Hemispheric asymmetries in cortical grey matter of gyri and sulci in modern human populations from South America

Structural asymmetries of brain regions associated with lateralised functions have been extensively studied. However, there are fewer morphometric analyses of asymmetries of the gyri and sulci of the entire cortex. The current study assessed cortical asymmetries in a sample of healthy adults (N = 175) from an admixed population from South America. Grey matter volume and surface area of 66 gyri and sulci were quantified on T1 magnetic resonance images. The departure from zero of the differences between left and right hemispheres (L-R), a measure of directional asymmetry (DA), the variance of L-R, and an index of fluctuating asymmetry (FA) were evaluated for each region. Significant departures from perfect symmetry were found for most cortical gyri and sulci. Regions showed leftward asymmetry at the population level in the frontal lobe and superior lateral parts of the parietal lobe. Rightward asymmetry was found in the inferior parietal, occipital, frontopolar, and orbital regions, and the cingulate (anterior, middle, and posterior-ventral). Despite this general pattern, several sulci showed the opposite DA compared to the neighbouring gyri, which remarks the need to consider the neurobiological differences in gyral and sulcal development in the study of structural asymmetries. The results also confirm the absence of DA in most parts of the inferior frontal gyrus and the precentral region. This study contributes with data on populations underrepresented in the databases used in neurosciences. Among its findings, there is agreement with previous results obtained in populations of different ancestry and some discrepancies in the middle frontal and medial parietal regions. A significant DA not reported previously was found for the volume of long and short insular gyri and the central sulcus of the insula, frontomarginal, transverse frontopolar, paracentral, and middle and posterior parts of the cingulate gyrus and sulcus, gyrus rectus, occipital pole, and olfactory sulcus, as well as for the volume and area of the transverse collateral sulcus and suborbital sulcus. Also, several parcels displayed significant variability in the left-right differences, which can be partially attributable to developmental instability, a source of FA. Moreover, a few gyri and sulci displayed ideal FA with non-significant departures from perfect symmetry, such as subcentral and posterior cingulate gyri and sulci, inferior frontal and fusiform gyri, and the calcarine, transverse collateral, precentral, and orbital sulci. Overall, these results show that asymmetries are ubiquitous in the cerebral cortex.

Classifying high-dimensional phenotypes with ensemble learning

Classification is a fundamental task in biology used to assign members to a class. While linear discriminant functions have long been effective, advances in phenotypic data collection are yielding increasingly high-dimensional datasets with more classes, unequal class covariances, and non-linear distributions. Numerous studies have deployed machine learning techniques to classify such distributions, but they are often restricted to a particular organism, a limited set of algorithms, and/or a specific classification task. In addition, the utility of ensemble learning or the strategic combination of models has not been fully explored.We performed a meta-analysis of 33 algorithms across 20 datasets containing over 20,000 high-dimensional shape phenotypes using an ensemble learning framework. Both binary (e.g., sex, environment) and multi-class (e.g., species, genotype, population) classification tasks were considered. The ensemble workflow contains functions for preprocessing, training individual learners and ensembles, and model evaluation. We evaluated algorithm performance within and among datasets. Furthermore, we quantified the extent to which various dataset and phenotypic properties impact performance.We found that discriminant analysis variants and neural networks were the most accurate base learners on average. However, their performance varied substantially between datasets. Ensemble models achieved the highest performance on average, both within and among datasets, increasing average accuracy by up to 3% over the top base learner. Higher class R2 values, mean class shape distances, and between- vs. within-class variances were positively associated with performance, whereas higher class covariance distances were negatively associated. Class balance and total sample size were not predictive.Learning-based classification is a complex task driven by many hyperparameters. We demonstrate that selecting and optimizing an algorithm based on the results of another study is a flawed strategy. Ensemble models instead offer a flexible approach that is data agnostic and exceptionally accurate. By assessing the impact of various dataset and phenotypic properties on classification performance, we also offer potential explanations for variation in performance. Researchers interested in maximizing performance stand to benefit from the simplicity and effectiveness of our approach made accessible via the R package pheble.

Influence of accessory sulci of the frontoparietal operculum on gray matter quantification

Introduction: The perisylvian region is the cortical core of language and speech. Several accessory sulci have been described in this area, whose presence could modify the results of the automatic quantification of gray matter by popularly used software. This study aimed to assess the expression of accessory sulci in the frontoparietal operculum (FPO) and to evaluate their influence on the gray matter volume estimated by an automatic parcellation of cortical gyri and sulci. Methods: Brain MRI scans of 100 healthy adult volunteers were visually analyzed. The existence of the triangular and diagonal sulci, and the number of accessory sulci in the frontoparietal operculum, were assessed on T1 images. Also, the gray matter volume of gyri and sulci was quantified by an automatized parcellation method. Interhemispheric differences in accessory sulci were evaluated with Chi-square and Wilcoxon paired tests. The effects of the hemisphere, sex, age, total intracranial volume, and accessory sulci on morphometric variables were assessed by linear models. Results: These sulci were found in more than half of the subjects, mostly in the left hemisphere, and showed a significant effect on the gray matter content of the FPO. In particular, the volume of the inferior frontal sulcus, pars opercularis of the inferior frontal gyrus, horizontal ramus of the lateral sulcus, angular gyrus, and postcentral gyrus showed a significant influence on the presence of accessory sulci. Discussion: The prevalence of tertiary sulci in the FPO is high, although their meaning is not yet known. Therefore, they should be considered to reduce the risk of misclassifications of normal variation.

