Fluctuating dental asymmetry as a measure of odontogenic canalization in man

No association between developmental instability as a general measure of stress and 2D:4D digit ratios in a non-western sample

OBJECTIVE

Random deviation from perfect symmetry of organisms is defined as fluctuating asymmetry. Fluctuating asymmetry is widely considered to be a marker of developmental instability, developmental noise and phenotypic quality at the population level. In this study, we investigated hypothesized relationship between 2D:4D digit ratios as a proxy measure of prenatal sex hormones and developmental instability levels in young adult humans.

MATERIALS AND METHODS

In the study, seven bilateral traits were measured for fluctuating asymmetry, as well as second and fourth digit lengths on both hands of 185 individuals, 87 men (mean age: 22.42 ± 3.37) and 97 women (mean age: 22.88 ± 3.87). The second digit length was divided by the fourth digit length and the resulting value was used as the digit ratio (2D:4D). Composite fluctuating asymmetry was calculated using five bilateral traits displaying fluctuating asymmetry.

RESULTS

Only the right hand 2D:4D ratio is lower in men than in women. However, this difference disappears when the effect of digit lengths is controlled. Composite fluctuating asymmetry results reveal that men are more asymmetrical than women. There was no significant relationship between digit ratios and composite fluctuating asymmetry.

CONCLUSION

In this study, the 2D:4D digit ratios appear to be influenced (at least in humans) by different digit lengths. This study also confirms previous studies that digit ratios as a proxy measure of exposure to either high testosterone or estrogen levels during early development does not appear to affect levels of developmental instability.

Developmental instability in wild Nigerian olive baboons (Papio anubis)

Background

Developmental instability in archaeological samples can be detected through analysis of skeletal and dental remains. During life, disruptions to biological internal homeostasis that occur during growth and development redirect bodily resources to returning to homeostasis and away from normal processes such as symmetrical development. Because dental enamel does not remodel in life, any deviations from normal development are left behind. Even subtle disturbances to developmental trajectory may be detected in asymmetrical development of traits, specifically a random variation in sides termed fluctuating asymmetry. Human dental fluctuating asymmetry studies are common, but here we investigate the permanent dentition of a non-human primate Papio anubis, for potential fluctuating asymmetry relative to sex, weaning, and reproductive maturity. The sample stems from an outlier population that lives in the wettest and most humid habitat of any studied baboon group.

Methods

The skulls of adult baboons were collected after their natural death in Gashaka Gumti National Park, Nigeria. The permanent dentition of antimeric teeth (paired) were measured for maximum length and breadth using standard methods. The metrics were analyzed to assess the presence of fluctuating asymmetry in adult permanent mandibular and maxillary dentition. Measurement error and other forms of asymmetry (antisymmetry, directional asymmetry) were considered and dental measures expressing true fluctuating asymmetry were used to address three research questions.

Results

Males exhibit greater fluctuating asymmetry than females, suggesting that males experience greater overall instability during the developmental period. While weaning is not more stressful than other life history stages for males and females (using the first molar fluctuating asymmetry index as a proxy compared to other teeth), it is more stressful for females than males. The onset of reproduction is also not more stressful than other life history stages for males and females (using the third molar fluctuating asymmetry index as a proxy compared to other teeth), but it is more stressful for males than females. We explore possible explanations for these findings in the discussion.

FLUCTUATING ODONTOMETRIC ASYMMETRY, MORPHOLOGICAL VARIABILITY, AND GENETIC MONOMORPHISM IN THE CHEETAH ACINONYX JUBATUS

The magnitudes of dimensional variability and fluctuating asymmetry in dental dimensions are reported for a sample of South African cheetah Acinonyx jubatus. To test the hypothesis that elevated levels of variability and asymmetry are associated with the increased developmental instability reported for this species, our results were contrasted to those for two other felids: Felis lybica and F. caracal. These findings suggest that dental dimensions in cheetahs are not significantly more variable or asymmetric. Hence, it is concluded that the cheetah may not be as developmentally unstable as was previously supposed.

