Canalization, developmental stability, and morphological integration in primate limbs

Mirror, mirror? An evaluation of identical twin mirroring in tooth crown morphology

It has been estimated that 25% of monozygotic ("identical") twin pairs exhibit reverse asymmetry (RA) or "mirroring" of minor anatomical features as a result of delayed zygote division. Here, we examine whether identical twin mirroring accounts for patterns of dental asymmetry in a sample of monozygotic and dizygotic ("fraternal") twins. We focus on crown morphology to approach the following question: is there an association between dental RA frequency and twin type suggestive of the presence of mirror image twins in our sample? Data were collected from 208 deciduous and 196 permanent dentitions of participants of the University of Adelaide Twin Study using Arizona State University Dental Anthropology System standards. RA frequencies were compared across morphological complexes (deciduous, permanent), twin types (monozygotic, dizygotic), and traits. Fisher's exact tests were performed to formally evaluate the association between twin type and dental RA. Across the entire dataset, RA rates failed to exceed 8% for any twin type. In monozygotic twins, deciduous mirroring totaled 5.3% of observed cases, while permanent mirroring totaled 7.8% of observed cases. We found no statistically significant association between RA and twin type for any morphological character (p-value range: 0.07-1.00). Our results suggest the timing of monozygotic twin division does not explain the structure of asymmetry for our morphology dataset and that published estimates of identical twin mirroring rates may be inflated or contingent upon phenotype. Instead, rates reported for this sample more closely align with the proposed etiology of this condition.

Reduced limb integration characterizes primate clades with diverse locomotor adaptations

Hominoids exhibit a strikingly diverse set of locomotor adaptations-including knuckle-walking, brachiation, quadrumanuous suspension, and striding bipedalism-while also possessing morphologies associated with forelimb suspension. It has been suggested that changes in limb element integration facilitated the evolution of diverse locomotor modes by reducing covariation between serial homologs and allowing the evolution of a greater diversity of limb lengths. Here, I compare limb element integration in hominoids with that of other primate taxa, including two that have converged with them in forelimb morphology, Ateles and Pygathrix. Ateles is part of a clade that, such as hominoids, exhibits diverse locomotor adaptations, whereas Pygathrix is an anomaly in a much more homogeneous (in terms of locomotor adaptations) clade. I find that all atelines (and possibly all atelids), not just Ateles, share reduced limb element integration with hominoids. Pygathrix does not, however, instead resembling other members of its own family. Indriids also seem to have higher limb integration than apes, despite using their forelimbs and hindlimbs in divergent ways, although there is more uncertainty in this group due to poor sample size. These results suggest that reduced limb integration is characteristic of certain taxonomic groups with high locomotor diversity rather than taxa with specific, specialized locomotor adaptations. This is consistent with the hypothesis that reduced integration serves to open new areas of morphospace to those clades while suggesting that derived locomotion with divergent demands on limbs is not necessarily associated with reduced limb integration.

New insights into patterns of integration in the femur and pelvis among catarrhines

OBJECTIVES

Integration reflects the level of coordinated variation of the phenotype. The integration of postcranial elements can be studied from a functional perspective, especially with regards to locomotion. This study investigates the link between locomotion, femoral structural properties, and femur-pelvis complex morphology.

MATERIALS AND METHODS

We measured (1) morphological integration between femoral and pelvic morphologies using geometric morphometrics, and (2) covariation between femoral/pelvic morphologies and femoral diaphyseal cross-sectional properties, which we defined as morpho-structural integration. Morphological and morpho-structural integration patterns were measured among humans (n = 19), chimpanzees and bonobos (n = 16), and baboons (n = 14), whose locomotion are distinct.

RESULTS

Baboons show the highest magnitude of morphological integration and the lowest of morpho-structural integration. Chimpanzees and bonobos show intermediate magnitude of morphological and morpho-structural integration. Yet, body size seems to have a considerable influence on both integration patterns, limiting the interpretations. Finally, humans present the lowest morphological integration and the highest morpho-structural integration between femoral morphology and structural properties but not between pelvic morphology and femur.

DISCUSSION

Morphological and morpho-structural integration depict distinct strategies among the samples. A strong morphological integration among baboon's femur-pelvis module might highlight evidence for long-term adaptation to quadrupedalism. In humans, it is likely that distinct selective pressures associated with the respective function of the pelvis and the femur tend to decrease morphological integration. Conversely, high mechanical loading on the hindlimbs during bipedal locomotion might result in specific combination of structural and morphological features within the femur.

Morphological variation, modularity and integration in the scapula and humerus of Lissotriton newts

The modular organization of tetrapod paired limbs and girdles, influenced by the expression of Hox genes is one of the primary driving forces of the evolution of animal locomotion. The increased morphological diversification of the paired limbs is correlated with reduced between-limb covariation, while correlation within the elements is usually higher than between the elements. The tailed amphibians, such as Lissotriton newts, have a biphasic lifestyle with aquatic and terrestrial environments imposing different constraints on limb skeleton. By employing the methods of computerized microtomography and 3D geometric morphometrics, we explored the pattern of morphological variation, disparity, modularity and morphological integration in the proximal parts of the anterior limbs of six species of Eurasian small bodied newts. Although the species significantly differ in limb shape, there is a great overlap in morphology of scapula and humerus, and there are no differences in morphological disparity. For the scapula, the shape differences related to the duration of the aquatic period are in length, depth and curvature. The shape of the humerus is not affected by the length of aquatic period, and shape differences between the species are related to robustness of the body. The length of aquatic period has statistically supported phylogenetic signal. The scapula and humerus are structures of varying modularity. For the humerus, the strongest support on the phylogenetic level was for the capitulum/shaft hypothesis, which can also be interpreted as functional modularity. For the scapula, the greatest support was for the antero-posterior hypothesis of modularity in case of Lissotriton vulgaris, which can be explained by different functional roles and muscle insertion patterns, while there was no phylogenetic modularity. The modularity patterns seem to correspond with the general tetrapod pattern, with modularity being more pronounced in the distal structure. The future research should include more salamandrid taxa with different habitat preferences and both adult and larval stages, in order to explore how size, phylogeny and ecology affect the morphology and covariation patterns of limbs.

Carpal variability and asymmetry in limb reduced Western lesser sirens (Siren nettingi)

Trait functionality can act as a constraint on morphological development. Traits that become vestigialized can exhibit unstable developmental patterns such as fluctuating asymmetry (FA) and variation in populations. We use clearing and staining along with morphometric analyzes to compare FA and allometry of limbs in Western lesser sirens (Siren nettingi) to Ouachita dusky salamanders (Desmognathus brimleyorum). Our results describe new carpal phenotypes and carpal asymmetry in our sample of S. nettingi. However, we found no significant evidence of limb length asymmetry in S. nettingi. The degree of relative limb asymmetry correlates inversely with body size in both of our samples. This work provides strong evidence of increased mesopodal variation within a population of S. nettingi. Our work provides a basis for further study of a broader range of morphological traits across salamanders.

Canalization reduces the nonlinearity of regulation in biological networks

Biological networks, such as gene regulatory networks, possess desirable properties. They are more robust and controllable than random networks. This motivates the search for structural and dynamical features that evolution has incorporated into biological networks. A recent meta-analysis of published, expert-curated Boolean biological network models has revealed several such features, often referred to as design principles. Among others, the biological networks are enriched for certain recurring network motifs, the dynamic update rules are more redundant, more biased, and more canalizing than expected, and the dynamics of biological networks are better approximable by linear and lower-order approximations than those of comparable random networks. Since most of these features are interrelated, it is paramount to disentangle cause and effect, that is, to understand which features evolution actively selects for, and thus truly constitute evolutionary design principles. Here, we compare published Boolean biological network models with different ensembles of null models and show that the abundance of canalization in biological networks can almost completely explain their recently postulated high approximability. Moreover, an analysis of random N-K Kauffman models reveals a strong dependence of approximability on the dynamical robustness of a network.

