First anatomical network analysis of fore- and hindlimb musculoskeletal modularity in bonobos, common chimpanzees, and humans

Evolutionary insights from an anatomical network analysis of the hyolaryngeal apparatus in extant archosaurs (birds and crocodilians)

Adaptive radiation of archosaurs, represented by crocodilians, non-avian dinosaurs, and birds, since the Mesozoic has been studied mainly based on their major skeletal elements (skull, vertebrae, and limbs). However, little is known about the evolution of their hyolaryngeal apparatus, which is involved with feeding, respiration, and vocalization, because of poor fossil preservation and the difficulty in determining the musculoskeletal homology of the apparatus. Network analysis is a framework to quantitatively characterize the topological organization of anatomical structures for comparing structural integration and modularity regardless of ambiguous homology. Herein, we modeled the musculoskeletal system of hyolarynx in six species of extant archosaurs and its sister-taxon turtle, and conducted a network analysis using network parameters, modular partition, and bone centrality in a phylogenetic framework. The network parameters reveal that ancestral archosaurs have reduced the numbers of elements and links and acquired complex networks as a whole domain with strong modularity in the hyolarynx. Furthermore, the modular partition and centrality reveal that the hyoids are highly evolvable, while the larynx is constrained and less evolvable. The archosaur hyolarynx exhibits different evolutionary trends: crocodilians with the system integration, basihyal simplification, and ceratobranchial centralization; and birds with the simplicity, weak integration, and modularity of the hyolarynx, laryngeal integration with cricoid centrality, and tongue-module expansion with the acquisition of paraglossal. Four hyolaryngeal bones (ceratobranchial, basihyal, paraglossal, and cricoid) have played important roles in archosaur evolution, and their fossil records are keys to understanding the two major archosaur lineages toward crocodilians and birds.

Morphological integration in the hominid midfoot

The calculation of morphological integration across living apes and humans may provide important insights into the potential influence of integration on evolutionary trajectories in the hominid lineage. Here, we quantify magnitudes of morphological integration among and within elements of the midfoot in great apes and humans to examine the link between locomotor differences and trait covariance. We test the hypothesis that the medial elements of the great ape foot are less morphologically integrated with one another compared to humans based on their abducted halluces, and aim to determine how adaptations for midfoot mobility/stiffness and locomotor specialization influence magnitudes of morphological integration. The study sample is composed of all cuneiforms, the navicular, the cuboid, and metatarsals 1-5 of Homo sapiens (n = 80), Pan troglodytes (n = 63), Gorilla gorilla (n = 39), and Pongo sp. (n = 41). Morphological integration was quantified using the integration coefficient of variation of interlandmark distances organized into sets of a priori-defined modules. Magnitudes of integration across these modules were then compared against sets of random traits from the whole midfoot. Results show that all nonhuman apes have less integrated medial elements, whereas humans have highly integrated medial elements, suggesting a link between hallucal abduction and reduced levels of morphological integration. However, we find considerable variation in magnitudes of morphological integration across metatarsals 2-5, the intermediate and lateral cuneiform, the cuboid, and navicular, emphasizing the influence of functional and nonfunctional factors in magnitudes of integration. Lastly, we find that humans and orangutans show the lowest overall magnitudes of integration in the midfoot, which may be related to their highly specialized functions, and suggest a link between strong diversifying selection and reduced magnitudes of morphological integration.

A network approach to the topological organization of the Brodmann map

Brain morphology is the result of functional factors associated with cortical areas, but it is also influenced by structural aspects due to physical and spatial constraints. Despite the noticeable advances in brain mapping, Brodmann's map is still used in many research fields that rely on macroscopic cortical features for practical or theoretical issues. Here, the topological relationships among the Brodmann areas were modelled according to the principles of network analysis, in order to provide a synthetic view of their spatial properties following a criterion of contiguity. The model evidences the importance of the parieto-temporal region in terms of biological burden and topological complexity. The retrosplenial region is particularly influenced by spatial constraints, and the cingulate cortex occupies a position that bridges the anterior and posterior topological blocks. Such spatial framework should be taken into account when dealing with brain morphology in both ontogeny and phylogeny. This article is protected by copyright. All rights reserved.