Morphological correspondence between brain and endocranial surfaces in mice exposed to undernutrition during development

The morphological changes of the brain and the skull are highly integrated as a result of shared developmental pathways and different types of interactions between them. Shared developmental trajectories between these two structures might be influenced by genetic and environmental factors. Although the effect of environmental factors on neural and craniofacial traits has been extensively studied, less is known about the specific impact of stressful conditions on the coordinated variation between these structures. Here, we test the effect of early nutrient restriction on morphological correspondence between the brain and the endocast. For this purpose, mice exposed to protein or calorie-protein restriction during gestation and lactation were compared with a control group in which dams were fed standard food ad libitum. High-resolution images were obtained after weaning to describe brain and endocranial morphology. By magnetic resonance imaging (MRI), brain volumes were obtained and endocasts were segmented from skull reconstructions derived from micro-computed tomography (microCT). Brain and endocranial volumes were compared to assess the correspondence in size. Shape changes were analyzed using a set of landmarks and semilandmarks on 3D surfaces. Results indicated that brain volume is relatively less affected by undernutrition during development than endocast volume. Shape covariation between the brain and the endocast was found to be quite singular for protein-restricted animals. Procrustes distances were larger between the brain and the endocast of the same specimens than between brains or endocasts of different animals, which means that the greatest similarity is by type of structure and suggests that the use of the endocast as a direct proxy of the brain at this intraspecific scale could have some limitations. In the same line, patterns of brain shape asymmetry were not directly estimated from endocranial surfaces. In sum, our findings indicate that morphological variation and association between the brain and the endocast is modulated by environmental factors and support the idea that head morphogenesis results from complex processes that are sensitive to the pervasive influence of nutrient intake.

Endocranial asymmetry in New World monkeys: a comparative phylogenetic analysis of morphometric data

Brain lateralization is a widespread phenomenon although its expression across primates is still controversial due to the reduced number of species analyzed and the disparity of methods used. To gain insight into the diversification of neuroanatomical asymmetries in non-human primates we analyze the endocasts, as a proxy of external brain morphology, of a large sample of New World monkeys and test the effect of brain size, home range and group sizes in the pattern and magnitude of shape asymmetry. Digital endocasts from 26 species were obtained from MicroCT scans and a set of 3D coordinates was digitized on endocast surfaces. Results indicate that Ateles, Brachyteles, Callicebus and Cacajao tend to have a rightward frontal and a leftward occipital lobe asymmetry, whereas Aotus, Callitrichinae and Cebinae have either the opposite pattern or no directional asymmetry. Such differences in the pattern of asymmetry were associated with group and home range sizes. Conversely, its magnitude was significantly associated with brain size, with larger-brained species showing higher inter-hemispheric differences. These findings support the hypothesis that reduction in inter-hemispheric connectivity in larger brains favors the lateralization and increases the structural asymmetries, whereas the patterns of shape asymmetry might be driven by socio-ecological differences among species.

Association between shape changes and bone remodeling patterns in the middle face during ontogeny in South American populations

The morphology of facial bones is modeled by processes of bone formation and resorption induced by interactions between tissues and compensatory responses. However, the role of remodeling patterns on the morphological changes within and among populations has been scarcely explored. Here, we assess the association between facial shape and the underlying bone cell activity throughout the ontogeny in two Amerindian populations that represent the extremes of craniofacial variation in South America. The sample comprises 71 individuals (36 adults and 35 subadults) representing hunter-gatherers from Patagonia and horticulturists from Northwest Argentina. We analyzed the shape and size of the zygomatic and the maxilla, and compared them with the patterns of bone formation and resorption. Bone formation and resorption were described by quantitative histological analysis of bone surfaces. Morphological changes were described by landmarks and semilandmarks digitized on 3D surfaces obtained from CT images. The results from multivariate statistics analysis show that the patterns of bone remodeling are associated with variation in the morphology of the middle face. We found a similar pattern of facial shape variation along the ontogenetic trajectory in the two samples, and a similar trend in the amount of formation and resorption activities across ages. The main differences between samples were found in the distribution of the areas of bone formation and resorption, possibly associated with mechanical bone response to masticatory loading. These findings provide clues about the processes and mechanisms of bone development involved in the facial morphological differentiation in human populations from southern South America.