Small body size is associated with increased evolutionary lability of wing skeleton proportions in birds

Birds are represented by 11,000 species and a great variety of body masses. Modular organisation of trait evolution across birds has facilitated simultaneous adaptation of different body regions to divergent ecological requirements. However, the role modularity has played in avian body size evolution, especially small-bodied, rapidly evolving and diverse avian subclades, such as hummingbirds and songbirds, is unknown. Modularity is influenced by the intersection of biomechanical restrictions, adaptation, and developmental controls, making it difficult to uncover the contributions of single factors such as body mass to skeletal organisation. We develop a novel framework to decompose this complexity, assessing factors underlying the modularity of skeletal proportions in fore-limb propelled birds distributed across a range of body masses. We demonstrate that differences in body size across birds triggers a modular reorganisation of flight apparatus proportions consistent with biomechanical expectations. We suggest weakened integration within the wing facilitates radiation in small birds. Our framework is generalisable to other groups and has the capacity to untangle the multi-layered complexity intrinsic to modular evolution.

Conserved patterns and locomotor-related evolutionary constraints in the hominoid vertebral column

The evolution of the hominoid lineage is characterized by pervasive homoplasy, notably in regions such as the vertebral column, which plays a central role in body support and locomotion. Few isolated and fewer associated vertebrae are known for most fossil hominoid taxa, but identified specimens indicate potentially high levels of convergence in terms of both form and number. Homoplasy thus complicates attempts to identify the anatomy of the last common ancestor of hominins and other taxa and stymies reconstructions of evolutionary scenarios. One way to clarify the role of homoplasy is by investigating constraints via phenotypic integration, which assesses covariation among traits, shapes evolutionary pathways, and itself evolves in response to selection. We assessed phenotypic integration and evolvability across the subaxial (cervical, thoracic, lumbar, sacral) vertebral column of macaques (n = 96), gibbons (n = 77), chimpanzees (n = 92), and modern humans (n = 151). We found a mid-cervical cluster that may have shifted cranially in hominoids, a persistent thoracic cluster that is most marked in chimpanzees, and an expanded lumbosacral cluster in hominoids that is most expanded in gibbons. Our results highlight the highly conserved nature of the vertebral column. Taxa appear to exploit existing patterns of integration and ontogenetic processes to shift, expand, or reduce cluster boundaries. Gibbons appear to be the most highly derived taxon in our sample, possibly in response to their highly specialized locomotion.

A map of signaling responses in the human airway epithelium

Receptor-mediated signaling plays a central role in tissue regeneration, and it is dysregulated in disease. Here, we build a signaling-response map for a model regenerative human tissue: the airway epithelium. We analyzed the effect of 17 receptor-mediated signaling pathways on organotypic cultures to determine changes in abundance and phenotype of epithelial cell types. This map recapitulates the gamut of known airway epithelial signaling responses to these pathways. It defines convergent states induced by multiple ligands and diverse, ligand-specific responses in basal cell and secretory cell metaplasia. We show that loss of canonical differentiation induced by multiple pathways is associated with cell-cycle arrest, but that arrest is not sufficient to block differentiation. Using the signaling-response map, we show that a TGFB1-mediated response underlies specific aberrant cells found in multiple lung diseases and identify interferon responses in COVID-19 patient samples. Thus, we offer a framework enabling systematic evaluation of tissue signaling responses. A record of this paper's transparent peer review process is included in the supplemental information.

Deciphering the origin of developmental stability: The role of intracellular expression variability in evolutionary conservation

Progress in evolutionary developmental biology (evo-devo) has deepened our understanding of how intrinsic properties of embryogenesis, along with natural selection and population genetics, shape phenotypic diversity. A focal point of recent empirical and theoretical research is the idea that highly developmentally stable phenotypes are more conserved in evolution. Previously, we demonstrated that in Japanese medaka (Oryzias latipes), embryonic stages and genes with high stability, estimated through whole-embryo RNA-seq, are highly conserved in subsequent generations. However, the precise origin of the stability of gene expression levels evaluated at the whole-embryo level remained unclear. Such stability could be attributed to two distinct sources: stable intracellular expression levels or spatially stable expression patterns. Here we demonstrate that stability observed in whole-embryo RNA-seq can be attributed to stability at the cellular level (low variability in gene expression at the cellular levels). We quantified the intercellular variations in expression levels and spatial gene expression patterns for seven key genes involved in patterning dorsoventral and rostrocaudal regions during early development in medaka. We evaluated intracellular variability by counting transcripts and found its significant correlation with variation observed in whole-embryo RNA-seq data. Conversely, variation in spatial gene expression patterns, assessed through intraindividual left-right asymmetry, showed no correlation. Given the previously reported correlation between stability and conservation of expression levels throughout embryogenesis, our findings suggest a potential general trend: the stability or instability of developmental systems-and the consequent evolutionary diversity-may be primarily anchored in intrinsic fundamental elements such as the variability of intracellular states.

Ancestral neural circuits potentiate the origin of a female sexual behavior

Courtship interactions are remarkably diverse in form and complexity among species. How neural circuits evolve to encode new behaviors that are functionally integrated into these dynamic social interactions is unknown. Here we report a recently originated female sexual behavior in the island endemic Drosophila species D. santomea, where females signal receptivity to male courtship songs by spreading their wings, which in turn promotes prolonged songs in courting males. Copulation success depends on this female signal and correlates with males' ability to adjust his singing in such a social feedback loop. Functional comparison of sexual circuitry across species suggests that a pair of descending neurons, which integrates male song stimuli and female internal state to control a conserved female abdominal behavior, drives wing spreading in D. santomea. This co-option occurred through the refinement of a pre-existing, plastic circuit that can be optogenetically activated in an outgroup species. Combined, our results show that the ancestral potential of a socially-tuned key circuit node to engage the wing motor program facilitates the expression of a new female behavior in appropriate sensory and motivational contexts. More broadly, our work provides insights into the evolution of social behaviors, particularly female behaviors, and the underlying neural mechanisms.

Deformity or variation? Phenotypic diversity in the zebrafish vertebral column

Vertebral bodies are composed of two types of metameric elements, centra and arches, each of which is considered as a developmental module. Most parts of the teleost vertebral column have a one-to-one relationship between centra and arches, although, in all teleosts, this one-to-one relationship is lost in the caudal fin endoskeleton. Deviation from the one-to-one relationship occurs in most vertebrates, related to changes in the number of vertebral centra or to a change in the number of arches. In zebrafish, deviations also occur predominantly in the caudal region of the vertebral column. In-depth phenotypic analysis of wild-type zebrafish was performed using whole-mount stained samples, histological analyses and synchrotron radiation X-ray tomographic microscopy 3D reconstructions. Three deviant centra phenotypes were observed: (i) fusion of two vertebral centra, (ii) wedge-shaped hemivertebrae and (iii) centra with reduced length. Neural and haemal arches and their spines displayed bilateral and unilateral variations that resemble vertebral column phenotypes of stem-ward actinopterygians or other gnathostomes as well as pathological conditions in extant species. Whether it is possible to distinguish variations from pathological alterations and whether alterations resemble ancestral conditions is discussed in the context of centra and arch variations in other vertebrate groups and basal actinopterygian species.