Dermatoglyphics in hypertension: a review

Hypertension is a major contributor to the global burden of disease and mortality. A major medical advancement would be a better means to ascertain which persons are at higher risk for becoming hypertensive beforehand. To that end, there have been a number of studies showing that certain dermatoglyphic markers are associated with hypertension. This association could be explained if the risk toward developing hypertension later on in life is somehow connected with fetal development of dermatoglyphics. It would be highly valuable from a clinical standpoint if this conjecture could be substantiated since dermatoglyphic markers could then be used for screening out individuals who might be at an elevated risk of becoming hypertensive. The aim of this review was to search for and appraise available studies that pertain to the association between hypertension and dermatoglyphics.

A systematic literature search conducted using articles from MEDLINE (PubMed), Trip, Cochran, Google scholar, and gray literature until December 2014. Of the 37 relevant publications, 17 were included in the review. The review performed according to the preferred reporting items for systematic reviews and meta-analyses (PRISMA) statement.

This review showed a fairly consistent finding of an increased frequency of whorl patterns along with a higher mean total ridge count in digital dermatoglyphic results in hypertensive samples compared to controls. However, it was imperative to discuss several limitations found in the studies that could make this association as yet unsettled.

Dimensions of central incisors, canines, and first molars in subjects with Down syndrome measured on cone-beam computed tomographs

INTRODUCTION

The literature on tooth dimensions in subjects with Down syndrome (DS) is scarce. To our knowledge, no studies have yet been published in which the morphometry of the teeth in DS subjects has been determined using computed tomography.

METHODS

The study group consisted of 40 subjects with DS, aged 10 to 40 years. An age- and sex-matched control group was selected. Cone-beam computed tomography images were retrieved from the archive of the Santiago de Compostela University in Spain. The maxillary central incisors, canines, and first molars were evaluated. The following variables were analyzed: overall tooth length, crown height, root length, mesiodistal diameter, vestibular-palatine diameter, crown-to-root ratio, and cervical circumference.

RESULTS

The teeth of subjects with DS were smaller than those of the controls, although the crown-to-root ratio was maintained. No clear sexual dimorphism was detected, but the root lengths of the incisors were greater in male subjects. Crown height, mesiodistal diameter, and crown-to-root ratio showed progressive reductions with age. There appeared to be a degree of "fluctuating dental asymmetry" with respect to vestibular-palatine diameter and cervical circumference.

CONCLUSIONS

Our findings confirm microdontia of the permanent teeth and progressive reductions in tooth sizes with age in persons with DS. These variations might be relevant to orthodontic treatment planning.

The second to fourth digit ratio and asymmetry

BACKGROUND

Deviations of physical characteristics from bilateral symmetry, in otherwise symmetric individuals, are supposed to result from environmental perturbations during development. One cause of such perturbations may be sex steroids such as testosterone and oestrogen.

AIM

The study examined the relationship between second to fourth digit ratio (2D:4D), a putative negative correlate with prenatal testosterone and a positive correlate with prenatal oestrogen, and asymmetry.

METHODS

Eleven traits (including the second and fourth finger lengths) were measured in a sample of 680 English children aged 2-18 years, and second to fifth finger lengths in samples of 120 Austrian and English undergraduate students aged from 17 to 30 years and 213 Polish adults aged from 26 to 90 years.

RESULTS

Significant U-shaped curvilinear associations between 2D:4D and all 11 traits were found in English children with the strongest associations between 2D:4D and composite asymmetry of second plus fourth digit, and second to fifth digits. Further investigation of the relation between 2D:4D and digit asymmetries in the sample of Austrian and English undergraduates and the Polish adults confirmed significant U-shaped relationships between 2D:4D and finger asymmetries.

CONCLUSION

Our data show that both low 2D:4D (a marker of high prenatal testosterone) and high 2D:4D (a marker of high prenatal oestrogen) are associated with elevated levels of asymmetry and this relationship applies particularly to finger asymmetry.

