Significant Directional Asymmetry Research Articles

Strong Genetic Control of Emergence of Human Primary Incisors

Our understanding of tooth eruption in humans remains incomplete. We hypothesized that genetic factors contribute significantly to phenotypic variation in the emergence of primary incisors. We applied model-fitting to data from Australian twins to quantify contributions of genetic and environmental factors to variation in timing of the emergence of human primary incisors. There were no significant differences in incisor emergence times between zygosity groups or sexes. Emergence times of maxillary central incisors and mandibular lateral incisors were less variable than those of maxillary lateral incisors and mandibular central incisors. Maxillary lateral incisors displayed significant directional asymmetry, the left side emerging earlier than the right. Variation in timing of the emergence of the primary incisors was under strong genetic control, with a small but significant contribution from the external environment. Estimates of narrow-sense heritability ranged from 82 to 94% in males and 71 to 96% in females.

Eye size in geckos: Asymmetry, allometry, sexual dimorphism, and behavioral correlates

The function of the vertebrate eye depends on its absolute size, and the size is presumably adapted to specific needs. We studied the variation of eye size at all levels, from intra-individual to inter-specific, in lid- less, spectacled, gecko lizards (Gekkonomorpha). We mea sured 1,408 museum specimens of 62 species, representing subfamilies Diplodactylinae, Gekkoninae, and Sphaerodactylinae. Intra-individually, eye size showed significant directional asymmetry in Stenodactylus sthenodactylus. A latitudinal study of six species confirmed that during postnatal ontogeny eye size undergoes conventional negative allometry; the slope is steeper among adults than among juveniles, expressing the need of juveniles for relatively larger eyes. Within species with sexual size dimorphism, commonly the larger sex possessed larger eyes in absolute terms but not relative to head-and-body length. Interspecifically, eye size showed negative allometry, with slope significantly steeper than those of intraspecific ontogenetic allometry, again expressing the need of juveniles for relatively larger eyes and showing that eye-size differences among species do not merely result from body-size differences. Finally, adult eye size varied interspecifically in correlation with parameters of behavioral ecology: eyes were significantly larger in nocturnal than in diurnal species, and significantly larger in cursorial than in scansorial species.

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.

Developmental instability and immune function in colour polymorphic pygmy grasshoppers

Random left minus right deviations from symmetry in otherwise bilaterally symmetric traits may arise due to developmental instability in response to environmental stress. Here we test for variation in developmental instability, measured as asymmetry of (femur) size, among individuals belonging to four different genetically encoded colour morphs of the pygmy grasshopper Tetrix undulata (Sow.) (Orthoptera: Tetrigidae). Such a difference is expected under the hypotheses that perturbed growth and development and concomitant morphological asymmetry may result from exposure to unfavourable temperatures or costs associated with melanization, or reflect a by-product of past disease in individuals with poor immune responsiveness. Results from mixed model two-way ANOVAs uncovered no statistically significant directional asymmetry in femur size, whereas non-directional asymmetry was significant in each of the four different colour morphs. The degree of intraindividual, interlateral variance in femur size varied significantly among individuals belonging to different colour morphs, being considerably higher in the black morph, which also suffers the greatest risk of overheating, compared to the brown, striped and grey morphs. Asymmetry in femur size was not associated with one measure of immune responsiveness, the encapsulation response of individuals experimentally implanted with a novel antigen consisting of a nylon mono-filament. These results are consistent with the notion that individual pigmentation pattern may indirectly influence developmental instability and morphological asymmetry, via the effects of coloration on body temperature.

Shape analysis of symmetric structures: quantifying variation among individuals and asymmetry.

Morphometric studies often consider parts with internal left-right symmetry, for instance, the vertebrate skull. This type of symmetry is called object symmetry and is distinguished from matching symmetry, in which two separate structures exist as mirror images of each other, one on each body side. We explain a method for partitioning the total shape variation of landmark configurations with object symmetry into components of symmetric variation among individuals and asymmetry. This method is based on the Procrustes superimposition of the original and a reflected copy of each landmark configuration and is compatible with the two-factor ANOVA model customary in studies of fluctuating asymmetry. We show a fully multivariate framework for testing the effects in the two-factor model with MANOVA statistics, which also applies to shapes with matching symmetry. We apply the new methods in a small case study of pharyngeal jaws of the Neotropical cichlid fish Amphilophus citrinellus. The analysis revealed that the symmetric component of variation in the pharyngeal jaws is dominated by the contrast between two alternative trophic morphs in this species and that there is subtle but statistically significant directional asymmetry. Finally, we provide some general recommendations for morphometric studies of symmetric shapes.

