in number of bones generated an evolutionary trend toward more complex skulls. This would imply that connectivity patterns among bones impose structural constraints on bone loss and fusion that increase bone burden due to the formation of new functional and developmental dependencies; thus, the higher the number of connections, the higher the burden. Here, we test this hypothesis by exploring plausible evolutionary scenarios based on selective versus random processes of bone loss and fusion. To do this, we have built a computational model that reduces iteratively the number of bones by loss and fusion, starting from hypothetical ancestral skulls represented as Gabriel networks in which bones are nodes and suture connections are links. Simulation results indicate that losses and fusions of bones affect skull structure differently whether they target bones at random or selectively depending on the number of bone connections. Our findings support a mixed scenario for Williston’s Law: the random loss of poorly connected bones and the selective fusion of the most connected ones. This evolutionary scenario offers a new explanation for the increase of morphological complexity in the tetrapod skull by reduction of bones during development.
- Dec 2013
Riedl's concept of burden neatly links development and evolution by ascertaining that structures that show a high degree of developmental co-dependencies with other structures are more constrained in evolution. The human skull can be precisely modeled as an articulated complex system of bones connected by sutures, forming a network of structural co-dependencies. We present a quantitative analysis of the morphological integration, modularity, and hierarchical organization of this human skull network model. Our overall results show that the human skull is a small-world network, with two well-delimited connectivity modules: one facial organized around the ethmoid bone, and one cranial organized around the sphenoid bone. Geometric morphometrics further support this two-module division, stressing the direct relationship between the developmental information enclosed in connectivity patterns and skull shape. Whereas the facial module shows a hierarchy of clustered blocks of bones, the bones of the cranial modules show a regular pattern of connections. We analyze the significance of these arrangements by hypothesizing specific structural roles for the most important bones involved in the formation of both modules, in the context of Riedl's burden. We conclude that it is the morphological integration of each group of bones that defines the semi-hierarchical organization of the human skull, reflecting fundamental differences in the ontogenetic patterns of growth and the structural constraints that generate each module. Our study also demonstrates the adequacy of network analysis as an innovative tool to understand the morphological complexity of anatomical systems. J. Exp. Zool. (Mol. Dev. Evol.) 9999B: XX-XX, 2013. © 2013 Wiley Periodicals, Inc.
- Apr 2019
The origin of the mammalian middle ear ossicles from the craniomandibular articulation of their synapsid ancestors is a key event in the evolution of vertebrates. The richness of the fossil record and the multitude of developmental studies have provided a stepwise reconstruction of this evolutionary innovation, highlighting the homology between the quadrate, articular, pre‐articular and angular bones of early synapsids with the incus, malleus, gonial and ectotympanic bones of derived mammals, respectively. There are several aspects involved in this functional exaptation: (i) an increase of the masticatory musculature; (ii) the separation of the quadrate bone from the cranium; and (iii) the disconnection of the post‐dentary bones from the dentary. Here, we compared the jaw‐otic complex for 43 synapsid taxa using anatomical network analysis, showing that the disconnection of mandibular bones was a key step in the mammalian middle ear evolution, changing the skull anatomical modularity concomitant to the acquisition of new functions. Furthermore, our analysis allows the identification of three types of anatomical modules evolving through five evolutionary stages during the anatomical transformation of the jawbones into middle ear bones, with the ossification and degradation of Meckel's cartilage in mammals as the key ontogenetic event leading the change of anatomical modularity.
- Aug 2018
- Digital Endocasts
Brain mapping has always been a priority in neurobiology and evolutionary neuroanatomy. In the last century, methodological issues and technical advances have generated a vivid debate on the parcellation and functions of the cortical territories. Brain structure is generally analyzed by considering the network of connections associated with neural pathways. Nonetheless, there is still a major debate on the recognition of the spatial and geometrical components of the cerebral cortex. The maps produced by Korbinian Brodmann in the early twentieth century on the basis of histological patterns represented a pioneering and decisive step in this sense, being a reference until the present day. Network models allow a numerical analysis of the spatial relationships among anatomical elements, supplying a quantitative tool to evaluate their reciprocal geometrical organization. This approach is able to analyze the spatial parameters associated with an anatomical system, characterized by the relationships of its elements. The network analysis of the spatial contiguity of Brodmann’s areas approximately describes the major cerebral lobes. A frontal cluster includes only the prefrontal areas. There is a large parieto-occipital block including also the precentral and paracentral cortex. The cortical areas identified by the model match different areas of craniocerebral relationships, namely, the anterior fossa influenced by the upper face (prefrontal cortex), the middle fossa influenced by cranial base and mandibular integration (temporal cortex), and the vault which is characterized by more linear brain-bone dynamics (parieto-occipital cortex). The maps of Brodmann, after one century of contributions, are now replaced by finer parcellations obtained with new technical approaches based on histology, biochemistry, and metabolism, enhanced by advances in brain imaging and digital biology. Besides issues associated with cognitive processing, structural factors can influence geometrical and mechanical properties of the cerebral morphology. Network theory, applied to alternative parcellation schemes or to specific brain districts, can provide essential information on evolutionary factors channeling or constraining the evolution of the brain spatial organization.