Quantifying the complexity of plant reproductive structures reveals a history of morphological and functional integration

Vascular plant reproductive structures have undoubtedly become more complex through time, evolving highly differentiated parts that interact in specialized ways. But quantifying these patterns at broad scales is challenging because lineages produce disparate reproductive structures that are often difficult to compare and homologize. We develop a novel approach for analysing interactions within reproductive structures using networks, treating component parts as nodes and a suite of physical and functional interactions among parts as edges. We apply this approach to the plant fossil record, showing that interactions have generally increased through time and that the concentration of these interactions has shifted towards differentiated surrounding organs, resulting in more compact, functionally integrated structures. These processes are widespread across plant lineages, but their extent and timing vary with reproductive biology; in particular, seed-producing structures show them more strongly than spore or pollen-producing structures. Our results demonstrate that major reproductive innovations like the origin of seeds and angiospermy were associated with increased integration through greater interactions among parts. But they also reveal that for certain groups, particularly Mesozoic gymnosperms, millions of years elapsed between the origin of reproductive innovations and increased interactions among parts within their reproductive structures.

Early life drought predicts components of adult body size in wild female baboons

OBJECTIVES

In many taxa, adverse early-life environments are associated with reduced growth and smaller body size in adulthood. However, in wild primates, we know very little about whether, where, and to what degree trajectories are influenced by early adversity, or which types of early adversity matter most. Here, we use parallel-laser photogrammetry to assess inter-individual predictors of three measures of body size (leg length, forearm length, and shoulder-rump length) in a population of wild female baboons studied since birth.

MATERIALS AND METHODS

Using >2000 photogrammetric measurements of 127 females, we present a cross-sectional growth curve of wild female baboons (Papio cynocephalus) from juvenescence through adulthood. We then test whether females exposed to several important sources of early-life adversity-drought, maternal loss, low maternal rank, or a cumulative measure of adversity-were smaller for their age than females who experienced less adversity. Using the "animal model," we also test whether body size is heritable in this study population.

RESULTS

Prolonged early-life drought predicted shorter limbs but not shorter torsos (i.e., shoulder-rump lengths). Our other measures of early-life adversity did not predict variation in body size. Heritability estimates for body size measures were 36%-67%. Maternal effects accounted for 13%-17% of the variance in leg and forearm length, but no variance in torso length.

DISCUSSION

Our results suggest that baboon limbs, but not torsos, grow plastically in response to maternal effects and energetic early-life stress. Our results also reveal considerable heritability for all three body size measures in this study population.

Rapid and biased evolution of canalization during adaptive divergence revealed by dominance in gene expression variability during Arctic charr early development

Adaptive evolution may be influenced by canalization, the buffering of developmental processes from environmental and genetic perturbations, but how this occurs is poorly understood. Here, we explore how gene expression variability evolves in diverging and hybridizing populations, by focusing on the Arctic charr (Salvelinus alpinus) of Thingvallavatn, a classic case of divergence between feeding habitats. We report distinct profiles of gene expression variance for both coding RNAs and microRNAs between the offspring of two contrasting morphs (benthic/limnetic) and their hybrids reared in common conditions and sampled at two key points of cranial development. Gene expression variance in the hybrids is substantially affected by maternal effects, and many genes show biased expression variance toward the limnetic morph. This suggests that canalization, as inferred by gene expression variance, can rapidly diverge in sympatry through multiple gene pathways, which are associated with dominance patterns possibly biasing evolutionary trajectories and mitigating the effects of hybridization on adaptive evolution.

Secular Trends in the Size and Shape of the Scapula among the Portuguese between the 19th and the 21st Centuries

Potential secular changes in the human scapula are fundamentally unbeknownst, with most of the preceding anatomical studies focusing on long-term changes in the long bones and the skull. As such, the cardinal purpose of this study pertains to the evaluation of secular trends on the shape and size of the scapula in a time period spanning from the 19th to the early 21st centuries. The study sample included 211 individuals (100 males and 111 females) from the Coimbra Identified Skeletal Collection and the 21st Century Identified Skeletal Collection. The size and shape of the scapula were evaluated using geometric morphometrics. Results show secular changes over a relatively short period of time in both the shape and size of the scapula in Portuguese nationals. Shape changes were observed in both sexes but expressed minimally, while a significant negative trend in the size of the scapula was detected in males. Scapular size decrement in males conceivably echoes general trends of the overall anatomy towards a narrower body associated with higher standards of living that include enhanced nutrition and universal healthcare, among other factors.

Morphology, evolution, and the whole organism imperative: Why evolutionary questions need multi-trait evolutionary quantitative genetics

Since Washburn's New Physical Anthropology, researchers have sought to understand the complexities of morphological evolution among anatomical regions in human and non-human primates. Researchers continue, however, to preferentially use comparative and functional approaches to examine complex traits, but these methods cannot address questions about evolutionary process and often conflate function with fitness. Moreover, researchers also tend to examine anatomical elements in isolation, which implicitly assumes independent evolution among different body regions. In this paper, we argue that questions asked in primate evolution are best examined using multiple anatomical regions subjected to model-bound methods built from an understanding of evolutionary quantitative genetics. A nascent but expanding number of studies over the last two decades use this approach, examining morphological integration, evolvability, and selection modeling. To help readers learn how to use these methods, we review fundamentals of evolutionary processes within a quantitative genetic framework, explore the importance of neutral evolutionary theory, and explain the basics of evolutionary quantitative genetics, namely the calculation of evolutionary potential for multiple traits in response to selection. Leveraging these methods, we demonstrate their use to understand non-independence in possible evolutionary responses across the limbs, limb girdles, and basicranium of humans. Our results show that model-bound quantitative genetic methods can reveal unexpected genetic covariances among traits that create a novel but measurable understanding of evolutionary complexity among multiple traits. We advocate for evolutionary quantitative genetic methods to be a standard whenever appropriate to keep studies of primate morphological evolution relevant for the next seventy years and beyond.

Adapting the pantograph limb: Differential robustness of fore- and hindlimb kinematics against genetically induced perturbation in the neural control networks and its evolutionary implications

The evolutionary transformation of limb morphology to the four-segmented pantograph of therians is among the milestones of mammalian evolution. But, it is still unknown if changes of the mechanical limb function were accompanied by corresponding changes in development and sensorimotor control. The impressive locomotor performance of mammals leaves no doubt about the high integration of pattern formation, neural control and mechanics. But, deviations from normal intra- and interlimb coordination (spatial and temporal) become evident in the presence of perturbations. We induced a perturbation in the development of the neural circuits of the spinal cord of mice (Mus musculus) using a deletion of the Wilms tumor suppressor gene Wt1 in a subpopulation of dI6 interneurons. These interneurons are assumed to participate in the intermuscular coordination within the limb and in left-right-coordination between the limbs. We describe the locomotor kinematics in mice with conditional Wt1 knockout and compare them to mice without Wt1 deletion. Unlike knockout neonates, knockout adult mice do not display severe deviations from normal (=control group) interlimb coordination, but the coordinated protraction and retraction of the limbs is altered. The forelimbs are more affected by deviations from the control than the hindlimbs. This observation appears to reflect a different degree of integration and resistance against the induced perturbation between the limbs. Interestingly, the observed effects are similar to locomotor deficits reported to arise when sensory feedback from proprioceptors or cutaneous receptors is impaired. A putative participation of Wt1 positive dI6 interneurons in sensorimotor integration is therefore considered.

