Why do some traits show higher fluctuating asymmetry than others? A test of hypotheses with tail feathers of birds

Genetic, morphometric, and molecular analyses of interspecies differences in head shape and hybrid developmental defects in the wasp genus Nasonia

Males in the parasitoid wasp genus Nasonia have distinct, species-specific, head shapes. The availability of fertile hybrids among the species, along with obligate haploidy of males, facilitates analysis of complex gene interactions in development and evolution. Previous analyses showed that both the divergence in head shape between Nasonia vitripennis and Nasonia giraulti, and the head-specific developmental defects of F2 haploid hybrid males, are governed by multiple changes in networks of interacting genes. Here, we extend our understanding of the gene interactions that affect morphogenesis in male heads. Use of artificial diploid male hybrids shows that alleles mediating developmental defects are recessive, while there are diverse dominance relationships among other head shape traits. At the molecular level, the sex determination locus doublesex plays a major role in male head shape differences, but it is not the only important factor. Introgression of a giraulti region on chromsome 2 reveals a recessive locus that causes completely penetrant head clefting in both males and females in a vitripennis background. Finally, a third species (N. longicornis) was used to investigate the timing of genetic changes related to head morphology, revealing that most changes causing defects arose after the divergence of N. vitripennis from the other species, but prior to the divergence of N. giraulti and N. longicornis from each other. Our results demonstrate that developmental gene networks can be dissected using interspecies crosses in Nasonia, and set the stage for future fine-scale genetic dissection of both head shape and hybrid developmental defects.

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.

Prenatal Developmental Trajectories of Fluctuating Asymmetry in Bat Humeri

Fluctuating asymmetry (random fluctuations between the left and right sides of the body) has been interpreted as an index to quantify both the developmental instabilities and homeostatic capabilities of organisms, linking the phenotypic and genotypic aspects of morphogenesis. However, studying the ontogenesis of fluctuating asymmetry has been limited to mostly model organisms in postnatal stages, missing prenatal trajectories of asymmetry that could better elucidate decoupled developmental pathways controlling symmetric bone elongation and thickening. In this study, we quantified the presence and magnitude of asymmetry during the prenatal development of bats, focusing on the humerus, a highly specialized bone adapted in bats to perform under multiple functional demands. We deconstructed levels of asymmetry by measuring the longitudinal and cross-sectional asymmetry of the humerus using a combination of linear measurements and geometric morphometrics. We tested the presence of different types of asymmetry and calculated the magnitude of size-controlled fluctuating asymmetry to assess developmental instability. Statistical support for the presence of fluctuating asymmetry was found for both longitudinal and cross-sectional asymmetry, explaining on average 16% of asymmetric variation. Significant directional asymmetry accounted for less than 6.6% of asymmetric variation. Both measures of fluctuating asymmetry remained relatively stable throughout ontogeny, but cross-sectional asymmetry was significantly different across developmental stages. Finally, we did not find a correspondence between developmental patterns of longitudinal and cross-sectional asymmetry, indicating that processes promoting symmetrical bone elongation and thickening work independently. We suggest various functional pressures linked to newborn bats’ ecology associated with longitudinal (altricial flight capabilities) and cross-sectional (precocial clinging ability) developmental asymmetry differentially. We hypothesize that stable magnitudes of fluctuating asymmetry across development could indicate the presence of developmental mechanisms buffering developmental instability.

Weak genetic signal for phenotypic integration implicates developmental processes as major regulators of trait covariation

Phenotypic integration is an important metric that describes the degree of covariation among traits in a population, and is hypothesized to arise due to selection for shared functional processes. Our ability to identify the genetic and/or developmental underpinnings of integration is marred by temporally overlapping cell-, tissue- and structure-level processes that serve to continually 'overwrite' the structure of covariation among traits through ontogeny. Here, we examine whether traits that are integrated at the phenotypic level also exhibit a shared genetic basis (e.g. pleiotropy). We micro-CT scanned two hard tissue traits, and two soft tissue traits (mandible, pectoral girdle, atrium and ventricle, respectively) from an F5 hybrid population of Lake Malawi cichlids, and used geometric morphometrics to extract 3D shape information from each trait. Given the large degree of asymmetric variation that may reflect developmental instability, we separated symmetric from asymmetric components of shape variation. We then performed quantitative trait loci (QTL) analysis to determine the degree of genetic overlap between shapes. While we found ubiquitous associations among traits at the phenotypic level, except for a handful of notable exceptions, our QTL analysis revealed few overlapping genetic regions. Taken together, this indicates developmental interactions can play a large role in determining the degree of phenotypic integration among traits, and likely obfuscate the genotype to phenotype map, limiting our ability to gain a comprehensive picture of the genetic contributors responsible for phenotypic divergence.