The concept of burden was developed around the 1970s by Austrian zoologists Rupert Riedl, based on morphological insights rooted in Karl Ernst von Baer’s embryological tradition. Burden’s main tenet is that, as a morphological character evolves, it develops more relationships with other characters, becoming more and more interconnected. Through this process, the morphological character acquires more biological “responsibilities” within the organism. Two main consequences of the burden hypothesis are that (1) a character’s evolvability will be limited by these responsibilities, and (2) a set of heavily burdened characters could be considered as part of the body plan of a taxonomic group. The concept of burden is intimately related to that of developmental constraint, and as such, it is central to evo-devo.
This chapter introduces the reader to anatomical network analysis (AnNA): a conceptual framework for the tolopological analysis of organismal form. AnNA focuses on the structural relations among anatomical parts, which allows for an evaluation of morphological organization in comparative analyses for both development and evolution. The nodes of the network represent anatomical elements, and the links that connect them represent structural relations or interactions among these elements. Network theory provides the methods to analyze these anatomical network models. The first and second sections present the historical and conceptual background of this framework. The third section explains the construction of anatomical networks and some of the basic parameters we can use to characterize the topology of these models and infer their morphological organization. The fourth section summarizes the interpretation of network parameters in terms of morphological complexity, hierarchy, integration, and modularity in the context of morphological evo-devo. The fifth section introduces the classical construction rules to build null models for networks and an example of the use of network null models in morphology. Finally, in the sixth section, we have explored some of the limits of AnNA.
The premature fusion of cranial bones, craniosynostosis, affects the correct development of the skull producing morphological malformations in newborns. To assess the susceptibility of each craniofacial articulation to close prematurely, we used a network model of the skull to quantify the link reliability (an index based on stochastic block modeling and Bayesian inference) of each articulation. We show that, of the 93 human skull articulations at birth, the few articulations that are associated with nonsyndromic craniosynostosis conditions have statistically significant lower reliability scores than the others. In a similar way, articulations that close during the normal postnatal development of the skull have also lower reliability scores than those articulations that persist through adult live. These results indicate a relationship between the architecture of the skull network and the specific articulations that close during normal development and in pathological conditions. Our findings suggest that the topological arrangement of skull bones might act as an epigenetic factor, predisposing some articulations to closure, both in normal and pathological development, and also affecting the long-term evolution of the skull.
- Jul 2016
Chondrichthyan teeth are capped with a hypermineralized tissue known as enameloid. Its microstructure displays a hierarchical organization that has increased in structural complexity from a homogenous single-crystallite enameloid (SCE) in early Chondricthyans to the complex multilayered enameloid found in modern sharks (consisting of bundles of crystallites arranged in intriguing patterns). Recent analyses of the enameloid microstructure in batoid fishes, focused on Myliobatiformes and fossil taxa, point to the presence of a bundled (or fibred) multilayered enameloid, a condition proposed as plesiomorphic for Batoidea. In this work, we provide further enameloid analysis for a selection of taxa covering the phylogeny of batoids. Our SEM analysis shows a superficial layer of SCE, where individualized crystallites are clearly discernable, capping the teeth in most of the species studied. A bundled double-layered enameloid was found only in a Rhinoidei, Rhina ancylostoma Bloch & Schneider, 1801. We conclude that the most widespread condition among extant batoids is a monolayer SCE lacking microstructural differentiation, probably plesiomorphic at least for crown batoidea. We suggest that the complex bundled enameloid present in other batoids is a convergent character that has appeared repeatedly during the evolution of batoids, probably as a mechanical adaptation towards moderate durophagous diets.