Dental fluctuating asymmetry in the Gullah: Tests of hypotheses regarding developmental stability in deciduous vs. permanent and male vs. female teeth

In this investigation, deciduous teeth (canines, c; first molars, m1; second molars, m2) and their permanent successors (canines, C; first premolars, P1; second premolars, P2) were used to test two related hypotheses about fluctuating asymmetry (FA). First, based on the biology of the developing dentition, it was predicted that deciduous teeth would be more developmentally stable and thus exhibit less dimensional FA than their permanent successors. Second, based on sex differences in tooth development, it was predicted that female canines would have greater developmental stability (less FA) than male canines. Bucco-lingual measurements were made on dental casts from a single Gullah population. Using a repeated-measures study design (n = 3 repeated measures), we tested these hypotheses on sample sizes ranging from 63-82 antimeric pairs. Neither hypothesis was supported by our data. In most cases, Gullah deciduous teeth did not exhibit statistically significantly less FA than their permanent successors; indeed, statistically significant differences were found for only 3 of 12 deciduous vs. permanent contrasts, and in two cases, the deciduous tooth had greater FA. Female mandibular canines exhibited statistically significantly greater FA than those of males, while there was no statistically significant sex difference in FA for the maxillary canine. FA in these Gullah samples is high when compared to Archaic and late prehistoric Ohio Valley Native Americans, consistent with historical and archaeological evidence that environmental stress was relatively higher in the Gullah population. We suggest that when environmental stress in a population is high, the impact of differences in tooth formation time spans and developmental buffering upon FA may be minor relative to the effect of developmental noise.

Dental Arch Asymmetry in an Isolated Adriatic Community

Developmental stability reflects the ability of a genotype to develop in the same way under varying environmental conditions. Deviations from developmental stability, arising from disruptive effects of environmental and genetic stresses, can be measured in terms of fluctuating asymmetry, a particularly sensitive indicator of the ability to cope with these stresses during ontogeny. In an inbred Adriatic island population, we expected dental arch fluctuating asymmetry 1) to be higher than in an outbred sample from the same island, and 2) within this population, to increase with the level of inbreeding. Due to environmental stress, we also expected to find higher fluctuating asymmetry in the outbred island population than in an urban reference group from the same country. The material consisted of 506 dental casts of 253 children from 1) the island of Hvar, and 2) Zagreb, Croatia. Three-dimensional coordinates of 26 landmarks spanning the arches were digitized. The analysis partitioned the asymmetry of arch forms into components for directional and fluctuating bilateral asymmetry, using the appropriate Procrustes method (geometric morphometrics). The results corroborated the hypotheses. Fluctuating asymmetry was found to be higher on the island than in Zagreb in all groups and in both jaws, and increased significantly with endogamy level in the lower jaw. There was no significant directional asymmetry in the Zagreb sample and likewise none in the upper jaws of the outbred island group, but significant directional asymmetry in both jaws of the inbred population and also in the lower jaws of the outbred island group. These results suggest an environmental as well as a genetic influence on dental arch asymmetry. Although the lower jaws expressed these two stresses almost additively, the upper jaws appeared to be better buffered. The role of directional asymmetry as a potential indicator of craniofacial developmental instability clearly merits further attention.

The mesiodistal crown diameters of primary dentition in Indonesian Javanese children

Dentition analysis of primary teeth is necessary for recognising and correcting occlusal problems in every stage of dental development to enable normal adult occlusion. To do this, normative data of mesiodistal tooth crown diameters from the same ethnic population are needed. The aims of this study were to gather normative data of mesiodistal crown diameters of primary dentition in Indonesian Javanese children and to compare this normative data with published data of other ethnic populations. Dental casts of 160 males and 137 females with acceptable occlusion, aged 3.25-6.58 years, were taken in Yogyakarta, Indonesia. Mesiodistal diameter was measured as the distance between the anatomic contact points using calipers with accuracy to within 0.05 mm. Each measurement was taken twice on different occasions. The results indicated that the magnitude of asymmetry between right and left teeth was larger in distal teeth within a tooth field, larger in males than females, and larger in mandibular than maxillary teeth. The stability of mesiodistal tooth crown diameters was less in males than in females, and was most prominent in the mandibular central incisor. Sexual dimorphisms were found in, the lateral incisor and first molar in the maxilla, and the canine, first and second molars in the mandible. Compared with other ethnic populations, Indonesian Javanese falls between Hong Kong Chinese and Australian Aboriginal.