Stability in gene expression and body-plan development leads to evolutionary conservation

BACKGROUND

Phenotypic evolution is mainly explained by selection for phenotypic variation arising from factors including mutation and environmental noise. Recent theoretical and experimental studies have suggested that phenotypes with greater developmental stability tend to have a constant phenotype and gene expression level within a particular genetic and environmental condition, and this positively correlates with stronger evolutionary conservation, even after the accumulation of genetic changes. This could reflect a novel mechanism that contributes to evolutionary conservation; however, it remains unclear whether developmental stability is the cause, or whether at least it contributes to their evolutionary conservation. Here, using Japanese medaka lines, we tested experimentally whether developmental stages and gene expression levels with greater stability led to their evolutionary conservation.

RESULTS

We first measured the stability of each gene expression level and developmental stage (defined here as the whole embryonic transcriptome) in the inbred F0 medaka population. We then measured their evolutionary conservation in the F3 generation by crossing the F0 line with the distantly related Japanese medaka line (Teradomori), followed by two rounds of intra-generational crossings. The results indicated that the genes and developmental stages that had smaller variations in the F0 generation showed lower diversity in the hybrid F3 generation, which implies a causal relationship between stability and evolutionary conservation.

CONCLUSIONS

These findings suggest that the stability in phenotypes, including the developmental stages and gene expression levels, leads to their evolutionary conservation; this most likely occurs due to their low potential to generate phenotypic variation. In addition, since the highly stable developmental stages match with the body-plan-establishment stage, it also implies that the developmental stability potentially contributed to the strict conservation of animal body plan.

Growth Regulation in the Larvae of the Lepidopteran Pieris brassicae: A Field Study

Size and shape are important determinants of fitness in most living beings. Accordingly, the capacity of the organism to regulate size and shape during growth, containing the effects of developmental disturbances of different origin, is considered a key feature of the developmental system. In a recent study, through a geometric morphometric analysis on a laboratory-reared sample of the lepidopteran Pieris brassicae, we found evidence of regulatory mechanisms able to restrain size and shape variation, including bilateral fluctuating asymmetry, during larval development. However, the efficacy of the regulatory mechanism under greater environmental variation remains to be explored. Here, based on a field-reared sample of the same species, by adopting identical measurements of size and shape variation, we found that the regulatory mechanisms for containing the effects of developmental disturbances during larval growth in P. brassicae are also effective under more natural environmental conditions. This study may contribute to better characterization of the mechanisms of developmental stability and canalization and their combined effects in the developmental interactions between the organism and its environment.

Mammalian forelimb evolution is driven by uneven proximal-to-distal morphological diversity

Vertebrate limb morphology often reflects the environment due to variation in locomotor requirements. However, proximal and distal limb segments may evolve differently from one another, reflecting an anatomical gradient of functional specialization that has been suggested to be impacted by the timing of development. Here, we explore whether the temporal sequence of bone condensation predicts variation in the capacity of evolution to generate morphological diversity in proximal and distal forelimb segments across more than 600 species of mammals. Distal elements not only exhibit greater shape diversity, but also show stronger within-element integration and, on average, faster evolutionary responses than intermediate and upper limb segments. Results are consistent with the hypothesis that late developing distal bones display greater morphological variation than more proximal limb elements. However, the higher integration observed within the autopod deviates from such developmental predictions, suggesting that functional specialization plays an important role in driving within-element covariation. Proximal and distal limb segments also show different macroevolutionary patterns, albeit not showing a perfect proximo-distal gradient. The high disparity of the mammalian autopod, reported here, is consistent with the higher potential of development to generate variation in more distal limb structures, as well as functional specialization of the distal elements.

Building a Co-ordinated Musculoskeletal System: The Plasticity of the Developing Skeleton in Response to Muscle Contractions

The skeletal musculature and the cartilage, bone and other connective tissues of the skeleton are intimately co-ordinated. The shape, size and structure of each bone in the body is sculpted through dynamic physical stimuli generated by muscle contraction, from early development, with onset of the first embryo movements, and through repair and remodelling in later life. The importance of muscle movement during development is shown by congenital abnormalities where infants that experience reduced movement in the uterus present a sequence of skeletal issues including temporary brittle bones and joint dysplasia. A variety of animal models, utilising different immobilisation scenarios, have demonstrated the precise timing and events that are dependent on mechanical stimulation from movement. This chapter lays out the evidence for skeletal system dependence on muscle movement, gleaned largely from mouse and chick immobilised embryos, showing the many aspects of skeletal development affected. Effects are seen in joint development, ossification, the size and shape of skeletal rudiments and tendons, including compromised mechanical function. The enormous plasticity of the skeletal system in response to muscle contraction is a key factor in building a responsive, functional system. Insights from this work have implications for our understanding of morphological evolution, particularly the challenging concept of emergence of new structures. It is also providing insight for the potential of physical therapy for infants suffering the effects of reduced uterine movement and is enhancing our understanding of the cellular and molecular mechanisms involved in skeletal tissue differentiation, with potential for informing regenerative therapies.

A map of signaling responses in the human airway epithelium

Receptor-mediated signaling plays a central role in tissue regeneration, and it is dysregulated in disease. Here, we build a signaling-response map for a model regenerative human tissue: the airway epithelium. We analyzed the effect of 17 receptor-mediated signaling pathways on organotypic cultures to determine changes in abundance and phenotype of all epithelial cell types. This map recapitulates the gamut of known airway epithelial signaling responses to these pathways. It defines convergent states induced by multiple ligands and diverse, ligand-specific responses in basal-cell and secretory-cell metaplasia. We show that loss of canonical differentiation induced by multiple pathways is associated with cell cycle arrest, but that arrest is not sufficient to block differentiation. Using the signaling-response map, we show that a TGFB1-mediated response underlies specific aberrant cells found in multiple lung diseases and identify interferon responses in COVID-19 patient samples. Thus, we offer a framework enabling systematic evaluation of tissue signaling responses.

Trabecular bone ontogeny tracks neural development and life history among humans and non-human primates

Trabecular bone-the spongy bone inside marrow cavities-adapts to its mechanical environment during growth and development. Trabecular structure can therefore be interpreted as a functional record of locomotor behavior in extinct vertebrates. In this paper, we expand upon traditional links between form and function by situating ontogenetic trajectories of trabecular bone in four primate species into the broader developmental context of neural development, locomotor control, and ultimately life history. Our aim is to show that trabecular bone structure provides insights into ontogenetic variation in locomotor loading conditions as the product of interactions between increases in body mass and neuromuscular maturation. Our results demonstrate that age-related changes in trabecular bone volume fraction (BV/TV) are strongly and linearly associated with ontogenetic changes in locomotor kinetics. Age-related variation in locomotor kinetics and BV/TV is in turn strongly associated with brain and body size growth in all species. These results imply that age-related variation in BV/TV is a strong proxy for both locomotor kinetics and neuromuscular maturation. Finally, we show that distinct changes in the slope of age-related variation in bone volume fraction correspond to the age of the onset of locomotion and the age of locomotor maturity. Our findings compliment previous studies linking bone development to locomotor mechanics by providing a fundamental link to brain development and life history. This implies that trabecular structure of fossil subadults can be a proxy for the rate of neuromuscular maturation and major life history events like locomotor onset and the achievement of adult-like locomotor repertoires.