Lack of head sparing following third-trimester caloric restriction among Tanzanian Maasai

The reduction of food intake during pregnancy is part of many cultural and religious traditions around the world. The impact of such practices on fetal growth and development are poorly understood. Here, we examined the patterns of diet intake among Maasai pregnant women and assessed their effect on newborn morphometrics. We recruited 141 mother-infant pairs from Ngorongoro Conservation Area (NCA) in Northern Tanzania and quantified dietary intake and changes in maternal diet during pregnancy. We obtained measurements of body weight (BW) and head circumference (HC) at birth. We found that Maasai women significantly reduced their dietary intake during the third trimester, going from an average of 1601 kcal/day during the first two trimesters to 799 kcal/day in the final trimester. The greatest proportion of nutrient reduction was in carbohydrates. Overall, 40% of HC Z-scores of the NCA sample were more than 2 standard deviations below the WHO standard. Nearly a third of neonates classify as low birth weight (< 2500g). HC was smaller relative to BW in this cohort than predicted using the WHO standard. This contrasts markedly to a Tanzanian birth cohort obtained at the same time in an urban context in which only 12% of infants exhibited low weight, only two individuals had HC Z-scores < 2 and HC's relative to birth weight were larger than predicted using the WHO standards. The surprising lack of head sparing in the NCA cohort suggests that the impact of third trimester malnutrition bears further investigation in both animal models and human populations, especially as low HC is negatively associated with long term health outcomes.

Integration and the Developmental Genetics of Allometry

Allometry refers to the ways in which organismal shape is associated with size. It is a special case of integration, or the tendency for traits to covary, in that variation in size is ubiquitous and evolutionarily important. Allometric variation is so commonly observed that it is routinely removed from morphometric analyses or invoked as an explanation for evolutionary change. In this case, familiarity is mistaken for understanding because rarely do we know the mechanisms by which shape correlates with size or understand their significance. As with other forms of integration, allometric variation is generated by variation in developmental processes that affect multiple traits, resulting in patterns of covariation. Given this perspective, we can dissect the genetic and developmental determinants of allometric variation. Our work on the developmental and genetic basis for allometric variation in craniofacial shape in mice and humans has revealed that allometric variation is highly polygenic. Different measures of size are associated with distinct but overlapping patterns of allometric variation. These patterns converge in part on a common genetic basis. Finally, environmental modulation of size often generates variation along allometric trajectories, but the timing of genetic and environmental perturbations can produce deviations from allometric patterns when traits are differentially sensitive over developmental time. These results question the validity of viewing allometry as a singular phenomenon distinct from morphological integration more generally.

Region-specific changes in Mus musculus brain size and cell composition under chronic nutrient restriction

Nutrition is one of the most influential environmental factors affecting the development of different tissues and organs. It is suggested that under nutrient restriction the growth of the brain is spared as a result of the differential allocation of resources from other organs. However, it is not clear whether this sparing occurs brain-wide. Here, we analyzed morphological changes and cell composition in different regions of the offspring mouse brain after maternal exposure to nutrient restriction during pregnancy and lactation. Using high-resolution magnetic resonance imaging, we found that brain regions were differentially sensitive to maternal protein restriction and exhibited particular patterns of volume reduction. The cerebellum was reduced in absolute and relative volume, while cortex volume was relatively preserved. Alterations in cell composition (examined by the isotropic fractionator method) and organization of white matter (measured by diffusor tensor images) were also region specific. These changes were not related to the metabolic rate of the regions and were only partially explained by their specific growth trajectories. This study is a first step towards understanding the mechanisms of regional brain sparing at microstructural and macrostructural levels resulting from undernutrition.

Mitochondrial DNA Diversity and Evolutionary History of Native Human Populations of Argentinean Northwest Patagonia

The genetic composition of Amerindian descendants from Patagonia has long been a focus of interest, although the information available is still scarce for many geographic areas. Here, we report the first analysis of the variation in the mitochondrial DNA (mtDNA) control region for an area of northwestern Patagonia, the North of Neuquén, with the aim of studying the processes and historical events that modeled the evolutionary history of these human groups. We analyzed 113 individuals from two localities of northern Neuquén, along with 6 from southern Neuquén and 223 previously published mtDNA sequences from neighboring areas in Argentina and Chile. We estimated the haplotypic variation and spatial structure of molecular variability. Amerindian subhaplogroups predominate in the two samples from northern Neuquén (n = 70), with D1g and C1b13 the most represented, although in different proportions. These samples exhibit Amerindian mtDNA haplotypes similar to the variants from neighboring areas. Most of haplotype variability was within group; variation among groups was relatively low and scarcely associated with geographical space. The most frequent subhaplogroups in northern Neuquén are characteristic of native populations from Patagonia and Chilean Araucanía, and probably originated in the region during the Late Pleistocene or Early Holocene. However, the spatial variation of mtDNA haplotypes departs from a latitudinal pattern and suggests differential levels of gene flow among areas during the Late Holocene, with moderate levels across the North of Neuquén as well as between this area and neighboring populations from Chile, the South of Neuquén, and Río Negro.