Embryological origins of developmental stability: size, shape and fluctuating asymmetry in prenatal random bred mice

Ontogenetic patterns of fluctuating asymmetry (FA) can be used to test models for the mechanisms underlying stability during embryonic development (developmental stability). In this study, we ask whether developmental processes initially show high levels of instability that are subsequently dampened through active compensatory mechanisms or passive properties of developmental systems or whether the effects of instability accumulate during embryonic development causing random drift away from an earlier stable state. Previous work on this question has dealt with postnatal skeletal growth and thus been unable to effectively distinguish developmental instability from the effects of mechanically mediated variation in bone modeling and remodeling. Here, we report that FA variances of limb skeletal elements in CD1 mice decrease with gestational age from day 14 to birth (day 20.5). Thus, in mouse limbs, skeletal development is characterized by a high level of developmental instability initially that is reduced during subsequent prenatal development. These results are consistent with the existence of active mechanisms that compensate for the effects of minor perturbations or deviations during development. However, they are also consistent with Soule's model of allomeric variation in which the variance of structures is reduced as the number of independent developmental events that produce them increases. This study illustrates that predictions based on morphometric analyses can yield insights into general properties of developmental systems in cases where specific developmental mechanisms are not yet known.

Canalization, developmental stability, and morphological integration in primate limbs

Canalization and developmental stability refer to the tendency of developmental processes to follow particular trajectories, despite external or internal perturbation. Canalization is the tendency for development of a specific genotype to follow the same trajectory under different conditions (different environments or different genetic backgrounds), while developmental stability is the tendency for the development of a specific genotype to follow the same trajectory under the same conditions. Morphological integration refers to the tendency for structures to show correlated variation because they develop in response to shared developmental processes or function in concert with other structures. All three phenomena are emergent properties of developmental systems that can affect the interaction of development and evolution. In this paper, we review the topics of canalization, developmental stability, and morphological integration and their relevance to primate and human evolution. We then test three developmentally motivated hypotheses about the patterning of variability components in the mammalian limb. We find that environmental variances and fluctuating asymmetries (FA) increase distally along the limb in adult macaques but not in fetal mice. We infer that the greater variability of more distal segments in macaques is due to postnatal mechanical effects. We also find that heritability and FA are significantly correlated when different limb measurements are compared in fetal mice. This supports the idea that the mechanisms underlying canalization and developmental stability are related. Finally, we report that the covariation structure of fore- and hindlimb skeletal elements shows evidence for morphological integration between serially homologous structures between the limbs. This is evidence for the existence of developmental modules that link structures between the limbs. Such modules would produce covariation that would need to be overcome by selection for divergence in hind- and forelimb morphology.

Assessment of soft tissue facial asymmetry in medically normal and syndrome-affected individuals by analysis of landmarks and measurements

We investigated soft tissue facial asymmetry in normal and syndrome-affected individuals ranging in age from 1 year to adulthood. The purposes of our study were to determine if facial asymmetry was greater in syndrome-affected individuals than in normal individuals and, if true, to distinguish those measurements that could be used in routine screening to identify the presence of syndromes in uncertain patients and, lastly, to investigate the causes of measurement asymmetry at the level of the landmarks. The last purpose was possible because we used a stereophotogrammetric method with which the three-dimensional (3D) landmark positions were obtained. In the statistically significantly different measurements, those from the right side were dominant, with one exception in each group, except normal males. In all groups the landmark analyses demonstrated the same trends, and while there was far less patterning in the 3D coordinates, these results were also consistent between the four groups. We compared the statistical findings of the 3D coordinates and measurements and found that there was no predictable relationship between significant findings in the landmarks and the measurements. In particular, we noted that statistical differences in measurements did not infer significant differences in the positions of the landmarks between the right and left sides of the face. Both the normal and syndrome-affected groups appeared to be equally canalized and similarly affected by developmental noise: When the bilateral measurement differences of each syndrome-affected subject were compared to the limits of normal asymmetry, less than 10% of the comparisons exceeded the norms.