Rethinking the Architecture of Attachment: New Insights into the Role for Oxytocin Signaling

Social attachments, the enduring bonds between individuals and groups, are essential to health and well-being. The appropriate formation and maintenance of social relationships depend upon a number of affective processes, including stress regulation, motivation, reward, as well as reciprocal interactions necessary for evaluating the affective state of others. A genetic, molecular, and neural circuit level understanding of social attachments therefore provides a powerful substrate for probing the affective processes associated with social behaviors. Socially monogamous species form long-term pair bonds, allowing us to investigate the mechanisms underlying attachment. Now, molecular genetic tools permit manipulations in monogamous species. Studies using these tools reveal new insights into the genetic and neuroendocrine factors that design and control the neural architecture underlying attachment behavior. We focus this discussion on the prairie vole and oxytocinergic signaling in this and related species as a model of attachment behavior that has been studied in the context of genetic and pharmacological manipulations. We consider developmental processes that impact the demonstration of bonding behavior across genetic backgrounds, the modularity of mechanisms underlying bonding behaviors, and the distributed circuitry supporting these behaviors. Incorporating such theoretical considerations when interpreting reverse genetic studies in the context of the rich ethological and pharmacological data collected in monogamous species provides an important framework for studies of attachment behavior in both animal models and studies of human relationships.

Morphological integration of the hominoid postcranium

Previous research has suggested that magnitudes of integration may be distinct in the postcranium of hominoids when compared to other primate species. To test this hypothesis, we estimated and compared magnitudes of integration of eight postcranial bones from three-dimensional surface scans for 57 Hylobates lar, 58 Gorilla gorilla, 60 Pan troglodytes, 60 Homo sapiens, 60 Chlorocebus pygerythrus, and 60 Macaca fascicularis. We tested the hypotheses that 1) magnitudes of integration would be distinct in the postcranium of hominoids compared to cercopithecoids, with the explicit prediction that magnitudes of integration would be lower in hominoids than in cercopithecoids, and 2) girdle elements (scapula, os coxa) would have lower magnitudes of integration across all taxa. Integration was quantified using the integration coefficient of variation from interlandmark distances reflecting anatomical and developmental modules defined according to a priori criteria. A resampling protocol was employed to generate distributions of integration values that were then compared statistically using Mann-Whitney U tests with Bonferroni adjustment. Support for hypothesis 1 was mixed: with the exception of Gorilla, hominoid taxa were less integrated than the cercopithecoids for all anatomical modules. However, Homo, Gorilla, and, to a lesser extent, Pan showed higher integration than Hylobates and the cercopithecoids for homologous limb elements, with magnitudes of integration for both modules being lowest for Hylobates. These results generally support the hypothesis of distinct patterns of magnitudes of integration in the hominoid postcranium. The high integration of Gorilla may be explained by the effects of overall body size. The results supported the predictions of the second hypothesis. Regardless of taxon, the os coxa and scapula were generally the least integrated skeletal elements, while the femur and radius were the most integrated. The lower integration of the girdle elements suggests that the geometric complexities of particular elements may significantly influence study outcomes.

An effect size for comparing the strength of morphological integration across studies

Understanding how and why phenotypic traits covary is a major interest in evolutionary biology. Biologists have long sought to characterize the extent of morphological integration in organisms, but comparing levels of integration for a set of traits across taxa has been hampered by the lack of a reliable summary measure and testing procedure. Here, we propose a standardized effect size for this purpose, calculated from the relative eigenvalue variance, V r e l $V_{rel}$ . First, we evaluate several eigenvalue dispersion indices under various conditions, and show that only V r e l $V_{rel}$ remains stable across samples size and the number of variables. We then demonstrate that V r e l $V_{rel}$ accurately characterizes input patterns of covariation, so long as redundant dimensions are excluded from the calculations. However, we also show that the variance of the sampling distribution of V r e l $V_{rel}$ depends on input levels of trait covariation, making V r e l $V_{rel}$ unsuitable for direct comparisons. As a solution, we propose transforming V r e l $V_{rel}$ to a standardized effect size (Z-score) for representing the magnitude of integration for a set of traits. We also propose a two-sample test for comparing the strength of integration between taxa, and show that this test displays appropriate statistical properties. We provide software for implementing the procedure, and an empirical example illustrates its use.

Morphological modularity in the anthropoid axial skeleton

Previous research has found that hominoids have stronger modularity between limb elements than other anthropoids, suggesting that there is less constraint on morphological diversification (e.g., limb proportions) in hominoids in terms of evolutionary independence. However, degrees of modularity in the axial skeleton have not been investigated across a broad range of anthropoid taxa. Thus, it is unknown whether hominoids also have stronger modularity in the axial skeleton than other anthropoids, which has implications for the evolution of diverse torso morphologies in Miocene apes as well as the evolution of novel characteristics in the skull and vertebrae of fossil hominins. In this study, 12 anthropoid genera were sampled to examine degrees of modularity between axial skeletal elements (i.e., cranium, mandible, vertebrae, and sacrum). Covariance ratio coefficients were calculated using variance/covariance matrices of interlandmark distances for each axial skeletal element to evaluate degrees of modularity. The results showed that Alouatta, Hylobates, Gorilla, Pan, and Homo showed generally stronger modularity than other anthropoid taxa when considering all axial skeletal elements. When only considering the vertebral elements (i.e., vertebrae and sacrum), Alouatta, Hylobates, Gorilla, and Pan showed generally stronger modularity than other anthropoid taxa. Humans showed stronger modularity between the skull and vertebrae than other hominoids. Thus, the evolution of novel characteristics in the skull and vertebral column may have been less constrained in fossil hominins due to the dissociation of trait covariation between axial skeletal elements in hominoid ancestors, thus fostering more evolutionary independence between the skull and vertebral column.

Investigating the reliability of metapodials as taxonomic Indicators for Beringian horses

The metapodials of extinct horses have long been regarded as one of the most useful skeletal elements to determine taxonomic identity. However, recent research on both extant and extinct horses has revealed the possibility for plasticity in metapodial morphology, leading to notable variability within taxa. This calls into question the reliability of metapodials in species identification, particularly for species identified from fragmentary remains. Here, we use ten measurements of metapodials from 203 specimens of four Pleistocene horse species from eastern Beringia to test whether there are significant differences in metapodial morphology that support the presence of multiple species. We then reconstruct the body masses for every specimen to assess the range in body size within each species and determine whether species differ significantly from one another in mean body mass. We find that that taxonomic groups are based largely on the overall size of the metapodial, and that all metapodial measurements are highly autocorrelated. We also find that mean body mass differs significantly among most, but not all, species. We suggest that metapodial measurements are unreliable taxonomic indicators for Beringian horses given evidence for plasticity in metapodial morphology and their clear reflection of differences in body mass. We recommend future studies use more reliable indicators of taxonomy to identify Beringian horse species, particularly from localities from which fossils of several species have been recovered.

Formal models for the study of the relationship between fluctuating asymmetry and fitness in humans

OBJECTIVES

To evaluate three of the main verbal models that have been proposed to explain the relationship between fluctuating asymmetry and fitness in humans: the "good genes," the "good development," and the "growth" hypotheses.

MATERIALS AND METHODS

A formal model was generated for each verbal model following three steps. First, based on the literature, a theoretical causal model and the theoretical object of inquiry were outlined. Second, an empirical causal model and the targets of inference were defined using observational data of facial asymmetries and life-history traits related to fitness. Third, generalized linear models and causal inference were used as the estimation strategy.

RESULTS

The results suggest that the theoretical and empirical assumptions of the "good genes" hypothesis should be reformulated. The results were compatible with most of the empirical assumptions of "the good development" hypothesis but suggest that further discussion of its theoretical assumptions is needed. The results were less informative about the "growth" hypothesis, both theoretically and empirically. There was a positive association between facial fluctuating asymmetry and the number of offspring that was not compatible with any of the empirical causal models evaluated.