Facial shape manifestations of growth faltering in Tanzanian children

Variation in the shape of the human face and in stature is determined by complex interactions between genetic and environmental influences. One such environmental influence is malnourishment, which can result in growth faltering, usually diagnosed by means of comparing an individual's stature with a set of age-appropriate standards. These standards for stature, however, are typically ascertained in groups where people are at low risk for growth faltering. Moreover, genetic differences among populations with respect to stature are well established, further complicating the generalizability of stature-based diagnostic tools. In a large sample of children aged 5-19 years, we obtained high-resolution genomic data, anthropometric measures and 3D facial images from individuals within and around the city of Mwanza, Tanzania. With genome-wide complex trait analysis, we partitioned genetic and environmental variance for growth outcomes and facial shape. We found that children with growth faltering have faces that look like those of older and taller children, in a direction opposite to the expected allometric trajectory, and in ways predicted by the environmental portion of covariance at the community and individual levels. The environmental variance for facial shape varied subtly but significantly among communities, whereas genetic differences were minimal. These results reveal that facial shape preserves information about exposure to undernourishment, with important implications for refining assessments of nutritional status in children and the developmental-genetics of craniofacial variation alike.

Body size and allometric variation in facial shape in children

OBJECTIVES

Morphological integration, or the tendency for covariation, is commonly seen in complex traits such as the human face. The effects of growth on shape, or allometry, represent a ubiquitous but poorly understood axis of integration. We address the question of to what extent age and measures of size converge on a single pattern of allometry for human facial shape.

METHODS

Our study is based on two large cross-sectional cohorts of children, one from Tanzania and the other from the United States (N = 7,173). We employ 3D facial imaging and geometric morphometrics to relate facial shape to age and anthropometric measures.

RESULTS

The two populations differ significantly in facial shape, but the magnitude of this difference is small relative to the variation within each group. Allometric variation for facial shape is similar in both populations, representing a small but significant proportion of total variation in facial shape. Different measures of size are associated with overlapping but statistically distinct aspects of shape variation. Only half of the size-related variation in facial shape can be explained by the first principal component of four size measures and age while the remainder associates distinctly with individual measures.

CONCLUSIONS

Allometric variation in the human face is complex and should not be regarded as a singular effect. This finding has important implications for how size is treated in studies of human facial shape and for the developmental basis for allometric variation more generally.

A quantitative approach for analysing bone modelling patterns from craniofacial surfaces in hominins

Bone size and shape arise throughout ontogeny as a result of the coordinated activity of osteoblasts and osteoclasts, responsible for bone deposition and resorption, and growth displacements. The modelling processes leave specific microstructural features on the bone surface, which can be used to infer the mechanisms shaping craniofacial traits in extinct and extant species. However, the analysis of bone surfaces from fossils and archaeological samples faces some difficulties related to the bone loss caused by taphonomic factors, and the lack of formal methods for estimating missing information and comparing the patterns of bone modelling among several specimens and samples. The present study provides a new approach for the quantitative analysis of bone formation and resorption patterns obtained from craniofacial surfaces. First, interpolation techniques were used to estimate missing data on high-resolution replicas of the left maxilla in a sample of sub-adult and adult modern humans and sub-adult fossil hominins. The performance of this approach was assessed by simulating variable amounts of missing data. Then, we applied measures of dispersion and central tendency to represent the variation and average pattern of bone modelling within samples. The spatial interpolation resulted in reliable estimations of the type of cell activity (deposition or resorption) in the missing areas, even when large extensions of the bone surface were lost. The quantification of the histological data allowed us to integrate the information of different specimens and depict the areas with higher and lower variation in the bone modelling pattern of the maxilla among specimens. Overall, the main advantages of the quantitative approach used here for generating bone modelling patterns are the high replicability and the possibility of incorporating variation among specimens into the comparisons among samples.