Independence between developmental stability and canalization in the skull of the house mouse

The relationship between the two components of developmental homeostasis, that is canalization and developmental stability (DS), is currently debated. To appraise this relationship, the levels and morphological patterns of interindividual variation and fluctuating asymmetry were assessed using a geometric morphometric approach applied to the skulls of laboratory samples of the house mouse. These three samples correspond to two random-bred strains of the two European subspecies of the house mouse and their F1 hybrids. The inter- and intraindividual variation levels were found to be smaller in the hybrid group compared to the parental ones, suggesting a common heterotic effect on skull canalization and DS. Both buffering mechanisms might then depend on the same genetic condition, i.e. the level of heterozygosity. However, related morphological patterns did not exhibit any congruence. In contradiction with previous studies on insect wing traits, we therefore suggest that canalization and DS may not act on the same morphological characters. The fact that this discrepancy could be related to the functional importance of the symmetry of the characters under consideration is discussed in the light of our knowledge of the genetic bases of both components of developmental homeostasis.

Deciduous tooth size and morphogenetic fields in children from Christ Church, Spitalfields

Deciduous tooth dimensions of a recent archaeological sample of 37 boys, 18 girls and 88 children of unknown sex from London were measured. Mesiodistal (maximum breadth at the contact point) and buccolingual measurements were recorded. A gradient of size variation was apparent with anterior teeth showing the highest variation and second molars the least. In general, the teeth of the children of Spitalfields were smaller than those of other groups from various archaeological and contemporary populations. Fluctuating asymmetry between left- and right-hand sides was greatest for the lateral incisor. Asymmetry of the mesiodistal dimension of the upper central incisor differed significantly from the buccolingual dimension (p < 0.01). Tooth size in both dimensions was generally larger in boys than in girls. The maxillary lateral incisor displayed significant dimorphism in the mesiodistal dimension (p < 0.05). The second molars (both upper and lower) were less variable and less asymmetrical than the first molars, suggesting that the second molar may be the key tooth of the deciduous molar field.

Fluctuating asymmetry and psychometric intelligence

Little is known about the genetic nature of human psychometric intelligence (IQ), but it is widely assumed that IQ's heritability is at loci for intelligence per se. We present evidence consistent with a hypothesis that interindividual IQ differences are partly due to heritable vulnerabilities to environmental sources of developmental stress, an indirect genetic mechanism for the heritability of IQ. Using fluctuating asymmetry (FA) of the body (the asymmetry resulting from errors in the development of normally symmetrical bilateral traits under stressful conditions), we estimated the relative developmental instability of 112 undergraduates and administered to them Cattell's culture fair intelligence test (CFIT). A subsequent replication on 128 students was performed. In both samples, FA correlated negatively and significantly with CFIT scores. We propose two non-mutually exclusive physiological explanations for this correlation. First, external body FA may correlate negatively with the developmental integrity of the brain. Second, individual energy budget allocations and/or low metabolic efficiency in high-FA individuals may lower IQ scores. We review the data on IQ in light of our findings and conclude that improving developmental quality may increase average IQ in future generations.

Fluctuating asymmetry and disorders of developmental origin

Environmental and/or genetic stresses may cause a breakdown in developmental homeostasis, resulting in increased bilateral asymmetry of morphological traits. The degree of these deviations (termed "fluctuating asymmetry") is thought to correlate with the severity of the stress. If these stresses also play a role in the appearance of developmental disorders, then increased morphological asymmetry may serve as a risk marker for disorders of developmental origin. This would be possible if 1) the environmental stress that caused a breakdown in developmental stability also contributed to the appearance of the disorder, and/or 2) the genetic predisposition (liability) to the disorder and increased susceptibility to fluctuating asymmetry have a common cause. Although a number of authors have reported associations between increased fluctuating asymmetry and disorders of presumed developmental origin, the usefulness of fluctuating asymmetry as a risk marker has not been established. One obstacle to this assessment is the lack of odds ratios reported by previous authors.

Developmental stability and its applications in ecotoxicology

: Developmental stability refers to the ability of a developing organism to produce a consistent phenotype in a given environment. It provides a simple, reliable method of detecting stressed populations and monitoring their recovery. The most common measure of developmental instability, fluctuating asymmetry, assesses minor deviations from perfect bilateral symmetry in traits that are normally symmetrical. Measures of developmental instability are based upon the concept of developmental invariance. The biotest approach consists of the simultaneous analysis of developmental instability (and related physiological instability) in a variety of species.