CONCLUSIONS

Although the three hypotheses focus on different aspects of the link between asymmetry and fitness, their overlap opens the possibility of a unified theory on the subject. The results of this study make explicit which assumptions need to be updated and discussed, facilitating the advancement of this area of research. Overall, this study elucidates the potential benefit of using formal models for theory revision and development.

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.

Cellular self-organization: An overdrive in Cambrian diversity?

During the early Cambrian period metazoan life forms diverged at an accelerated rate to occupy multiple ecological niches on earth. A variety of explanations have been proposed to address this major evolutionary phenomenon termed the "Cambrian explosion." While most hypotheses address environmental, developmental, and ecological factors that facilitated evolutionary innovations, the biological basis for accelerated emergence of species diversity in the Cambrian period remains largely conjectural. Herein, we posit that morphogenesis by self-organization enables the uncoupling of genomic mutational landscape from phenotypic diversification. Evidence is provided for a two-tiered interpretation of genomic changes in metazoan animals wherein mutations not only impact upon function of individual cells, but also alter the self-organization outcome during developmental morphogenesis. We provide evidence that the morphological impacts of mutations on self-organization could remain repressed if associated with an unmet negative energetic cost. We posit that accelerated morphological diversification in transition to the Cambrian period has occurred by emergence of dormant (i.e., reserved) morphological novelties whose molecular underpinnings were seeded in the Precambrian period.

Potential contribution of intrinsic developmental stability toward body plan conservation

Background

Despite the morphological diversity of animals, their basic anatomical patterns—the body plans in each animal phylum—have remained highly conserved over hundreds of millions of evolutionary years. This is attributed to conservation of the body plan-establishing developmental period (the phylotypic period) in each lineage. However, the evolutionary mechanism behind this phylotypic period conservation remains under debate. A variety of hypotheses based on the concept of modern synthesis have been proposed, such as negative selection in the phylotypic period through its vulnerability to embryonic lethality. Here we tested a new hypothesis that the phylotypic period is developmentally stable; it has less potential to produce phenotypic variations than the other stages, and this has most likely led to the evolutionary conservation of body plans.

Results

By analyzing the embryos of inbred Japanese medaka embryos raised under the same laboratory conditions and measuring the whole embryonic transcriptome as a phenotype, we found that the phylotypic period has greater developmental stability than other stages. Comparison of phenotypic differences between two wild medaka populations indicated that the phylotypic period and its genes in this period remained less variational, even after environmental and mutational modifications accumulated during intraspecies evolution. Genes with stable expression levels were enriched with those involved in cell-cell signalling and morphological specification such as Wnt and Hox, implying possible involvement in body plan development of these genes.

Conclusions

This study demonstrated the correspondence between the developmental stage with low potential to produce phenotypic variations and that with low diversity in micro- and macroevolution, namely the phylotypic period. Whereas modern synthesis explains evolution as a process of shaping of phenotypic variations caused by mutations, our results highlight the possibility that phenotypic variations are readily limited by the intrinsic nature of organisms, namely developmental stability, thus biasing evolutionary outcomes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01276-5.

Evolution of Bacterial Persistence to Antibiotics during a 50,000-Generation Experiment in an Antibiotic-Free Environment

Failure of antibiotic therapies causes > 700,000 deaths yearly and involves both bacterial resistance and persistence. Persistence results in the relapse of infections by producing a tiny fraction of pathogen survivors that stay dormant during antibiotic exposure. From an evolutionary perspective, persistence is either a ‘bet-hedging strategy’ that helps to cope with stochastically changing environments or an unavoidable minimal rate of ‘cellular errors’ that lock the cells in a low activity state. Here, we analyzed the evolution of persistence over 50,000 bacterial generations in a stable environment by improving a published method that estimates the number of persister cells based on the growth of the reviving population. Our results challenged our understanding of the factors underlying persistence evolution. In one case, we observed a substantial decrease in persistence proportion, suggesting that the naturally observed persistence level is not an unavoidable minimal rate of ‘cellular errors’. However, although there was no obvious environmental stochasticity, in 11 of the 12 investigated populations, the persistence level was maintained during 50,000 bacterial generations.

Cranial-Vertebral-Maxillary Morphological Integration in Down Syndrome

Background: Morphological integration refers to the tendency of anatomical structures to show correlated variations because they develop in response to shared developmental processes or function in concert with other structures. The objective of this study was to determine the relationships between the dimensions of different cranial-cervical-facial structures in patients with Down syndrome (DS). Methodology: The study group consisted of 41 individuals with DS who had undergone cone-beam computed tomography (CBCT) at the Dental Radiology Unit of the University of Santiago de Compostela (Spain). In the historical archive of this same unit, 41 CBCTs belonging to individuals with no known systemic disorders or severe malformations of the maxillofacial region were selected, forming an age and sex-matched control group. Twenty-nine measurements were performed on each participant’s CBCT images, which were grouped into three blocks: atlantoaxial dimensions, craniovertebral dimensions and cephalometric dimensions. To determine whether there were significant differences between the dimensions obtained in the DS and control groups, we applied multiple analysis of variance and linear discriminant analysis tests. The analysis of the association between blocks (in pairs) was performed with the canonical correlation analysis test. Results: The dimensions evaluated in the three blocks of variables of individuals with DS differ significantly from those of nonsyndromic controls (p < 0.001). The highest discriminative capacity to identify controls and patients with DS was obtained with the cephalometric dimensions (87.5%). With regard to the association between blocks (two-by-two measurements), we found no significant relationship in the DS group. However, we confirmed a statistically significant correlation between all pairs of blocks of variables in the controls, especially between the atlantoaxial and cephalometric dimensions (p < 0.001) and between the craniovertebral and cephalometric dimensions (p < 0.001). Conclusions: Our results confirm a very poor morphological integration of the cranial-cervical-maxillary complex in individuals with DS. This finding reinforces the proposal that gene overload enhances the channeling process.

The effect of ultraviolet radiation on the incidence and severity of major mental illness using birth month, birth year, and sunspot data

Background and objectives

The evaluation of the severity of patients afflicted with major mental illness (MMI) has been problematic because of confounding variables and genetic variability. There have been multiple studies that suggest several human diseases, especially schizophrenia, are predisposed to be born in certain months or seasons. This observation implied an epigenetic effect of sunlight, likely ultraviolet radiation (UVR), which is damaging to DNA, especially in an embryo. This paper outlines a method to evaluate the severity of schizophrenia (SZ), bipolar disorder (BPD), and schizoaffective disorder (SZ-AFF) using the month/year of birth of those affected compared to the month/year of birth of the general population (GP).

Relevance

Our previous research found that more intense UVR (equal to or greater than 90 sunspot number (SSN)) had a negative effect on the average human lifespan. Also, human birth rates vary in frequency by month of birth reflecting variables like availability of food, sunlight, and other unknown epigenetic factors. We wanted to see if the patient month of birth varied from the average birth months of the general population and if UVR has an epigenetic effect promoting these diseases.

Methods

We obtained the month and year of birth of 1,233 patients admitted over a 15-year period to Maine's largest state psychiatric hospital and counted the months of birth for each diagnosis of SZ, BPD, and SZ-AFF, and compared these results to the general population's birth months of 4,265,555 persons from U. S. Census Year 2006. The number of patients in each month was normalized to August and compared with the normalized birth months of the general population (GP). Plots of the normalized months were considered rates of change (e.g., derivatives) and their respective integrals gave domains of each mental illness relative to the GP. Normalizing the GP to unity was then related to the factor 1.28, e.g., 28% more entropy, deduced from the Sun's fractal dimension imprinted on biological organisms.

Results

The percent of patients meeting our criterion for severity: SZ = 27%; BPD = 26%; SZ-AFF = 100%.