Fluctuating asymmetry and feather growth bars as biomarkers to assess the habitat quality of shade coffee farming for avian diversity conservation

Shade coffee farming has been promoted as a means of combining sustainable coffee production and biodiversity conservation. Supporting this idea, similar levels of diversity and abundance of birds have been found in shade coffee and natural forests. However, diversity and abundance are not always good indicators of habitat quality because there may be a lag before population effects are observed following habitat conversion. Therefore, other indicators of habitat quality should be tested. In this paper, we investigate the use of two biomarkers: fluctuating asymmetry (FA) of tarsus length and rectrix mass, and feather growth bars (average growth bar width) to characterize the habitat quality of shade coffee and natural forests. We predicted higher FA and narrower feather growth bars in shade coffee forest versus natural forest, indicating higher quality in the latter. We measured and compared FA in tarsus length and rectrix mass and average growth bar width in more than 200 individuals of five bird species. The extent of FA in both tarsus length and rectrix mass was not different between the two forest types in any of the five species. Similarly, we found no difference in feather growth between shade coffee and natural forests for any species. Therefore, we conclude our comparison of biomarkers suggests that shade coffee farms and natural forests provide similar habitat quality for the five species we examined.

Developmental pathways inferred from modularity, morphological integration and fluctuating asymmetry patterns in the human face

Facial asymmetries are usually measured and interpreted as proxies to developmental noise. However, analyses focused on its developmental and genetic architecture are scarce. To advance on this topic, studies based on a comprehensive and simultaneous analysis of modularity, morphological integration and facial asymmetries including both phenotypic and genomic information are needed. Here we explore several modularity hypotheses on a sample of Latin American mestizos, in order to test if modularity and integration patterns differ across several genomic ancestry backgrounds. To do so, 4104 individuals were analyzed using 3D photogrammetry reconstructions and a set of 34 facial landmarks placed on each individual. We found a pattern of modularity and integration that is conserved across sub-samples differing in their genomic ancestry background. Specifically, a signal of modularity based on functional demands and organization of the face is regularly observed across the whole sample. Our results shed more light on previous evidence obtained from Genome Wide Association Studies performed on the same samples, indicating the action of different genomic regions contributing to the expression of the nose and mouth facial phenotypes. Our results also indicate that large samples including phenotypic and genomic metadata enable a better understanding of the developmental and genetic architecture of craniofacial phenotypes.

Influence of physiological stress on the presence of hypoplasia and fluctuating asymmetry in a medieval population from the village of Sypniewo

This study aims to estimate the levels of physiological stress in the medieval rural population of Sypniewo by evaluating patterns of fluctuating asymmetry (FA) and enamel hypoplasia (EH), and provide information on the influence of physiological stress during the prenatal and perinatal period on early childhood development. Stress is defined as any external or internal condition that challenges homeostasis of an organism. FA is associated with physiological stress occurring mainly during prenatal development and early childhood. The level of FA is thought to reflect the intensity of the stressor(s). EH is caused by physiological stress such as nutritional instability during the first years of life. The studied material consisted of 126 skulls from the village of Sypniewo (Poland). Cranial radiographs were taken in postero-anterior (P-A) and basal views. The images were scanned and calibrated. Measurements of the cranium were used to estimate FA. The presence of EH was assessed using standard anthropological methods The highest levels of FA were observed in the region of the cranial base. EH was observed in 29% of individuals from the rural skeletal series. There was no statistically significant correlation between FA and EH occurrence or between sex and the studied stress indicators.

Mercury exposure may influence fluctuating asymmetry in waterbirds

Variation in avian bilateral symmetry can be an indicator of developmental instability in response to a variety of stressors, including environmental contaminants. The authors used composite measures of fluctuating asymmetry to examine the influence of mercury concentrations in 2 tissues on fluctuating asymmetry within 4 waterbird species. Fluctuating asymmetry increased with mercury concentrations in whole blood and breast feathers of Forster's terns (Sterna forsteri), a species with elevated mercury concentrations. Specifically, fluctuating asymmetry in rectrix feather 1 was the most strongly correlated structural variable of those tested (wing chord, tarsus, primary feather 10, rectrix feather 6) with mercury concentrations in Forster's terns. However, for American avocets (Recurvirostra americana), black-necked stilts (Himantopus mexicanus), and Caspian terns (Hydroprogne caspia), the authors found no relationship between fluctuating asymmetry and either whole-blood or breast feather mercury concentrations, even though these species had moderate to elevated mercury exposure. The results indicate that mercury contamination may act as an environmental stressor during development and feather growth and contribute to fluctuating asymmetry of some species of highly contaminated waterbirds. Environ Toxicol Chem 2017;36:1599-1605. Published 2016 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