Directional bilateral asymmetry in human sacral morphology

AbstractDirectional bilateral asymmetries in human gross skeletal morphology are largely attributable to differential mechanical loading from handedness during endochondral bone growth. While much has been done in the way of identifying directional asymmetries of the upper limb in relation to handedness, comparatively little research has focused on asymmetry in the lower body. The present paper analyses asymmetry in the human sacrum in a sample of 238 modern individuals. Measurements of right and left sides of the lateral and posterior breadths of the alae and the maximum height of the auricular surface were used. Asymmetry was calculated as: [(left side − right side) ÷ right side] * 100. Directional asymmetry was identified using a one‐sample t‐test against a hypothesized mean of zero. Significant directional asymmetry (P < 0.05) was found in all three dimensions. The pattern of asymmetry is consistent with models describing the influence of right‐handedness on the lower body, which predicts that left side dimensions will be larger. Copyright © 2002 John Wiley & Sons, Ltd.

Left-right asymmetry of fly wings and the evolution of body axes.

The body plan of Drosophila, and presumably that of other insects, develops under the control of anterio-posterior and dorsal ventral axes, but no evidence for a left-right axis has yet been found. We used geometric morphometrics to study the wings in three species of flies: Drosophila melanogaster, Musca domestica and Glossina palpalis gambiensis. In all three species, we found that both size and shape showed subtle, but statistically significant directional asymmetry. For size, these asymmetries were somewhat inconsistent within and between species, but for shape, highly significant directional asymmetry was found in all samples examined. These systematic left-right differences imply the existence of a left-right axis that conveys distinct positional identities to the wing imaginal discs on either body side. Hence, the wing discs of Drosophila may be a new model to study the developmental genetics of left-right asymmetry. The asymmetries of shape were similar among species, suggesting that directional asymmetry has been evolutionarily conserved since the three lineages diverged. We discuss the implications of this evolutionary conservatism in conjunction with results from earlier studies that showed a lack of genetic variation for directional asymmetry in Drosophila.

Structural Asymmetries in the Human Forebrain and the Forebrain of Non-human Primates and Rats

ZILLES, K., A. DABRINGHAUS, S. GEYER, K. AMUNTS, M. QÜ, A. SCHLEICHER, E. GILISSEN, G. SCHLAUG, H. STEINMETZ. Structural asymmetries in the human forebrain and the forebrain of non-human primates and rats. NEUROSCI BIOBEHAV REV 20(4)593-605, 1996.—Possible asymmetries of the following structures were studied: volumes of total human hemispheres, cortex and white matter volumes in post-mortem- (unknown handedness) and living brains (male right-handers); volumes of the rat total primary visual cortex, its mon- and binocular subfields, its layer IV and the density of myelinated fibres in layer IV; transmitter receptor densities (NMDA, AMPA, kainate and GABA A receptors) in sensorimotor regions of the rat cortex; volume of the motor cortex and the 3D-extent of the central sulcus in the post-mortem- (unknown handedness) and living human brain (male right-handers); petalia of the hemispheres in human (male right- and left-handers) and chimpanzee brains. Histological, MRI and receptor autoradiographic techniques were used. With the notable exceptions of the transmitter receptors and the total primary visual cortex in rats and the hemispheres in chimpanzees, which do not show any significant directional asymmetry, all other parameters studied are asymmetrically distributed between the right- and left hemispheres. The regional distribution pattern and the degree of asymmetry of frontal and occipital petalia in living human brains differ between right- and left-handers. Copyright © 1996 Elsevier Science Ltd.

Three-dimensional quantitative analysis of hemispheric asymmetry in the human superior temporal region.