Genetic structure of phenotypic robustness in the collaborative cross mouse diallel panel

Developmental stability and canalization describe the ability of developmental systems to minimize phenotypic variation in the face of stochastic micro-environmental effects, genetic variation and environmental influences. Canalization is the ability to minimize the effects of genetic or environmental effects, whereas developmental stability is the ability to minimize the effects of micro-environmental effects within individuals. Despite much attention, the mechanisms that underlie these two components of phenotypic robustness remain unknown. We investigated the genetic structure of phenotypic robustness in the collaborative cross (CC) mouse reference population. We analysed the magnitude of fluctuating asymmetry (FA) and among-individual variation of cranial shape in reciprocal crosses among the eight parental strains, using geometric morphometrics and a diallel analysis based on a Bayesian approach. Significant differences among genotypes were found for both measures, although they were poorly correlated at the level of individuals. An overall positive effect of inbreeding was found for both components of variation. The strain CAST/EiJ exerted a positive additive effect on FA and, to a lesser extent, among-individual variance. Sex- and other strain-specific effects were not significant. Neither FA nor among-individual variation was associated with phenotypic extremeness. Our results support the existence of genetic variation for both developmental stability and canalization. This finding is important because robustness is a key feature of developmental systems. Our finding that robustness is not related to phenotypic extremeness is consistent with theoretical work that suggests that its relationship to stabilizing selection is not straightforward.

Chronic Protein Restriction in Mice Impacts Placental Function and Maternal Body Weight before Fetal Growth

Mechanisms of resource allocation are essential for maternal and fetal survival, particularly when the availability of nutrients is limited. We investigated the responses of feto-placental development to maternal chronic protein malnutrition to test the hypothesis that maternal low protein diet produces differential growth restriction of placental and fetal tissues, and adaptive changes in the placenta that may mitigate impacts on fetal growth. C57BL/6J female mice were fed either a low-protein diet (6% protein) or control isocaloric diet (20% protein). On embryonic days E10.5, 17.5 and 18.5 tissue samples were prepared for morphometric, histological and quantitative RT-PCR analyses, which included markers of trophoblast cell subtypes. Potential endocrine adaptations were assessed by the expression of Prolactin-related hormone genes. In the low protein group, placenta weight was significantly lower at E10.5, followed by reduction of maternal weight at E17.5, while the fetuses became significantly lighter no earlier than at E18.5. Fetal head at E18.5 in the low protein group, though smaller than controls, was larger than expected for body size. The relative size and shape of the cranial vault and the flexion of the cranial base was affected by E17.5 and more severely by E18.5. The junctional zone, a placenta layer rich in endocrine and energy storing glycogen cells, was smaller in low protein placentas as well as the expression of Pcdh12, a marker of glycogen trophoblast cells. Placental hormone gene Prl3a1 was altered in response to low protein diet: expression was elevated at E17.5 when fetuses were still growing normally, but dropped sharply by E18.5 in parallel with the slowing of fetal growth. This model suggests that nutrients are preferentially allocated to sustain fetal and brain growth and suggests the placenta as a nutrient sensor in early gestation with a role in mitigating impacts of poor maternal nutrition on fetal growth.

Technical note: Performance of semi and fully automated approaches for registration of 3D surface coordinates in geometric morphometric studies

OBJECTIVES

One of the biggest challenges in the study of complex morphologies is to adequately describe shape variation. Here, we assess how the random sampling of surface points automatically obtained performs, when compared with observer-guided sampling procedures, and also evaluate the effect of sliding surface points by bending energy and minimum Procrustes distance.

MATERIAL AND METHODS

Three datasets comprising structures with disparate levels of complexity and intrasample variation are as follows: mouse molars, mouse brains, and primate endocasts. Different configurations of 3D coordinates on curves and surfaces were digitized from MRI images and CT scans using semi and fully automated procedures. Shape variables were obtained by Generalized Procrustes Superpositions before and after sliding the pseudolandmarks. Multivariate analyses were used to summarize and compare shape variation.

RESULTS

For the primate endocast, the semiautomated and automated strategies yield similar ordinations of specimens. Conversely, the semiautomated strategy better discriminates molar shapes between mouse groups. Shape differences among specimens are not adequately represented by the PCs calculated with surface pseudolandmarks. This is improved when the points are converted into semilandmarks by a sliding criterion.

DISCUSSION

Surface semilandmarks automatically obtained from 3D models are promising although they should be used with some caution in complex structures. This approach can be taken as complementary of semiautomated procedures which perform better for assessing shape variation in localized traits previously selected while automated procedures are suitable in studies aimed at comparing overall variation in shape and when there is no previous information about highly variable anatomical regions.

Effects of growth hormone on the ontogenetic allometry of craniofacial bones

Organism size is controlled by interactions between genetic and environmental factors mediated by hormones with systemic and local effects. As changes in size are usually not isometric, a considerable diversity in shape can be generated through modifications in the patterns of ontogenetic allometry. In this study we evaluated the role of timing and dose of growth hormone (GH) release on growth and correlated shape changes in craniofacial bones. Using a longitudinal study design, we analyzed GH deficient mice treated with GH supplementation commencing pre- and post-puberty. We obtained 3D in vivo micro-CT images of the skull between 21 and 60 days of age and used geometric morphometrics to analyze size and shape changes among control and GH deficient treated and non-treated mice. The variable levels of circulating GH altered the size and shape of the adult skull, and influenced the cranial base, vault, and face differently. While cranial base synchondroses and facial sutures were susceptible to either the direct or indirect effect of GH supplementation, its effect was negligible on the vault. Such different responses support the role of intrinsic growth trajectories of skeletal components in controlling the modifications induced by systemic factors. Contrary to the expected, the timing of GH treatment did not have an effect on catch-up growth. GH levels also altered the ontogenetic trajectories by inducing changes in their location and extension in the shape space, indicating that differences arose before 21 days and were further accentuated by a truncation of the ontogenetic trajectories in GHD groups.