How do the various anatomical parts (modules) of the animal body evolve into very different integrated forms (integration) yet still function properly without decreasing the individual's survival? This long-standing question remains unanswered for multiple reasons, including lack of consensus about conceptual definitions and approaches, as well as a reasonable bias toward the study of hard tissues over soft tissues. A major difficulty concerns the non-trivial technical hurdles of addressing this problem, specifically the lack of quantitative tools to quantify and compare variation across multiple disparate anatomical parts and tissue types. In this paper we apply for the first time a powerful new quantitative tool, Anatomical Network Analysis (AnNA), to examine and compare in detail the musculoskeletal modularity and integration of normal and abnormal human upper and lower limbs. In contrast to other morphological methods, the strength of AnNA is that it allows efficient and direct empirical comparisons among body parts with even vastly different architectures (e.g. upper and lower limbs) and diverse or complex tissue composition (e.g. bones, cartilages and muscles), by quantifying the spatial organization of these parts—their topological patterns relative to each other—using tools borrowed from network theory. Our results reveal similarities between the skeletal networks of the normal newborn/adult upper limb vs. lower limb, with exception to the shoulder vs. pelvis. However, when muscles are included, the overall mus-culoskeletal network organization of the upper limb is strikingly different from that of the lower limb, particularly that of the more proximal structures of each limb. Importantly, the obtained data provide further evidence to be added to the vast amount of paleontological, gross anatomical, developmental, molecular and embryological data recently obtained that
- Sep 2015
Bone fusion has occurred repeatedly during skull evolution in all tetrapod lineages, leading to a reduction in the number of bones and an increase in their morphological complexity. The ontogeny of the human skull includes also bone fusions as part of its normal developmental process. However, several disruptions might cause premature closure of cranial sutures (craniosynostosis), reducing the number of bones and producing new skull growth patterns that causes shape changes. Here, we compare skull network models of a normal newborn with different craniosynostosis conditions, the normal adult stage, and phylogenetically reconstructed forms of a primitive tetrapod, a synapsid, and a placental mammal. Changes in morphological complexity of newborn-to-synostosed skulls are two to three times less than in newborn-to-adult; and even smaller when we compare them to the increases among the reconstructed ancestors in the evolutionary transitions. In addition, normal, synostosed, and adult human skulls show the same connectivity modules: facial and cranial. Differences arise in the internal structure of these modules. In the adult skull the facial module has an internal hierarchical organization, whereas the cranial module has a regular network organization. However, all newborn forms, normal and synostosed, do not reach such kind of internal organization. We conclude that the subtle changes in skull complexity at the developmental scale can change the modular substructure of the newborn skull to more integrated modules in the adult skull, but is not enough to generate radical changes as it occurs at a macroevolutionary scale. The timing of closure of craniofacial sutures, together with the conserved patterns of morphological modularity, highlights a potential relation between the premature fusion of bones and the evolution of the shape of the skull in hominids.
Modularity and complexity go hand in hand in the evolution of the skull of primates. Because analyses of these two parameters often use different approaches, we do not know yet how modularity evolves within, or as a consequence of, an also-evolving complex organization. Here we use a novel network theory-based approach (Anatomical Network Analysis) to assess how the organization of skull bones constrains the co-evolution of modularity and complexity among primates. We used the pattern of bone contacts modeled as networks to identify connectivity modules and quantify morphological complexity. We analyzed whether modularity and complexity evolved coordinately in the skull of primates. Specifically, we tested Herbert Simon’s general theory of near-decomposability, which states that modularity promotes the evolution of complexity. We found that the skulls of extant primates divide into one conserved cranial module and up to three labile facial modules, whose composition varies among primates. Despite changes in modularity, statistical analyses reject a positive feedback between modularity and complexity. Our results suggest a decoupling of complexity and modularity that translates to varying levels of constraint on the morphological evolvability of the primate skull. This study has methodological and conceptual implications for grasping the constraints that underlie the developmental and functional integration of the skull of humans and other primates.
The different manifestations of equivalence and similarity in structure throughout evolution suggest a continuous and hierarchical process that starts out with the origin of a morphological novelty, unit, or homologue. Once a morphological unit has originated, its properties change subsequently into variants that differ, in magnitude, from the original properties found in the common ancestor. We will look into the nature of morphological units and their degrees of modification, which will provide the starting point for restructuring the concept of "homology," keeping the use of homology as the identity of an anatomical part, and homogeny, as the specific variation of that anatomical part during evolution. We will also show that parallelism has a distinct placement within an evolutionary continuum between homology and homoplasy, whereas the phenomenon of evolutionary convergence is left outside this continuum. We will then provide some epistemological and developmental criteria to justify these distinctions, showing that there is a direct relation between the nature of these concepts and the constraints that developmental mechanisms impose on evolution. Finally, we will propose a hierarchical model that places homology, homogeny, homoplasy, and parallelism, as distinct phenomena within an evolutionary continuum. J. Exp. Zool. (Mol. Dev. Evol.) 00B: 1-13, 2015. © 2015 Wiley Periodicals, Inc. © 2015 Wiley Periodicals, Inc.
Mosaic evolution is a key mechanism that promotes robustness and evolvability in living beings. For the human head, to have a modular organization would imply that each phenotypic module could grow and function semi-independently. Delimiting the boundaries of head modules, and even assessing their existence, is essential to understand human evolution. Here we provide the first study of the human head using anatomical network analysis (AnNA), offering the most complete overview of the modularity of the head to date. Our analysis integrates the many biological dependences that tie hard and soft tissues together, arising as a consequence of development, growth, stresses and loads, and motion. We created an anatomical network model of the human head, where nodes represent anatomical units and links represent their physical articulations. The analysis of the human head network uncovers the presence of 10 musculoskeletal modules, deep-rooted in these biological dependences, of developmental and evolutionary significance. In sum, this study uncovers new anatomical and functional modules of the human head using a novel quantitative method that enables a more comprehensive understanding of the evolutionary anatomy of our lineage, including the evolution of facial expression and facial asymmetry.