The recent observations of overall symmetry of the caudal infrasylvian region by Steinmetz et al. (1990) and Witelson and Kigar (1991, 1992) diverge from earlier findings of leftward asymmetry in this region (Geschwind and Levitsky, 1968; Galaburda et al., 1987; Larsen et al., 1989). To address this inconsistency, we measured the entire infrasylvian surface posterior to Heschl's gyrus from coronal magnetic resonance images of 10 young, normal, right-handed subjects. Computer models were constructed by tracing contours of this region and then interpolating a 3D triangle mesh between each pair of adjacent contours. Measurements of these models showed no significant directional asymmetry. The same contour set was used to obtain measurements with a conventional algorithm that does not interpolate a surface between contours. The results obtained with the second method showed significant leftward asymmetry. These results suggest that in some cases, unbalanced distortions due to folding differences of the hemispheres are sufficient to obtain spurious findings of left-right asymmetry. This supports the claim of Steinmetz and Witelson that leftward asymmetry is restricted to the temporal bank of the caudal infrasylvian surface, and is balanced by rightward asymmetry of the parietal bank.

Selectivity for orientation and direction of motion of single neurons in cat striate and extrastriate visual cortex

1. We consider the consequences of the orientation selectivity shown by most cortical neurons for the nature of the signals they can convey about the direction of stimulus movement. On theoretical grounds we distinguish component direction selectivity, in which cells are selective for the direction of movement of oriented components of a complex stimulus, from pattern direction selectivity, or selectivity for the overall direction of movement of a pattern irrespective of the directions of its components. We employed a novel test using grating and plaid targets to distinguish these forms of direction selectivity. 2. We studied the responses of 280 cells from the striate cortex and 107 cells from the lateral suprasylvian cortex (LS) to single sinusoidal gratings to determine their orientation preference and directional selectivity. We tested 73 of these with sinusoidal plaids, composed of two sinusoidal gratings at different orientations, to study the organization of the directional mechanisms within the receptive field. 3. When tested with single gratings, the directional tuning of 277 oriented cells in area 17 had a mean half width of 20.6 degrees, a mode near 13 degrees, and a range of 3.8-58 degrees. Simple cells were slightly more narrowly tuned than complex cells. The selectivity of LS neurons for the direction of moving gratings is not markedly different from that of neurons in area 17. The mean direction half width was 20.7 degrees. 4. We evaluated the directional selectivity of these neurons by comparing responses to stimuli moved in the optimal direction with those elicited by a stimulus moving in the opposite direction. In area 17 about two-thirds of the neurons responded less than half as well to the non-preferred direction as to the preferred direction; two-fifths of the units responded less than one-fifth as well. Complex cells showed a somewhat greater tendency to directional bias than simple cells. LS neurons tended to have stronger directional asymmetries in their response to moving gratings: 83% of LS neurons showed a significant directional asymmetry. 5. Neurons in both areas responded independently to each component of the plaid. Thus cells giving single-lobed directional-tuning curves to gratings showed bilobed plaid tuning curves, with each lobe corresponding to movement in an effective direction by one of the two component gratings within the plaid. The two best directions for the plaids were those at which one or other single grating would have produced an optimal response when presented alone.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of total and partial callosal agenesis on the development of morphological brain asymmetries in the BALB/cCF mouse

The corpus callosum fails to develop in some mice of the BALB/cCF strain. We report here a study on cerebral morphological asymmetry in males of this particular strain in order to test the hypothesis that the normal development of the corpus callosum is responsible for the establishment of brain asymmetries. In 46 animals the dorsal area and the weight of each hemisphere were measured. In order to identify the animals with callosal defects and check for size anomalies of the anterior commissure, the hemispheres were cut into sagittal sections and stained with cresyl violet. Measures of sagittal area of the anterior commissure did not support the hypothesis that this commissure is enlarged when the corpus callosum is reduced or absent. In normal animals, the mean difference between left and right dorsal cortical areas showed a significant directional asymmetry, the left being consistently greater than the right. A similar pattern was found for weight. In mice with callosal defects there were no significant differences, either between the dorsal areas or between the weights of the hemispheres. However, data on the absolute values of the hemispheric differences indicated the presence of a nondirectional asymmetry not only in normal animals, but also in mice with callosal defects. Therefore, our data suggest that the ontogenesis of the corpus callosum plays a role in directing the development of cerebral asymmetries.