Variables affecting dental fluctuating asymmetry in human isolates

The aims of the present study are to 1) determine and describe levels of dental fluctuating asymmetry (FA) in a highly endogamous human group; 2) evaluate the effects of various FA measures on perceived FA levels and their interrelationships; 3) study the connections between dental variables (tooth size, class, position, type, location and dimension) and FA levels; and 4) estimate the interrelationships between dental FA measures. The study was carried out on 242 Bedouin boys aged 5 to 14 years. The results demonstrate that the main variables influencing dental FA levels within this population are tooth class (incisors, canine, premolars, molars) and position (mesial, distal) and that the interaction between the two is significant. When sample sizes are large enough and individual measures are needed for the statistical analysis, the use of a computational method based on absolute values is legitimate. Clear relationships between some FA dental traits are discerned through principal-components analysis.

Efficiency of cranial bilateral measurements in separating human populations

A set of 31 nonconventional paired cranial measurements, as well as six conventional nonpaired measures, were taken on 266 skulls, representing two related populations: Bedouins of the Israeli Negev Desert and Bedouins of the Sinai. The data were subjected to univariate and discriminant analyses to determine the relative efficacy of paired vs. conventional measures in sorting individuals according to tribal and sex affiliation. It was found that paired measures have greater discriminatory power (87%) than conventional ones (47%) in terms of classifying individuals belonging to human isolates derived from a common ancestor and sharing similar environmental conditions. This greater discrimination attests to the value of the level of "developmental noise" (a measure provided by fluctuating asymmetry) in sorting human populations. Possible explanations are proffered for the above finding.

Study of genetic variance in the fluctuating asymmetry of anthropometrical traits

We have studied the fluctuating asymmetry (FA) of 8 bilateral morphometric traits in two-parent families, comprising 216 families with one newborn baby, and 60 families with two children (age range 5-18 years). Heritability was assessed by: (1) multiple regression analyses of the children's measurements on the mother's and father's measurements; (2) midparent-child regressions; and (3) sibling correlations. The extent of genetic determination of individual FA measurements was generally low, albeit statistically significant in some cases. However, even these correlations were inconsistent between samples and relatives. However, the mean FA values for all 8 studied traits showed positive and significant correlation between parents and children in two samples and in total. Additive genetic variance, calculated from multiple regression analyses and midparent-child correlations, was estimated to be between 0.25-0.30. Three multiple regressions (two for the separate group and one for the total sample) yielded a statistically significant value (between 0.21-0.33) also for the non-additive genetic component.

Fluctuating odontometric asymmetry in an urban South African black population

The magnitude of fluctuating odontometric asymmetry is reported for a group of 106 urban South African Blacks by use of re-scaled asymmetry values and Euclidean map analyses. When these results were contrasted with those reported for South African Caucasoids and Paraguayan Lengua Indians, Blacks were found to be significantly more asymmetric. It is suggested that the disproportionately high levels of dental asymmetry may be ascribed to the high disease and malnutrition burden of South African Blacks and to decreased individual buffering ability. The present study failed to support Garn's model of x-chromosomal odontogenetic buffering in females, and also failed to confirm significant arcadal differences in the magnitude of fluctuating odontometric asymmetry.

Decreased developmental stability as assessed by fluctuating asymmetry of morphometric traits in preterm infants

Fluctuating asymmetry (FA) of 8 morphometric traits was studied in 113 preterm infants (26-36 wk of gestation), 103 term infants (37-41 wk), and their respective parents. With 3 different measures of FA, the highest values were obtained from extremely preterm infants (26-29 wk), and the lowest from the group of term infants. The estimates of FA values among parents, particularly mothers, showed a similar, albeit less pronounced, trend. Multiple regression analysis of individual mean FA values, calculated in infants for the 8 studied bilateral traits, documented a significant inverse correlation with gestational age and with the health status of the infants and their mothers, as well as a positive correlation with the mothers' mean FA values.