Canalization and developmental instability of the fetal skull in a mouse model of maternal nutritional stress

Nutritional imbalance is one of the main sources of stress in both extant and extinct human populations. Restricted availability of nutrients is thought to disrupt the buffering mechanisms that contribute to developmental stability and canalization, resulting in increased levels of fluctuating asymmetry (FA) and phenotypic variance among individuals. However, the literature is contradictory in this regard. This study assesses the effect of prenatal nutritional stress on FA and among-individual variance in cranial shape and size using a mouse model of maternal protein restriction. Two sets of landmark coordinates were digitized in three dimensions from skulls of control and protein restricted specimens at E17.5 and E18.5. We found that, by the end of gestation, maternal protein restriction resulted in a significant reduction of skull size. Fluctuating asymmetry in size and shape exceeded the amount of measurement error in all groups, but no significant differences in the magnitude of FA were found between treatments. Conversely, the pattern of shape asymmetry was affected by the environmental perturbation since the angles between the first eigenvectors extracted from the covariance matrix of shape asymmetric component of protein restricted and control groups were not significantly different from the expected for random vectors. In addition, among-individual variance in cranial shape was significantly higher in the protein restricted than the control group at E18.5. Overall, the results obtained from a controlled experiment do not support the view of fluctuating asymmetry of cranial structures as a reliable index for inferring nutritional stress in human populations.

Developmental plasticity in covariance structure of the skull: effects of prenatal stress

Environmental perturbations of many kinds influence growth and development. Little is known, however, about the influence of environmental factors on the patterns of phenotypic integration observed in complex morphological traits. We analyze the changes in phenotypic variance-covariance structure of the rat skull throughout the early postnatal ontogeny (from birth to weaning) and evaluate the effect of intrauterine growth retardation (IUGR) on this structure. Using 2D coordinates taken from lateral radiographs obtained every 4 days, from birth to 21 days old, we show that the pattern of covariance is temporally dynamic from birth to 21 days. The environmental perturbation provoked during pregnancy altered the skull growth, and reduced the mean size of the IUGR group. These environmental effects persisted throughout lactancy, when the mothers of both groups received a standard diet. More strikingly, the effect grew larger beyond this point. Altering environmental conditions did not affect all traits equally, as revealed by the low correlations between covariance matrices of treatments at the same age. Finally, we found that the IUGR treatment increased morphological integration as measured by the scaled variance of eigenvalues. This increase coincided and is likely related to an increase in morphological variance in this group. This result is expected if somatic growth is a major determinant of covariance structure of the skull. In summary, our findings suggest that environmental perturbations experienced in early ontogeny alter fundamental developmental processes and are an important factor in shaping the variance-covariance structure of complex phenotypic traits.

Ontogeny of robusticity of craniofacial traits in modern humans: a study of South American populations

To date, differences in craniofacial robusticity among modern and fossil humans have been primarily addressed by analyzing adult individuals; thus, the developmental basis of such differentiation remains poorly understood. This article aims to analyze the ontogenetic development of craniofacial robusticity in human populations from South America. Geometric morphometric methods were used to describe cranial traits in lateral view by using landmarks and semilandmarks. We compare the patterns of variation among populations obtained with subadults and adults to determine whether population-specific differences are evident at early postnatal ontogeny, compare ontogenetic allometric trajectories to ascertain whether changes in the ontogeny of shape contribute to the differentiation of adult morphologies, and estimate the amount of size change that occurs during growth along each population-specific trajectory. The results obtained indicate that the pattern of interpopulation variation in shape and size is already established at the age of 5 years, meaning that processes acting early during ontogeny contribute to the adult variation. The ontogenetic allometric trajectories are not parallel among all samples, suggesting the divergence in the size-related shape changes. Finally, the extension of ontogenetic trajectories also seems to contribute to shape variation observed among adults.