Conclusions

High UVR intensity or a rapid increase in UVR in early gestation are likely epigenetic triggers of major mental illness. BPD is more epigenetically affected than SZ or SZ-AFF disorders. We found that 52% of 1,233 patients comprised the core function of a tertiary-care psychiatric hospital. Also, mental illness exacerbated when the median SSN doubled. This work also validates the Kraeplinian dichotomy.

What is new in this research

This paper offers a new paradigm for evaluating the severity of MMI and supports significant epigenetic effects from UVR.

Skull morphological variation in a British stranded population of false killer whale (Pseudorca crassidens): a three-dimensional geometric morphometric approach

The false killer whale (Pseudorca crassidens (Owen, 1846)) is a globally distributed delphinid that shows geographical differentiation in its skull morphology. We explored cranial morphological variation in a sample of 85 skulls belonging to a mixed sex population stranded in the Moray Firth, Scotland, in 1927. A three-dimensional digitizer (Microscribe 2GX) was used to record 37 anatomical landmarks on the cranium and 25 on the mandible to investigate size and shape variation and to explore sexual dimorphism using geometric morphometric. Males showed greater overall skull size than females, whereas no sexual dimorphism could be identified in cranial and mandibular shape. Allometric skull changes occurred in parallel for both males and females, supporting the lack of sexual shape dimorphism for this particular sample. Also, fluctuating asymmetry did not differ between crania of males and females. This study confirms the absence of sexual shape dimorphism and the presence of a sexual size dimorphism in this false killer whale population.

Long bone shape variation in the forelimb of Rhinocerotoidea: relation with size, body mass and body proportions

In quadrupeds, limb bones are strongly affected by functional constraints linked to weight support, but few studies have addressed the complementary effects of mass, size and body proportions on limb bone shape. During their history, Rhinocerotoidea have displayed a great diversity of body masses and relative size and proportions of limb bones, from small tapir-like forms to giant species. Here, we explore the evolutionary variation of shapes in forelimb bones and its relationship with body mass in Rhinocerotoidea. Our results indicate a general increase in robustness and greater development of muscular insertions in heavier species, counteracting the higher weight loadings induced by an increased body mass. The shape of the humerus changes allometrically and exhibits a strong phylogenetic signal. Shapes of the radius and ulna display a stronger link with body mass repartition than with the absolute mass itself. Congruent shape variation between the humerus and the proximal part of the ulna suggests that the elbow joint is comprised of two strongly covariant structures. In addition, our work confirms the uniqueness of giant Paraceratheriidae among Rhinocerotoidea, whose shape variation is related to both a high body mass and a cursorial forelimb construction.

Covariation between the cranium and the cervical vertebrae in hominids

The analysis of patterns of integration is crucial for the reconstruction and understanding of how morphological changes occur in a taxonomic group throughout evolution. These patterns are relatively constant; however, both patterns and the magnitudes of integration may vary across species. These differences may indicate morphological diversification, in some cases related to functional adaptations to the biomechanics of organisms. In this study, we analyze patterns of integration between two functional and developmental structures, the cranium and the cervical spine in hominids, and we quantify the amount of divergence of each anatomical element through phylogeny. We applied these methods to three-dimensional data from 168 adult hominid individuals, summing a total of more than 1000 cervical vertebrae. We found the atlas (C1) and axis (C2) display the lowest covariation with the cranium in hominids (Homo sapiens, Pan troglodytes, Pan paniscus, Gorilla gorilla, Gorilla beringei, Pongo pygmaeus). H. sapiens show a relatively different pattern of craniocervical correlation compared with chimpanzees and gorillas, especially in variables implicated in maintaining the balance of the head. Finally, the atlas and axis show lower magnitude of shape change during evolution than the rest of the cervical vertebrae, especially those located in the middle of the subaxial cervical spine. Overall, results suggest that differences in the pattern of craniocervical correlation between humans and gorillas and chimpanzees could reflect the postural differences between these groups. Also, the stronger craniocervical integration and larger magnitude of shape change during evolution shown by the middle cervical vertebrae suggests that they have been selected to play an active role in maintaining head balance.

Lower Levels of Vestibular Developmental Stability in Slow-Moving than Fast-Moving Primates

The vestibular system of the mammalian inner ear senses angular and linear velocity of the head and enables animals to maintain their balance. Vestibular anatomy has been studied extensively in order to link its structure to particular kinds of locomotion. Available evidence indicates that, in primates, slow-moving species show higher levels of vestibular variation than fast-moving taxa. We analysed intraspecific morphological variation and fluctuating asymmetry (FA) levels in the semicircular canal systems of six species of lorisiform primates: three slow-moving lorisids and three fast-moving galagids. Our results showed clear differences in levels of intraspecific variation between slow-moving and fast-moving taxa. Higher levels of variation were responsible for deviations from coplanarity for synergistic pairs of canals in slower taxa. Lorisids also presented higher levels of FA than galagids. FA is a better indicator of agility than intraspecific variation. These results suggest that in order to function efficiently in fast taxa, semicircular canal systems must develop as symmetrically as possible, and should minimise the deviation from coplanarity for synergistic pairs. Higher levels of variation and asymmetry in slow-moving taxa may be related to lower levels of stabilising selection on the vestibular system, linked to a lower demand for rapid postural changes.

Canalization and developmental stability of the yellow-necked mouse (Apodemus flavicollis) mandible and cranium related to age and nematode parasitism

BACKGROUND

Mammalian mandible and cranium are well-established model systems for studying canalization and developmental stability (DS) as two elements of developmental homeostasis. Nematode infections are usually acquired in early life and increase in intensity with age, while canalization and DS of rodent skulls could vary through late postnatal ontogeny. We aimed to estimate magnitudes and describe patterns of mandibular and cranial canalization and DS related to age and parasite intensity (diversity) in adult yellow-necked mice (Apodemus flavicollis).

RESULTS

We found the absence of age-related changes in the levels of canalization for mandibular and cranial size and DS for mandibular size. However, individual measures of mandibular and cranial shape variance increased, while individual measures of mandibular shape fluctuating asymmetry (FA) decreased with age. We detected mandibular and cranial shape changes during postnatal ontogeny, but revealed no age-related dynamics of their covariance structure among and within individuals. Categories regarding parasitism differed in the level of canalization for cranial size and the level of DS for cranial shape. We observed differences in age-related dynamics of the level of canalization between non-parasitized and parasitized animals, as well as between yellow-necked mice parasitized by different number of nematode species. Likewise, individual measures of mandibular and cranial shape FA decreased with age for the mandible in the less parasitized category and increased for the cranium in the most parasitized category.

CONCLUSIONS

Our age-related results partly agree with previous findings. However, no rodent study so far has explored age-related changes in the magnitude of FA for mandibular size or mandibular and cranial FA covariance structure. This is the first study dealing with the nematode parasitism-related canalization and DS in rodents. We showed that nematode parasitism does not affect mandibular and cranial shape variation and covariance structure among and within individuals. However, parasite intensity (diversity) is related to ontogenetic dynamics of the levels of canalization and DS. Overall, additional studies on animals from natural populations are required before drawing some general conclusions.