The enameloid microstucture of chondrichthyan teeth has been studied for decades and it has proven to be a useful taxonomic tool. Changes in enameloid organization have been related to the emergence of new trophic strategies and Mesozoic radiation of the neoselachian crown group. However, in contrast to the abundance of these data on tooth enameloid, descriptions of chondrichthyan scale enameloid are almost nonexistent. The topology and microstructure of scale enameloid in particular euselachian groups: fossil Mesozoic Hybodontiformes and living neoselachians, including batoids and sharks, are described. It is shown that a thick layer of single crystallite enameloid (SCE) covers all studied scales. Although the enameloid of scales clearly does not reach high levels of microstructural differentiation present in the dental enameloid of some neoselachians, we found some degree of organization, such as oriented crystallites, differentiation into sublayers, and the presence of poorly structured sets of densely arranged parallel crystallites. As scales lack feeding functions of teeth, we suggest that the emergence of microstructural organization/differentiation of chondrichthyan enameloid can be understood as consequence of a self-organizing process rather than adaptive pressure.
- Jun 2014
Craniofacial sutures and synchondroses form the boundaries between bones in the human skull, providing functional, developmental and evolutionary information. Bone articulations in the skull arise due to interactions between genetic regulatory mechanisms and epigenetic factors such as functional matrices (soft tissues and cranial cavities), which mediate bone growth. These matrices are largely acknowledged for their influence on shaping the bones of the skull; however, it is not fully understood to what extent functional matrices mediate the formation of bone articulations. Aiming to identify whether or not functional matrices are key developmental factors guiding the formation of bone articulations, we have built a network null model of the skull that simulates unconstrained bone growth. This null model predicts bone articulations that arise due to a process of bone growth that is uniform in rate, direction and timing. By comparing predicted articulations with the actual bone articulations of the human skull, we have identified which boundaries specifically need the presence of functional matrices for their formation. We show that functional matrices are necessary to connect facial bones, whereas an unconstrained bone growth is sufficient to connect non-facial bones. This finding challenges the role of the brain in the formation of boundaries between bones in the braincase without neglecting its effect on skull shape. Ultimately, our null model suggests where to look for modified developmental mechanisms promoting changes in bone growth patterns that could affect the development and evolution of the head skeleton.
Despite the many reconstructions of fossil material that have recently appeared in the literature, dinosaur eggshells have never been reconstructed using computing techniques. Using the EMAC 3-D modelling methodology, we reconstruct a section of the Late Cretaceous Megaloolithus siruguei eggshell, which has a particularly complex pore system, exhibiting an intricate network of vertical, oblique, and horizontal pores.
- Jan 2013
Despite the many reconstructions of fossil material that have recently appeared in the literature, dinosaur eggshells have never been reconstructed using computing techniques. Using the EMAC 3-D modelling methodology, we reconstruct a section of the Late Cretaceous Megaloolithus siruguei eggshell, which has a particularly complex pore system, exhibiting an intricate network of vertical, oblique, and horizontal pores.
- Jun 2012
Ever since the appearance of the first land vertebrates, the skull has undergone a simplification by loss and fusion of bones in all major groups. This well-documented evolutionary trend is known as “Williston’s Law”. Both loss and fusion of bones are developmental events that generate, at large evolutionary scales, a net reduction in the number of skull bones. We reassess this evolutionary trend by analyzing the patterns of skull organization captured in network models in which nodes represent bones and links represent suture joints. We also evaluate the compensatory process of anisomerism (bone specialization) suggested to occur as a result of this reduction by quantifying the heterogeneity and the ratio of unpaired bones in real skulls. Finally, we perform simulations to test the differential effect of bone losses in skull evolution. We show that the reduction in bone number during evolution is accompanied by a trend toward a more complex organization, rather than toward simplification. Our results indicate that the processes by which bones are lost or fused during development are central to explain the evolution of the morphology of the skull. Our simulations suggest that the evolutionary trend of increasing morphological complexity can be caused as a result of a structural constraint, the systematic loss of less connected bones during development.
Network theory has been extensively used to model the underlying structure of biological processes. From genetics to ecology, network thinking is changing our understanding of complex systems, specifically how their internal structure determines their overall behavior. Concepts such as hubs, scale-free or small-world networks, common in the complexity literature, are now used more and more in sociology, neurosciences, as well as other anthropological fields. Even though the use of network models is nowadays so widely applied, few attempts have been carried out to enrich our understanding in the classical morphological sciences such as in comparative anatomy or physical anthropology. The purpose of this article is to introduce the usage of network tools in morphology; specifically by building anatomical networks, dealing with the most common analyses and problems, and interpreting their outcome.