Ecological and evolutionary factors in dental morphological diversification among modern human populations from southern South America

The knowledge of processes involved in morphological variation requires the integrated analysis of evolutionary and ecological factors. Here, we investigate the factors responsible for dental variation among human populations from southern South America. The aim of this work is to test the correspondence of dental size and shape variation with geographical, molecular (i.e. mtDNA) and ecological (i.e. climate, diet and food preparation) variables employing comparative phylogenetic methods, which have not previously been extensively applied at a within-species level. The results of the Procrustes analysis show a significant association of shape variables with molecular distance and geography, whereas dental size is not associated with molecular or geographical distances among groups. Phylogenetic generalized least-squares analysis, which takes into account the evolutionary autocorrelation among populations, shows a significant relationship between dental size variation and diet, while temperature and pottery do not correspond with dental size or shape. Specifically, groups with diets rich in carbohydrates, as well as the maritime hunter-gatherers, have the smallest teeth. In summary, our results support ecological factors as the dominant factor on dental size diversification in this region, while evolutionary relationships account for variation in dental shape.

Spatial patterns and evolutionary processes in southern South America: a study of dental morphometric variation

The purpose of this article is to examine the patterns of evolutionary relationships between human populations from the later Late Holocene (1,500-100 years BP) of southern South America on the basis of dental morphometric data. We tested the hypotheses that the variation observed in this region would be explained by the existence of populations with different phylogenetic origin or differential action of gene flow and genetic drift. In this study, we analyzed permanent teeth from 17 samples of male and female adult individuals from throughout southern South America. We measured mesiodistal and buccolingual diameters at the base of the crown, along the cement-enamel junction. The results of multiple regression analysis and a mantel correlogram indicate the existence of spatial structure in dental shape variation, as the D(2) Mahalanobis distance between samples increases with increasing geographical distance between them. In addition, the correlation test results show a trend toward reduction of the internal variation of samples with increasing latitude. The detected pattern of dental variation agrees with the one expected as an outcome of founder serial effects related to an expansion of range during the initial occupation of southern South America.

Discrepancy between cranial and DNA data of early Americans: implications for American peopling

Currently, one of the major debates about the American peopling focuses on the number of populations that originated the biological diversity found in the continent during the Holocene. The studies of craniometric variation in American human remains dating from that period have shown morphological differences between the earliest settlers of the continent and some of the later Amerindian populations. This led some investigators to suggest that these groups--known as Paleomericans and Amerindians respectively--may have arisen from two biologically different populations. On the other hand, most DNA studies performed over extant and ancient populations suggest a single migration of a population from Northeast Asia. Comparing craniometric and mtDNA data of diachronic samples from East Central Argentina dated from 8,000 to 400 years BP, we show here that even when the oldest individuals display traits attributable to Paleoamerican crania, they present the same mtDNA haplogroups as later populations with Amerindian morphology. A possible explanation for these results could be that the craniofacial differentiation was a local phenomenon resulting from random (i.e. genetic drift) and non-random factors (e.g. selection and plasticity). Local processes of morphological differentiation in America are a probable scenario if we take into consideration the rapid peopling and the great ecological diversity of this continent; nevertheless we will discuss alternative explanations as well.

Geometric morphometric approach to sex estimation of human pelvis

Sex estimation of skeletal remains is an important issue in both forensics and bioarchaeology. The chance of attaining a high level of accuracy regarding sex allocations is related to the skeletal component analyzed and the ability of the techniques employed to describe shape and size differences among the sexes. Current opinion regards the hip bone as the most reliable sex indicator because it is the most dimorphic bone, particularly in adult individuals. The aim of this study was therefore to analyze the greater sciatic notch and the ischiopubic complex morphology by employing geometric morphometric techniques, based on semilandmark and multivariate statistical methods, in order to develop a reliable and accurate technique for adult sex estimation. The sample analyzed consisted of 121 adult left hip bones randomly selected from the collection of documented skeletons housed at the Museu Antropologico de Coimbra. Morphometric analysis was based on coordinates of landmarks and semilandmarks of the ilium and ischiopubic regions that were digitized on 2D photographic images. Discriminant analysis with leave-one-out cross-validation and k-means clustering of shape and shape-size variables were used in order to classify individuals by sex. For the greater sciatic notch, average accuracy of 90.9% was achieved with both multivariate analyses based on shape variables. For the ischiopubic complex, the values obtained with shape variables were 93.4% and 90.1% for discriminant and k-means, respectively. Females were misclassified more frequently than males, especially for the ischiopubic complex. When multivariate statistical analyses were performed using shape-size variables, the percentages of correct classifications were lower than those obtained with shape variables. We conclude that the use of geometric morphometrics and multivariate statistics is a reliable method to quantify pelvic shape differences between the sexes and could be applied to discriminate between females and males.

