Facial asymmetry and genetic ancestry in Latin American admixed populations.

Fluctuating and directional asymmetry are aspects of morphological variation widely used to infer environmental and genetic factors affecting facial phenotypes. However, the genetic basis and environmental determinants of both asymmetry types is far from being completely known. The analysis of facial asymmetries in admixed individuals can be of help to characterize the impact of a genome's heterozygosity on the developmental basis of both fluctuating and directional asymmetries. Here we characterize the association between genetic ancestry and individual asymmetry on a sample of Latin-American admixed populations. To do so, three-dimensional (3D) facial shape attributes were explored on a sample of 4,104 volunteers aged between 18 and 85 years. Individual ancestry and heterozygosity was estimated using more than 730,000 genome-wide markers. Multivariate techniques applied to geometric morphometric data were used to evaluate the magnitude and significance of directional and fluctuating asymmetry (FA), as well as correlations and multiple regressions aimed to estimate the relationship between facial FA scores and heterozygosity and a set of covariates. Results indicate that directional and FA are both significant, the former being the strongest expression of asymmetry in this sample. In addition, our analyses suggest that there are some specific patterns of facial asymmetries characterizing the different ancestry groups. Finally, we find that more heterozygous individuals exhibit lower levels of asymmetry. Our results highlight the importance of including ancestry-admixture estimators, especially when the analyses are aimed to compare levels of asymmetries on groups differing on socioeconomic levels, as a proxy to estimate developmental noise.

The relationship between facial shape asymmetry and attractiveness in Mexican students.

OBJECTIVES: It has been postulated that symmetric faces are considered more attractive than asymmetric ones because symmetry may signal high quality due to developmental stability. However, other studies showed that both symmetric and slightly asymmetric faces are considered attractive. Here we aim to explore this discrepancy, beginning with the analysis of the normal prevalence of facial symmetry in a population as a necessary first step prior to any attractiveness assessment. METHODS: We collected facial landmarks from two-dimensional digital images of a sample of Mexican individuals (280 females and 285 males aged 18-68 years) that were analyzed using geometric morphometric methods. Then, we chose a subsample of 100 photographs (50 females and 50 males aged 18-27 years) selected to represent a broad range of asymmetrical variation, in order to evaluate attractiveness using a sex-opposite test. Finally, we analyzed the linear correlation between attractiveness and asymmetry. RESULTS: We found that every evaluated subject presents some degree of facial asymmetry, and that both fluctuating asymmetry and directional asymmetry were significant (P < 0.0001) components of total facial asymmetry. Fluctuating asymmetry was slightly associated with age (r = 0.0858, P = 0.0414) and there were no differences between geographical regions (P = 0.413). Attractiveness was not correlated to levels of asymmetry in either sex (males: P = 0.0973; females P = 0.7415). CONCLUSIONS: Asymmetry was a prevalent feature in the present sample, and preferences for symmetric faces were not operating in the studied population.

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.

Collective Rhythm as an Emergent Property During Human Social Coordination.

The literature on social interactions has shown that participants coordinate not only at the behavioral but also at the physiological and neural levels, and that this coordination gives a temporal structure to the individual and social dynamics. However, it has not been fully explored whether such temporal patterns emerge during interpersonal coordination beyond dyads, whether this phenomenon arises from complex cognitive mechanisms or from relatively simple rules of behavior, or which are the sociocultural processes that underlie this phenomenon. We review the evidence for the existence of group-level rhythmic patterns that result from social interactions and argue that the complexity of group dynamics can lead to temporal regularities that cannot be predicted from the individual periodicities: an emergent collective rhythm. Moreover, we use this interpretation of the literature to discuss how taking into account the sociocultural niche in which individuals develop can help explain the seemingly divergent results that have been reported on the social influences and consequences of interpersonal coordination. We make recommendations on further research to test these arguments and their relationship to the feeling of belonging and assimilation experienced during group dynamics.

Ontogeny of the facial phenotypic variability in Mexican patients with 22q11.2 deletion syndrome.