- Sep 2009
Evo‐Devo, the science that puts together in a common framework the dynamics of evolution with the processes of embryonic development is inherently multiscale. The hierarchical organization of life phenomena also contributes to the possibility of reducing its complexity to workable modules. However, the emphasis on a compositional, or blocks‐within‐blocks kind of hierarchy, implies a reductionistic perspective on multiscaling that ignores the irreducibility of some levels. The notion of generative hierarchies tackles this problem, introducing an organicist perspective that, while keeping levels of organization, acknowledges the existence of breaks in the hierarchy at the genomic, cellular, individual, and species levels. Whereas independent modeling in development or evolution has been done at each scale of organization, no multiscale approaches have so far been worked out that can account for the relationship between these two fundamental mechanisms that have shaped biodiversity throughout the history of life on Earth.
- Jan 2009
Evo-Devo, the science that puts together in a common framework the dynamics of evolution with the processes of embryonic development is inherently multiscale. The hierarchical organization of life phenomena also contributes to the possibility of reducing its complexity to workable modules. However, the emphasis on a compositional, or blocks-within-blocks kind of hierarchy, implies a reductionistic perspective on multiscaling that ignores the irreducibility of some levels. The notion of generative hierarchies tackles this problem, introducing an organicist perspective that, while keeping levels of organization, acknowledges the existence of breaks in the hierarchy at the genomic, cellular, individual, and species levels. Whereas independent modeling in development or evolution has been done at each scale of organization, no multiscale approaches have so far been worked out that can account for the relationship between these two fundamental mechanisms that have shaped biodiversity throughout the history of life on Earth. Organization and Complexity in Evo-devo Departing from evolutionary theory and developmental theory, the field of evolutionary developmental biology (evo-devo) has flourished in recent years, fuelled by the discovery of the so called "developmental genetic toolkit," a suit of genes used during development shared by most animals . Before this renaissance, evo-devo had also a rich research tradition starting back in the 19th century with the French anatomist Etienne Geoffroy St Hilaire, and the German embryologists Karl von Baer and Ernst Haeckel (both famous for their law-like semiempirical developmental observations that ended up in Haeckel's linkage of ontogeny with phytogeny), and later in the 20th century, outstanding figures such as Garstang, De Beer, Waddington, Gould, and Alberch [2,3 and references therein]. In essence, this "old" evo-devo, was focused on morphological issues in a comparative framework. The emphasis of this morphological evo-devo was on how anatomical parts differed in related species as a result of specific growth rates. This kind of research was subsumed under the all encompassing theme of heterochrony, which has still a very active role in the field. However, the morphological evo-devo tradition has re-invented itself, going beyond heterochrony as a main focus, to embrace other issues such as modularity, innovation and emergence of morphological traits, and phenotypic plasticity . In the background of this tradition is the question of biological organization and biological complexity. Whereas organization has been "solved" by resourcing to hierarchy and modularity, biological complexity is one of those concepts for which there are no universal metrics; hence, it has rarely been used as a proxy for evolution and never to infer systematic relationships [5,6,7]. An important contribution to the debate on complexity was Herbert Simon's article "On
- Jan 2009
- Adaptación y Evolución. 150 años después del Origen de las Especies
El concepto de cambio morfológico direccional jugó un papel clave en los orígenes de la teoría evolutiva. Su vigencia hoy en día se ha visto menoscabada por el neutralismo y el equilibrio puntuado, dos teorías procedentes de campos tan distantes como la genética de poblaciones y la paleontología. Sin embargo, el estudio del cambio direccional sigue contribuyendo de manera importante a la teoría evolutiva en su conjunto. La descripción y formalización de tendencias y patrones morfológicos direccionales a lo largo del proceso evolutivo ha estado tradicionalmente ligada al enunciado de leyes empíricas. Ejemplos de ello son la Ley de Cope de incremento de tamaño, la ley de Dollo de irreversibilidad, el establecimiento de las reglas de proporción alométrica, las reglas ecomorfológicas relativas al tamaño y a la superficie del cuerpo y las reglas de simetría o de repetición de segmentos corporales en la arquitectura animal. Las tendencias pueden generarse como resultado de la acción concertada de diversos procesos internos y externos. Entre los primeros destacan las consecuencias de las restricciones impuestas por el programa de desarrollo y el cambio en las tasas de desarrollo. Una hipótesis reciente identifica a la tendencia al aumento del tamaño corporal en algún momento de la historia evolutiva de un linaje como nexo de unión entre las distintas escalas de organización biológica. Según dicha hipótesis, las tendencias evolutivas se suceden dentro de un ciclo causal que liga la complejidad genómica y morfológica, el tamaño corporal y el tamaño poblacional. La acción concertada de estos tres factores integra a los procesos externos (procedentes del ambiente) e internos (como parte de la dinámica del desarrollo) como motores del cambio evolutivo.