Role of wild plant foods among late Holocene hunter-gatherers from Central and North Patagonia (South America): An approach from dental evidence

This study evaluates the role of plant foods in the subsistence of hunter-gatherers that inhabited the Central East, Northwest, and Northeast Patagonia (Argentina) during the late Holocene (ca. 3,000-500 years BP). The goal of the present study is to assess the temporal variation of dental caries ratio and wear rate in skeletal samples to ascertain if the biological information supports the dietary shift toward greater consumption of wild plant foods around 1,500 years BP, suggested by other types of evidence. The authors registered caries, antemortem and postmortem tooth loss, and tooth wear from eight samples belonging to hunter-gatherers from Patagonia for which chronological sequences from early late Holocene (ca. 3,000-1,500 years BP) up to final late Holocene (ca. 1,500-500 years BP) are available. The results indicate that caries percentages in Patagonian samples fall within the range established for hunter-gatherers but there are significant geographical differences. In addition, caries ratio does not change significantly through time, so the amount of carbohydrates consumed seems to have remained fairly constant since 3,000 years BP. In contrast, there is a marked temporal trend toward the reduction of wear rates in the three areas, suggesting a faster rate in early late Holocene than in final late Holocene. These results would reflect a change to less hard and/or abrasive diets in the final late Holocene, probably owing to differences in food processing methods employed.

Morphological differentiation of aboriginal human populations from Tierra del Fuego (Patagonia): Implications for South American peopling

This study aims to integrate the craniofacial morphological variation of southern South American populations with the results of mtDNA haplogroup variation, to discuss the South America peopling. Because the causes of morphological differentiation of Fueguian populations are still a controversial subject, the comparison with neutral variation could contribute to elucidate them. Samples of human remains from South America regions were used to analyze the evolutionary relationships. Several craniofacial traits observed in frontal and lateral view were analyzed by means of geometric morphometrics techniques, and the evolutionary relationships based on morphological and molecular data were established in base to ordination analyses. The results from the facial skeleton agree with those obtained from mtDNA haplogroup frequencies, with La Pampa/Chaco samples detached from the Patagonian samples. Hence, the same mechanism that accounts for the pattern of frequency of haplogroups could explain the variation found in facial skeleton among the samples. It is suggested that such geographic pattern of craniofacial and molecular diversity may reflect the effect of genetic drift that occurred in the small founding populations isolated by distance or geographic barriers. Conversely, the results obtained using the traits from the lateral view slightly differ from the molecular results, showing differences between southernmost Patagonian and the other samples. Therefore, mechanisms other than genetic drift (e.g., natural selection) could have acted to shape the pattern observed in some craniofacial structures present in the lateral view, characterized by the fact that the southernmost Patagonian samples display the most robust and dolichocephalic crania.

Differences between sliding semi-landmark methods in geometric morphometrics, with an application to human craniofacial and dental variation

Over the last decade, geometric morphometric methods have been applied increasingly to the study of human form. When too few landmarks are available, outlines can be digitized as series of discrete points. The individual points must be slid along a tangential direction so as to remove tangential variation, because contours should be homologous from subject to subject whereas their individual points need not. This variation can be removed by minimizing either bending energy (BE) or Procrustes distance (D) with respect to a mean reference form. Because these two criteria make different assumptions, it becomes necessary to study how these differences modify the results obtained. We performed bootstrapped-based Goodall's F-test, Foote's measurement, principal component (PC) and discriminant function analyses on human molars and craniometric data to compare the results obtained by the two criteria. Results show that: (1) F-scores and P-values were similar for both criteria; (2) results of Foote's measurement show that both criteria yield different estimates of within- and between-sample variation; (3) there is low correlation between the first PC axes obtained by D and BE; (4) the percentage of correct classification is similar for BE and D, but the ordination of groups along discriminant scores differs between them. The differences between criteria can alter the results when morphological variation in the sample is small, as in the analysis of modern human populations.

Variation and causal factors of craniofacial robusticity in Patagonian hunter-gatherers from the late Holocene

Fueguian-Patagonian skulls have been characterized as some of the most robust of any modern crania. However, the causal factors of such robusticity remain unsettled. We assess within- and among-sample cranial robusticity of seven samples from continental Patagonia and Tierra del Fuego, using geometric morphometric techniques. In addition, the biomechanical, phylogenetic, and climatic hypotheses proposed to account for robusticity in such samples are discussed. Two Amerindian samples of farmers and two early middle Holocene samples from South America were included. The results show: 1) large variation in craniofacial robusticity among Patagonian samples, with the highest robusticity in samples from south continental Patagonia and Isla Grande of Tierra del Fuego, whereas central and north Patagonian samples display the same degree of robusticity as farmer samples; 2) that early middle Holocene samples display lower levels of robusticity than South Patagonian samples; and 3) strong association between latitude and craniofacial robusticity, with the most robust craniofacial morphologies occurring at the highest latitudes. In consequence, neither masticatory stress nor retention of ancestral features is supported by the morphological evidence analyzed. Hence it is hypothesized that endocrine changes related to cold climate may be a plausible explanation for several craniofacial features found in Fueguian and south continental Patagonian samples, such as their large masticatory component, and pronounced supraorbital ridge and glabellar region.