Anatomical comparison across heads, fore- and hindlimbs in mammals using network models

Animal body parts evolve with variable degrees of integration that nonetheless yield functional adult phenotypes: but, how? The analysis of modularity with Anatomical Network Analysis (AnNA) is used to quantitatively determine phenotypic modules based on the physical connection among anatomical elements, an approach that is valuable to understand developmental and evolutionary constraints. We created anatomical network models of the head, forelimb, and hindlimb of two taxa considered to represent a 'generalized' eutherian (placental: mouse) and metatherian (marsupial: opossum) anatomical configuration and compared them with our species, which has a derived eutherian configuration. In these models, nodes represent anatomical units and links represent their physical connection. Here, we aimed to identify: (1) the commonalities and differences in modularity between species, (2) whether modules present a potential phylogenetic character, and (3) whether modules preferentially reflect either developmental or functional aspects of anatomy, or a mix of both. We predicted differences between networks of metatherian and eutherian mammals that would best be explained by functional constraints, versus by constraints of development and/or phylogeny. The topology of contacts between bones, muscles, and bones + muscles showed that, among all three species, skeletal networks were more similar than musculoskeletal networks. There was no clear indication that humans and mice are more alike when compared to the opossum overall, even though their musculoskeletal and skeletal networks of fore- and hindlimbs are slightly more similar. Differences were greatest among musculoskeletal networks of heads and next of forelimbs, which showed more variation than hindlimbs, supporting previous anatomical studies indicating that in general the configuration of the hindlimbs changes less across evolutionary history. Most observations regarding the anatomical networks seem to be best explained by function, but an exception is the adult opossum ear ossicles. These ear bones might form an independent module because the incus and malleus are involved in forming a functional primary jaw that enables the neonate to attach to the teat, where this newborn will complete its development. Additionally, the human data show a specialized digit 1 module (thumb/big toe) in both limb types, likely the result of functional and evolutionary pressures, as our ape ancestors had highly movable big toes and thumbs.

Delimiting the boundaries of sesamoid identities under the network theory framework

Sesamoid identity has long been the focus of debate, and how they are linked to other elements of the skeleton has often been considered relevant to their definition. A driving hypothesis of our work was that sesamoids’ nature relies deeply on their connections, and thus we propose an explicit network framework to investigate this subject in Leptodactylus latinasus (Anura: Leptodactylidae). Through the dissection of L. latinasus’ skeleton, we modeled its anatomical network where skeletal elements were considered nodes while joints, muscles, tendons, and aponeurosis were considered links. The skeletal elements were categorized into canonical skeletal pieces, embedded sesamoids, and glide sesamoids. We inquired about the general network characterization and we have explored further into sesamoid connectivity behavior. We found that the network is structured in a modular hierarchical organization, with five modules on the first level and two modules on the second one. The modules reflect a functional, rather than a topological proximity clustering of the skeleton. The 25 sesamoid pieces are members of four of the first-level modules. Node parameters (centrality indicators) showed that: (i) sesamoids are, in general terms, peripheral elements of the skeleton, loosely connected to the canonical bone structures; (ii) embedded sesamoids are not significantly distinguishable from canonical skeletal elements; and (iii) glide sesamoids exhibit the lowest centrality values and strongly differ from both canonical skeletal elements and embedded sesamoids. The loose connectivity pattern of sesamoids, especially glides, could be related to their evolvability, which in turn seems to be reflected in their morphological variation and facultative expression. Based on the connectivity differences among skeletal categories found in our study, an open question remains: can embedded and glide sesamoids be defined under the same criteria? This study presents a new approach to the study of sesamoid identity and to the knowledge of their morphological evolution.

First use of anatomical networks to study modularity and integration of heads, forelimbs and hindlimbs in abnormal anencephalic and cyclopic vs normal human development

The ill-named "logic of monsters" hypothesis of Pere Alberch - one of the founders of modern evo-devo - emphasized the importance of "internal rules" due to strong developmental constraints, linked teratologies to developmental processes and patterns, and contradicted hypotheses arguing that birth defects are related to a chaotic and random disarray of developmental mechanisms. We test these hypotheses using, for the first time, anatomical network analysis (AnNA) to study and compare the musculoskeletal modularity and integration of both the heads and the fore- and hindlimbs of abnormal cyclopic trisomy 18 and anencephalic human fetuses, and of normal fetal, newborn, and adult humans. Our previous works have shown that superficial gross anatomical analyses of these specimens strongly support the "logic of monsters" hypothesis, in the sense that there is an 'order' or 'logic' within the gross anatomical patterns observed in both the normal and abnormal individuals. Interestingly, the results of the AnNA done in the present work reveal a somewhat different pattern: at least concerning the musculoskeletal modules obtained in our AnNA, we observe a hybrid between the "logic of monsters" and the "lack of homeostasis" hypotheses. For instance, as predicted by the latter hypothesis, we found a high level of left-right asymmetry in the forelimbs and/or hindlimbs of the abnormal cyclopic trisomy 18 and anencephalic human fetuses. That is, a network analysis of the organization of/connection between the musculoskeletal structures of these fetuses reveals a more "chaotic" pattern than that detected by superficial gross anatomical comparisons. We discuss the broader developmental, evolutionary, and medical implications of these results.