Pere Alberch's work did not take the path of molecular biology. As a result, his contributions to a morphological evo-devo have been neglected by the molecular evodevo community. I will show, in the following sections, the key elements that, in my opinion, form the core of how molecular biologists embraced this specifi c kind of evodevo, which, at present, is overshadowing Alberch’s conceptual legacy. I will analyze the main theme upon which molecular evo-devo has been built, namely, the discovery of shared developmental genes across animal phyla. In doing so, I will show that by putting emphasis on cell diff erentiation, molecular evo-devo became a developmental biology with an evolutionary fl avour, concentrating on the establishment of animal Bauplan and the early specifi cation of animal symmetry axes, providing molecular answers to old morphological questions. I will fi nally look at some points in common that can make them coalesce into an integrated whole that will deepen our understanding of the causal relationship between development and evolution (for a complementary glimpse of the nature of morphological evo-devo see Etxeberria and Nuño de la Rosa, and De Renzi, this book, as well as Pere Alberch’s own articles in this book).
- Jun 2008
The theory of Punctuated Equilibria challenges the neo-Darwinian tenet that evolution is a uniform process. Recently, an article by Hunt has found that directional change during the evolution of a lineage is relatively small (occurring only in 5% of 250 analyzed traits). Of those traits that were shown to follow a trend, size was more likely to show gradual changes, whereas shape changes were more random. Here, we provide a short view of the nature of evolutionary trends, showing that directional change within lineages and among clades provides valuable evolutionary information about the processes involved in their generation.
We describe a new methodology for rapid 2D and 3D computer analysis and visualisation of gene expression and gene product pattern in the context of anatomy and tissue architecture. It is based on episcopic imaging of embryos and tissue samples, as they are physically sectioned, thereby producing inherently aligned digital image series and volume data sets, which immediately permit the generation of 3D computer representations. The technique uses resin as embedding medium, eosin for unspecific tissue staining, and colour reactions (beta-galactosidase/Xgal or BCIP/NBT) for specific labelling of gene activity and mRNA pattern. We tested the potential of the method for producing high-resolution volume data sets of adult human and porcine tissue samples and of specifically and unspecifically stained mouse, chick, quail, frog, and zebrafish embryos. The quality of the episcopic images resembles the quality of digital images of true histological sections with respect to resolution and contrast. Specifically labelled structures can be extracted using simple thresholding algorithms. Thus, the method is capable of quickly and precisely detecting molecular signals simultaneously with anatomical details and tissue architecture. It has no tissue restrictions and can be applied for analysis of human tissue samples as well as for analysis of all developmental stages of embryos of a wide variety of biomedically relevant species.
Gradient formation is a fundamental patterning mechanism during embryo development, commonly related to secreted proteins that move along an existing field of cells. Here, we mathematically address the feasibility of gradients of mRNAs and non-secreted proteins. We show that these gradients can arise in growing tissues whereby cells dilute and transport their molecular content as they divide and grow, a mechanism we termed 'cell lineage transport.' We provide an experimental test by unveiling a distal-to-proximal gradient of Hoxd13 in the vertebrate developing limb bud driven by cell lineage transport, corroborating our model. Our study indicates that gradients of non-secreted molecules exhibit a power-law profile and can arise for a wide range of biologically relevant parameter values. Dilution and nonlinear growth confer robustness to the spatial gradient under changes in the cell cycle period, but at the expense of sensitivity in the timing of gradient formation. We expect that gradient formation driven by cell lineage transport will provide future insights into understanding the coordination between growth and patterning during embryonic development.
- Nov 2005
Nodal cilia dynamics is a key factor for left/right axis determination in mouse embryos through the induction of a leftward fluid flow. So far it has not been clearly established how such dynamics is able to induce the asymmetric leftward flow within the node. Herein we propose that an asymmetric two-phase nonplanar beating cilia dynamics that involves the bending of the ciliar axoneme is responsible for the leftward fluid flow. We support our proposal with a host of hydrodynamic arguments, in silico experiments and in vivo video microscopy data in wild-type embryos and inv mutants. Our phenomenological modeling approach underscores how the asymmetry and speed of the flow depends on different relevant parameters. In addition, we discuss how the combination of internal and external mechanisms might cause the two-phase beating cilia dynamics.
We present a vector field method for obtaining the spatial organization of three-dimensional patterns of gene expression based on gradients and lines of force obtained by numerical integration. The convergence of these lines of force in local maxima are centers of gene expression, providing a natural and powerful framework to characterize the organization and dynamics of biological structures. We apply this methodology to analyze the expression pattern of the enhanced green fluorescent protein (EGFP) driven by the promoter of light chain myosin II during zebrafish heart formation.
We present a vector field method for obtaining the spatial organization of three-dimensional patterns of gene expression based on gradients and lines of force obtained by numerical integration. The convergence of these lines of force in local maxima are centers of gene expression, providing a natural and powerful framework to characterize the organization and dynamics of biological structures. We apply this methodology to analyze the expression pattern of the enhanced green fluorescent protein EGFP driven by the promoter of light chain myosin II during zebrafish heart formation.