The fluctuating asymmetry of medieval and modern human skulls

The analysis of fluctuating asymmetry (FA) allows for estimating the influence of stress factors on human development and allows to evaluate resistance to stress. FA is often used as a marker of prenatal stress. The aim of this work is to estimate the symmetry of skulls from selected historic human populations and to analyse changes in their morphology which have taken place over centuries. The studied material consisted of two skull samples - a modern sample containing 82 skulls and a medieval sample of 77 skulls from Gródek on the Bug River. Radiographs were taken in postero-anterior (P-A) and base projections. Images were scanned and calibrated by means of MicroStation 95 Academic Edition software. Measurements of the skull images were used to estimate FA. All data were analysed statistically. The skulls in both samples showed asymmetry. The levels of FA varied in different skull regions. A high level of FA in the calvaria and a low asymmetry for the facial part of skull is characteristic of modern skulls. In medieval skulls these relations are inverted. The higher value of FA in modern skulls is an evidence of a higher level of developmental stress in the modern population as well as of its lesser abilities to resist stress.

Fluctuating asymmetry and stress in a medieval Nubian population

Fluctuating asymmetry is commonly used as a bioindicator of developmental stress. This study addresses asymmetry under nutritional/systemic stress in the human craniofacial skeleton and its utility as an indicator of developmental instability. Crania from the diachronic Christian cemeteries at Kulubnarti (Sudanese Nubia) were chosen as a model for nutrition/systemic stress. Previous studies indicate that individuals from the Early Christian cemetery were subjected to greater developmental stress when compared with individuals from the Late Christian cemetery. Therefore, crania from the Early Christian cemetery should display a greater magnitude of fluctuating asymmetry than crania from the Late Christian cemetery. Thirty adult crania of comparable age and sex were selected from each population. Landmark coordinates were digitized in two separate trials and averaged to minimize error. Euclidean distance matrix analysis (EDMA) was used to measure and compare the magnitude of fluctuating asymmetry in each sample. Results indicate that crania from the Early Christian cemetery display greater amounts of fluctuating asymmetry than those from the Late Christian cemetery, as predicted. The degree of fluctuating asymmetry for each linear distance is highly correlated between the cemeteries, suggesting that all humans may share common patterns of fluctuating asymmetry in the skull. In contrast, there is little correlation between magnitude of fluctuating asymmetry and length of linear distance, between-subject variability, or measurement error. These results support the hypothesis that poor nutrition/systemic stress increases developmental instability in the human skull and that increased fluctuating asymmetry constitutes morphological evidence of this stress.

Evolution of morphological integration: developmental accommodation of stress-induced variation

Extreme environmental change during growth often results in an increase in developmental abnormalities in the morphology of an organism. The evolutionary significance of such stress-induced variation depends on the recurrence of a stressor and on the degree to which developmental errors can be accommodated by an organism's ontogeny without significant loss of function. We subjected populations of four species of soricid shrews to an extreme environment during growth and measured changes in the patterns of integration and accommodation of stress-induced developmental errors in a complex of mandibular traits. Adults that grew under an extreme environment had lower integration of morphological variation among mandibular traits and highly elevated fluctuating asymmetry in these traits, compared to individuals that grew under the control conditions. However, traits differed strongly in the magnitude of response to a stressor--traits within attachments of the same muscle (functionally integrated traits) had lower response and changed their integration less than other traits. Cohesiveness in functionally integrated complexes of traits under stress was maintained by close covariation of their developmental variation. Such developmental accommodation of stress-induced variation might enable the individual's functioning and persistence under extreme environmental conditions and thus provides a link between individual adaptation to stress and the evolution of stress resistance.

Evolution of morphological integration. I. Functional units channel stress-induced variation in shrew mandibles

Stress-induced deviations from normal development are often assumed to be random, yet their accumulation and expression can be influenced by patterns of morphological integration within an organism. We studied within-individual developmental variation (fluctuating asymmetry) in the mandible of four shrew species raised under normal and extreme environments. Patterns of among-individual variation and fluctuating asymmetry were strongly concordant in traits that were involved in the attachment of the same muscles (i.e., functionally integrated traits), and fluctuating asymmetry was closely integrated among these traits, implying direct developmental interactions among traits involved in the same function. Stress-induced variation was largely confined to the directions delimited by functionally integrated groups of traits in the pattern that was concordant with species divergence--species differed most in the same traits that were most sensitive to stress within each species. These results reveal a strong effect of functional complexes on directing and incorporating stress-induced variation during development and might explain the historical persistence of sets of traits involved in the same function in shrew jaws despite their high sensitivity to environmental variation.