Biomechanical Evaluation on the Bilateral Asymmetry of Complete Humeral Diaphysis in Chinese Archaeological Populations

Diaphyseal cross-sectional geometry (CSG) is an effective indicator of humeral bilateral asymmetry. However, previous studies primarily focused on CSG properties from limited locations to represent the overall bilateral biomechanical performance of humeral diaphysis. In this study, the complete humeral diaphyses of 40 pairs of humeri from three Chinese archaeological populations were scanned using high-resolution micro-CT, and their biomechanical asymmetries were quantified by morphometric mapping. Patterns of humeral asymmetry were compared between sub-groups defined by sex and population, and the representativeness of torsional rigidity asymmetry at the 35% and 50% cross-sections (J35 and J50 asymmetry) was testified. Inter-group differences were observed on the mean morphometric maps, but were not statistically significant. Analogous distribution patterns of highly asymmetrical regions, which correspond to major muscle attachments, were observed across nearly all the sexes and populations. The diaphyseal regions with high variability of bilateral asymmetry tended to present a low asymmetrical level. The J35 and J50 asymmetry were related to the overall humeral asymmetry, but the correlation was moderate and they could not reflect localized asymmetrical features across the diaphysis. This study suggests that the overall asymmetry pattern of humeral diaphysis is more complicated than previously revealed by individual sections.

Cranial shape diversification in horses: variation and covariation patterns under the impact of artificial selection

The potential of artificial selection to dramatically impact phenotypic diversity is well known. Large-scale morphological changes in domestic species, emerging over short timescales, offer an accelerated perspective on evolutionary processes. The domestic horse (Equus caballus) provides a striking example of rapid evolution, with major changes in morphology and size likely stemming from artificial selection. However, the microevolutionary mechanisms allowing to generate this variation in a short time interval remain little known. Here, we use 3D geometric morphometrics to quantify skull morphological diversity in the horse, and investigate modularity and integration patterns to understand how morphological associations contribute to cranial evolvability in this taxon. We find that changes in the magnitude of cranial integration contribute to the diversification of the skull morphology in horse breeds. Our results demonstrate that a conserved pattern of modularity does not constrain large-scale morphological variations in horses and that artificial selection has impacted mechanisms underlying phenotypic diversity to facilitate rapid shape changes. More broadly, this study demonstrates that studying microevolutionary processes in domestic species produces important insights into extant phenotypic diversity.

Patterns of skeletal integration in birds reveal that adaptation of element shapes enables coordinated evolution between anatomical modules

Birds show tremendous ecological disparity in spite of strong biomechanical constraints imposed by flight. Modular skeletal evolution is generally accepted to have facilitated this, with distinct body regions showing semi-independent evolutionary trajectories. However, this hypothesis has received little scrutiny. We analyse evolutionary modularity and ecomorphology using three-dimensional data from across the entire skeleton in a phylogenetically broad sample of extant birds. We find strongly modular evolution of skeletal element sizes within body regions (head, trunk, forelimb and hindlimb). However, element shapes show substantially less modularity, have stronger relationships to ecology, and provide evidence that ecological adaptation involves coordinated evolution of elements across different body regions. This complicates the straightforward paradigm in which modular evolution facilitated the ecological diversification of birds. Our findings suggest the potential for undetected patterns of morphological evolution in even well-studied groups, and advance the understanding of the interface between evolutionary integration and ecomorphology.

Studying factors influencing facial developmental instability

BACKGROUND

Developmental instability is a component of non-genetic variation that results from random variation in developmental processes. It is considered a sensitive indicator of the physiological state of individuals. It is reflected in various ways, but in this study we focussed on its reflection in fluctuating asymmetry (FA) and morphological integration.

AIM

To assess how, if at all, variations of facial morphology mirror developmental instability across childhood with respect to sex, growth rate and socioeconomic/environmental factors.

SUBJECTS AND METHODS

A set of 210 three-dimensional facial models (of children aged between 6.3 and 14.3 years) originating from the FIDENTIS 3D Face Database was subjected to landmark-based methods of geometric morphometrics to quantify the degree of facial asymmetry and facial morphological integration. In addition, the association with age, sex, and socioeconomic factors was assessed.

RESULTS

Our results showed a nonlinear increase of FA with age up to the age of 14 years. The pattern of sex-related variants in facial FA differed in relation to age, as girls exhibited higher values of FA than boys up to the age of 9 years. We found that a signal of modularity based on functional demands and organisation of the face is of particular importance. Here, girls exhibited higher morphological covariation among modules. During more rapid adolescence-related growth, however, covariation among modules at the asymmetrical level decreased in both sexes.

CONCLUSION

We can conclude that facial morphology was shown to be strongly integrated, particularly until adolescence. This covariation can facilitate an increase of FA. In addition, the results of this study indicate there is a weak association between socioeconomic stress and facial asymmetries. In contrast, sex and growth rate are reflected in developmental instability.

Functional signals and covariation in triquetrum and hamate shape of extant primates using 3D geometric morphometrics

In this study, we want to investigate the covariation in the shape of two carpal bones, the triquetrum and hamate, and the possible association with locomotor behavior in a broad range of primate taxa. We applied 3D Geometric Morphometrics on a large data set comprising 309 anthropoid primates of 12 different genera. Principal component analyses were performed on the covariance matrix of 18 (triquetrum) and 23 (hamate) Procrustes-aligned surface landmarks. A two-block partial least square analysis was done to test the covariance between triquetrum and hamate shape, without relying on the predictive models implicit in regression analyses. The results show that the carpal shape of quadrupedal anthropoids, which mainly use their wrist under compressive conditions, differs from that of suspensory primates as their wrist is possibly subjected to tensile and torsional forces. Within the hominids, differences in shape also distinguish more terrestrial from more arboreal species. Even within the great apes, we are able to capture shape differences between species of the same genus. In combination with behavioral and biomechanical studies, the results of this research can be used to establish form-function relationships of the primate hand which will aid the functional interpretation of primate fossil remains.

Phenotypic plasticity, canalisation and developmental stability of Triatoma infestans wings: effects of a sublethal application of a pyrethroid insecticide

BACKGROUND

Triatomine control campaigns have traditionally consisted of spraying the inside of houses with pyrethroid insecticides. However, exposure to sublethal insecticide doses after the initial application is a common occurrence and may have phenotypic consequences for survivors. Here, using Triatoma infestans (the main vector of Chagas disease in the Southern Cone of South America) as a model species, we quantified the effects of exposure to a sublethal dose of pyrethroid insecticide on wing morphology. We tested if the treatment (i) induced a plastic effect (change in the character mean); (ii) altered environmental canalisation (higher individual variation within genotypes); (iii) altered genetic canalisation (higher variation among genotypes); and (iv) altered developmental stability (higher fluctuating asymmetry [FA]).

METHODS

Each of 25 full-sib families known to be susceptible to pyrethroid insecticides were split in two groups: one to be treated with a sublethal dose of deltamethrin (insecticide-treated group) and the other to be treated with pure acetone (control group). Wings of the emerging adults were used in a landmark-based geometric morphometry analysis to extract size and shape measurements. Average differences among treatments were measured. Levels of variation among families, among individuals within families and among sides within individuals were computed and compared among treatments.

RESULTS

Wing size and shape were affected by a sublethal dose of deltamethrin. The treated insects had larger wings and a more variable wing size and shape than control insects. For both wing size and shape, genetic variation was higher in treated individuals. Individual variations and variations in FA were also greater in deltamethrin-treated insects than in control ones for all full-sib families; however, the patterns of shape variation associated with genetic variation, individual variation and FA were different.

CONCLUSIONS

Insects exposed to a sublethal dose of deltamethrin presented larger, less symmetrical and less canalised wings. The insecticide treatment jointly impaired developmental stability and genetic and environmental canalisation. The divergent patterns of shape variation suggest that the related developmental buffering processes differed at least partially. The morphological modifications induced by a single sublethal exposure to pyrethroids early in life may impinge on subsequent flight performance and consequently affect the dynamics of house invasion and reinfestation, and the effectiveness of triatomine control operations.