- May 2004
Morphospaces are theoretical tools to explore the morphological organization of living and fossil organisms. They have been used mostly by the paleontological community in an effort to get the most out of one of the only pieces of evidence that fossil material usually provide: the morphology of hard parts. The expectation with the establishment of theoretical morphospaces is that, by abstracting and modeling the fundamental parts of form, the multiple processes that generate the phenotypes of embryonic and adult structures will be better understood. In this essay, we suggest that ontogenetic trajectories can be used as the generative functions that build morphospaces, and propose approaches to build theoretical models for the establishment of left-right asymmetries during vertebrate heart embryogenesis.
During vertebrate embryo development, the breaking of the initial bilateral symmetry is translated into asymmetric gene expression around the node and/or in the lateral plate mesoderm. The earliest conserved feature of this asymmetric gene expression cascade is the left-sided expression of Nodal, which depends on the activity of the Notch signalling pathway. Here we present a mathematical model describing the dynamics of the Notch signalling pathway during chick embryo gastrulation, which reveals a complex and highly robust genetic network that locally activates Notch on the left side of Hensen's node. We identify the source of the asymmetric activation of Notch as a transient accumulation of extracellular calcium, which in turn depends on left-right differences in H+/K+-ATPase activity. Our results uncover a mechanism by which the Notch signalling pathway translates asymmetry in epigenetic factors into asymmetric gene expression around the node.
El Bauplan de los animales bilaterales se caracteriza por la demarcación en estadíos tempranos de tres ejes embrionarios. Uno de estos ejes define el plano de simetría bilateral del futuro adulto, el cual divide a estos organismos, en teoría, en dos mitades especulares: izquierda y derecha. Las especies de este grupo se distancian de un modo significativo y robusto de ese Bauplan bilateral ideal, exhibiendo un conjunto de caracteres asimétricos. Este artículo caracteriza los diferentes patrones y procesos en donde se presenta la asimetría, destacando aspectos tanto filogenéticos como ontogenéticos de esta característica básica de los Bilateria. Se apunta, también, el posible papel de la simetría y la asimetría como vehículos para incrementar la cantidad de complejidad en la organización animal. The body plan of bilateral animals is characterized by the early specification of three embryonic axes. One of these axes defines the plane of bilateral symmetry of the future adult, ideally dividing these organisms in two mirror-image halves, left and right. Most species of this group exhibit significant and robust departures from this ideal bilateral plan, showing a suite of asymmetric characters. This paper characterizes different patterns and processes of asymmetry, highlighting both phylogenetic and ontogenetic aspects of such a basic feature of Bilateria. In addition, the role of symmetry and asymmetry as a way to increase the amount of complexity in animal organization is explored.
[EN] The body plan of bilateral animals is characterized by the early specification of three embryonic axes. One of these axes defines the plane of bilateral symmetry of the future adult, ideally dividing these organisms in two mirror-image halves, left and right. Most species of this group exhibit significant and robust departures from this ideal bilateral plan, showing a suite of asymmetric characters. This paper characterizes different patterns and processes of asymmetry, highlighting both phylogenetic and ontogenetic aspects of such a basic feature of Bilateria. In addition, the role of symmetry and asymmetry as a way to increase the amount of complexity in animal organization is explored [ES] El Bauplan de los animales bilaterales se caracteriza por la demarcación en estadíos tempranos de tres ejes embrionarios. Uno de estos ejes define el plano de simetría bilateral del futuro adulto, el cual divide a estos organismos, en teoría, en dos mitades especulares: izquierda y derecha. Las especies de este grupo se distancian de un modo significativo y robusto de ese Bauplan bilateral ideal, exhibiendo un conjunto de caracteres asimétricos. Este artículo caracteriza los diferentes patrones y procesos en donde se presenta la asimetría, destacando aspectos tanto filogenéticos como ontogenéticos de esta característica básica de los Bilateria. Se apunta, también, el posible papel de la simetría y la asimetría como vehículos para incrementar la cantidad de complejidad en la organización animal. This paper has been partly funded by grant PB98- 0813 of the DGYCIT. Peer reviewed
Theoretical models of skeletal structures provide suitable frameworks to assess macroevolutionary patterns of form change. We discuss three theoretical approaches to account for morphological patterns of the pelvic girdle in archosaurs. Every approach targets a different level of organization within the concept of morphospace. First, we build a morphocline by applying a mathematical transformation to the outline of the hip of the theropod dinosaur Deinonychus antirrhopus, in order to look at theoretical paths of evolutionary change based on changes of proportion. Second, we analyze the variability of a sample of 86 hips within a theoretical construction that incorporates information about the spatial orientation of the three paired bones that build this skeletal compound. Finally, we look at boundary patterns within these hips as a basis for generating a formalism based on graph theory. Insights about the evolution and development of the archosaur triradiate pelvis and its morphological trends are suggested in the light of each theoretical approach, with a special focus on the convergent evolution of a retroverted pubis in ornithischians and birds.