BACKGROUND: 22q11.2 deletion syndrome is a medical condition that results from genomic loss at chromosome 22. Affected patients exhibit large variability that ranges from a severe condition to mild symptoms. In addition, the spectrum of clinical features differs among populations and even within family members. The facial features related to this syndrome are not an exception, and although part of its variation arises through development, few studies address this topic in order to understand the intra and inter-population heterogeneities. Here, we analyze the ontogenetic dynamics of facial morphology of Mexican patients with del22q11.2 syndrome. METHODS: Frontal facial photographs of 37 patients (mean age = 7.65 ± 4.21 SE) with del22q11.2DS and 200 control subjects (mean age = 7.69 ± 4.26 SE) were analyzed using geometric morphometric methods. Overall mean shape and size differences between patients and controls were analyzed, as well as differences in ontogenetic trajectories (i.e. development, growth, and allometry). RESULTS: We found that Mexican patients show typical traits that have been reported for the Caucasian population. Additionally, there were significant differences between groups in the facial shape and size when all the ontogenetic stages were considered together and, along ontogeny. The developmental and allometric trajectories of patients and controls were similar, but they differed in allometric scaling. Finally, patients and controls showed different growth trajectories. CONCLUSION: The results suggest that the typical face of patients with del22q11.2DS is established prenatally; nonetheless, the postnatal ontogeny could influence the dysmorphology and its variability through size-related changes.

Effects of an herbicide on physiology, morphology, and fitness of the dung beetle Euoniticellus intermedius (Coleoptera: Scarabaeidae).

Some agrochemical compounds threaten nontarget organisms and their functions in the ecosystem. The authors experimentally evaluated the effects of one of the most common herbicide mixtures used worldwide, containing 2,4-dichlorophenoxyacetic acid and picloram, on dung beetles, which play fundamental roles in the function of natural and managed ecosystems. The present study employed techniques of physiology and geometric morphometrics, besides including fitness measurements, to assess the effects of the herbicide in the introduced beetle Euoniticellus intermedius. Because herbicide components promote oxidative stress and affect survival in certain insects, the authors predicted negative effects on the beetles. Unexpectedly, no effect of herbicide concentration was found on clutch size, sex ratio, and fluctuating asymmetry, and it even increased physiological condition and body size in exposed beetles. Because the studied species presents 2 male morphs, the authors, for the first time, evaluated the effect of a pollutant on the ratio of these morphs. Contrary to the prediction, the herbicide mixture increased the proportion of major males. Thus, the herbicide does not threaten populations of the studied beetles. The present study discusses how both negative and positive effects of pollutants on wild animals modify natural and sexual selection processes occurring in nature, which ultimately impact population dynamics. The authors recommend the use of physiological and geometric morphometrics techniques to assess the impact of pollutants on nontarget animals. Environ Toxicol Chem 2017;36:96-102. © 2016 SETAC.

Quantitative assessment of the facial features of a Mexican population dataset.

The present study describes the morphological variation of a large database of facial photographs. The database comprises frontal (386 female, 764 males) and lateral (312 females, 666 males) images of Mexican individuals aged 14-69 years that were obtained under controlled conditions. We used geometric morphometric methods and multivariate statistics to describe the phenotypic variation within the dataset as well as the variation regarding sex and age groups. In addition, we explored the correlation between facial traits in both views. We found a spectrum of variation that encompasses broad and narrow faces. In frontal view, the latter is associated to a longer nose, a thinner upper lip, a shorter lower face and to a longer upper face, than individuals with broader faces. In lateral view, antero-posteriorly shortened faces are associated to a longer profile and to a shortened helix, than individuals with longer faces. Sexual dimorphism is found in all age groups except for individuals above 39 years old in lateral view. Likewise, age-related changes are significant for both sexes, except for females above 29 years old in both views. Finally, we observed that the pattern of covariation between views differs in males and females mainly in the thickness of the upper lip and the angle of the facial profile and the auricle. The results of this study could contribute to the forensic practices as a complement for the construction of biological profiles, for example, to improve facial reconstruction procedures.