- Jan 2000
THEORETICAL MORPHOLOGY: THE CONCEPT AND ITS APPLICATIONS, by George R. McGhee, 1999. Columbia University Press, New York, 316 p. ISBN 0–231–10616–5 (hardback), $60.00; ISBN 0–231–10617–3 (paperback), $26.50
The comparison of bone homology between the manus of an Early Cretaceous fossil crocodile and that of the extant species Alligator mississippiensis supports explicidy, for the first time, the hypothesis of carpal loss in crocodilian limb evolution. This hypothesis, based on a developmental model of the organization of the tetrapod limb, is in accordance with the fossil evidence, and may supersede traditional Haeckelian views based on recapitulatory paradigms. The homologous relationships of carpal elements reveal the existence of two carpal patterns- one plesiomorphic and one apomorphic-in the crocodilian lineage. Phylogenetic change is explained causally by alterations of the osteogenesis of the distal carpals 2 and 3, which remain unossified in extant crocodile adults. This implies that crocodilian limb evolution is constrained by a process of paedomorphosis. This modification of the architecture of the crocodilian hand is a terminal event of its evolutionary history, affecting only eusuchian crocodiles. The results of this study contest the traditional view that the skeletal pattern of the crocodilian limb has been conserved unchanged since the Triassic.
The comparison of bone homology between the manus of an Early Cretaceous fossil crocodile and that of the extant speciesAlligator mississippiensissupports explicitly, for the first time, the hypothesis of carpal loss in crocodilian limb evolution. This hypothesis, based on a developmental model of the organization of the tetrapod limb, is in accordance with the fossil evidence, and may supersede traditional Haeckelian views based on recapitulatory paradigms. The homologous relationships of carpal elements reveal the existence of two carpal patterns—one plesiomorphic and one apomorphic—in the crocodilian lineage. Phylogenetic change is explained causally by alterations of the osteogenesis of the distal carpals 2 and 3, which remain unossified in extant crocodile adults. This implies that crocodilian limb evolution is constrained by a process of paedomorphosis. This modification of the architecture of the crocodilian hand is a terminal event of its evolutionary history, affecting only eusuchian crocodiles. The results of this study contest the traditional view that the skeletal pattern of the crocodilian limb has been conserved unchanged since the Triassic.
A feathered skeleton of a Lower Cretaceous enantiornithine bird from Spain indicates that the modified diapsid skull of modern birds did not evolve until late in their evolution: Basal birds retained an essentially primitive diapsid design. The fossil provides data clarifying long-standing debates on the cranial morphology of the basalmost bird, Archaeopteryx. It also reemphasizes the notion that the early morphological transformations of birds were focused on the flight apparatus. This fossil was a nestling and suggests that early postnatal developments in the Cretaceous enantiornithine birds and those in their extant counterparts are comparable.
Affine change might be used to seek general patterns of variation or to envisage new ways of describing and evaluating form change. In this paper, we assume a theoretical perspective and show a strategy for generating affine morphospaces (ordered collections of affine-transformed images of a base form), by means of a new computer program called D’ARCYGRAPH. We propose the use of affine transformations as models of change that might induce the search for new explanations based on the constraints imposed by the properties of affine change, that is, their invariants. We introduce a technique to elaborate affine morphospaces; as an example, we build the morphospace of pelvic girdles generated by using as a base form the pelvis of Deinonychus antirrhopus (Dinosauria, Theropoda). We explore possible transformation paths from this base form, which is considered to be the closest sister group of the major clade Ayes. A discussion of the type of variables (called “dispositionals”) that can be used and explained by the affine model is provided. We conclude that it is possible to build an affine trend that simulates the fossil record transition, Theropoda—Aves, according to these dispositional variables. Also, we introduce an interactive strategy for the superimposition of forms, without any automatic optimality criteria. A comparison with other superimposition methods, based on tridactyl dinosaur footprints is discussed.
THE Lower Cretaceous lithographic limestones from Las Hoyas (province of Cuenca, Spain) have yielded important vertebrate fossil remains. We report here a new specimen, the first ornithomimosaur theropod found in Europe. Pelecanimimus polyodon gen. et sp. nov., has some striking elements preserved, such as the hyoid, sternum and integumentary impressions. The fossil has revealed other unexpected features, including a derived hand in an ancient ornithomimosaur, and a large number of teeth (over 200) with a distinctive morphology. This specimen suggests an alternative evolutionary process towards the toothless condition in Ornithomi-mosauria, which could be explained by an exaptation. Pelecanimimus polyodon stresses the relationship between Troodontidae and Ornithomimosauria.