Showing papers on "Fluctuating asymmetry published in 1980"

Dental asymmetry in Australian Aboriginals.

Abstract: Fluctuating dental asymmetry was studied in the permanent dentition of Australian Aboriginals. Asymmetry scores were greater for males than females, greater for maxillary teeth than mandibular and greater for mesiodistal dimensions than buccolingual. No evidence was found for either a significant genetic component of asymmetry or for compensatory interactions between developing teeth. The findings for Aboriginals are compared with reports of dental asymmetry in other populations. Bilateral structures rarely exhibit perfect symmetry even though their genetic control appears to be identical (Adams and Niswander, 1967; Potter et al. 1976). Asymmetry is said to be directional when one side regularly displays greater development than the other, probably due to consistent differences in local environments of the developing organ on each side. On the other hand, random, non-directional differences between sides may indicate an inability of the individual to buffer against developmental disturbances (Van Valen, 1962). The degree of this type of asymmetry, which is termed fluctuating, is thought to reflect the magnitude of local developmental disturbances, or as Waddington (1957) described it, developmental "noise". Some individuals, depending on their genotypes, are better buffered against "noise" than others and display greater developmental homeostasis. The dentition provides a useful model system to study asymmetry, particularly in relation to phylogeny and ontogeny, as maximum crown dimensions are determined well before eruption. Asymmetry comparisons can be made within and between tooth classes as well as between individual teeth. Crown measurements can be obtained directly, or indirectly from plaster casts and, preferably, they should be made on recently emerged teeth to avoid the bias introduced by attritional wear and other age-related changes affecting the dentition. Studies of modern and prehistoric dentitions indicate that fluctuating asymmetry tends to increase with inbreeding or an adverse environment, suggesting an association with genetic and environmental stress (Bailit et al. 1970; Suarez, 1974; Doyle and Johnston, 1977; Perzigian, 1977). 1 Faculty of Dentistry, The University of Adelaide, Adelaide, South Australia 5000 Human Biology , December 1980 , Vol. 52, No. 4, pp. 661-673. ® Wayne State University Press, 1980 This content downloaded from 157.55.39.249 on Wed, 03 Aug 2016 04:45:04 UTC All use subject to http://about.jstor.org/terms 662 Grant С. Townsend and T asman Brown Polygenic systems may buffer developmental processes against environmental insults, while deleterious genes may reduce the buffering below a threshold level (Shapiro, 1970, 1975). This view is supported by observations that dental asymmetry is increased in individuals with genetic disorders, for example, cleft lip (Adams and Niswander, 1967; Sofaer, 1979) and Down's syndrome (Garn et al. 1970). For some characters the ability to buffer against environmental stress appears to differ between sexes (Blanco et al. 1974; Tanner, 1978), but studies of dental asymmetry are equivocal in this respect. Dental asymmetry was found to be greater in females by Niswander and Chung (1965), greater in males by Garn et al. (1966, 1967) and similar in each sex by Bailit et al. (1970) and Perzigian (1977). Furthermore, there is little evidence to support a genetic basis for fluctuating dental asymmetry according to the heritability estimates obtained for related individuals by Bailit et al. (1970) and the results of a twin study of Potter and Nance (1976). Dental asymmetry has also been studied within the context of morphogenetic fields as proposed by Butler (1939) and subsequently modified by Dahlberg (1945). Asymmetry tends to follow the expected pattern, that is, increasing variability from the mesial or "key" tooth to the more distal members of a tooth class (Lundstrom, I960; Garn et al. 1966, 1967; Bailit et al. 1970; Perzigian, 1977). The dentai studies referred to above were carried out on human populations where relationships between asymmetry, genotype and environmental stress are still unclear. However, a number of associations have been noted in animal experiments. For example, dental asymmetry appears to increase in inbred mice (Bader, 1965) and also in response to specific environmental stress such as noise, cold and heat (Siegel and Smookler, 1973; Siegel and Doyle, 1975; Siegel et al. 1977). Recently, the interactions of a number of stress-inducing agencies were assessed in relation to dental asymmetry (Sciulli et al. 1979). This paper provides the first report of dental asymmetry in Australian Aboriginals. Specifically the aims of the study were to quantify the extent of asymmetry in the permanent dentition of Australian Aboriginals and to compare results with those published for other groups, to compare asymmetry between tooth dimensions, between sexes and between teeth; to test the presence of a genetic component of asymmetry and to examine evidence for compensatory interaction between adjacent tooth germs as reflected by specific patterns of correlations for asymmetry between tooth pairs. This content downloaded from 157.55.39.249 on Wed, 03 Aug 2016 04:45:04 UTC All use subject to http://about.jstor.org/terms Dental Asymmetry in Australian Aboriginals 663 Materials and Methods Mesiodistal and buccolingual tooth diameters of the permanent teeth were recorded to an accuracy of 0.10mm from plaster casts of 208 male and 184 female Aboriginals enrolled in a longitudinal growth study at Yuendumu, situated about 285km north-west of Alice Springs in the Northern Territory of Australia. Descriptions of Yuendumu, its inhabitants and marriage patterns, details of the cast selection, and the method of semi-automatic tooth measurement have been reported previously (Brown and Barrett, 1971; Townsend and Brown, 1978a, 1979). Concordance in size between pairs of antimeric teeth may be estimated for an individual by the intraclass correlation coefficient, r (Bailit et al. 1970). This correlation increases with greater symmetry and decreases with greater asymmetry. Accordingly, intraclass correlations were estimated for each Aboriginal subject as a measure of dental asymmetry. For the Yuendumu population the mean asymmetry score was derived by transforming the individual r values into Fisher's z-values and averaging. Comparisons within the Yuendumu population and with other groups were made using Student s t-test applied to mean asymmetry scores. To detect the presence of a genetic component of asymmetry, correlations for asymmetry scores were derived between brother-brother, sister-sister, and brother-sister pairs selected at random. A similar procedure was performed for half-siblings. These correlations were compared with results for 50 pairs of unrelated males and 50 pairs of unrelated females. In addition to the asymmetry scores for individuals, the population averages for antimeric differences in size were computed for each tooth. The mean difference, Xdiff, was calculated as:

Fluctuating Asymmetry in the Deciduous Dentition

Abstract: The magnitude of fluctuating dental asymmetry has been reported for the permanent dentition by a number of authors. Comparable data for the deciduous dentition have been published for the mesiodistal dimensions only (Moorrees and Reed, Arch Oral Biol 9:685-697, 1964). The data presented here are based on 125 white children who participated in the University School Growth Study of the University of Michigan. These individuals were selected because they had complete deciduous dentitions. Mesiodistal and buccolingual measurements of the deciduous teeth were made with aid of the OPTOCOM (Van Der Linden et al., J Dent Res 41:1100, 1972). T-tests of mean antimeric dimensions indicated no significant differences; therefore, any asymmetry is fluctuating rather than directional. The statistic used to measure symmetry is the correlation coefficient, r. Asymmetry is equal to l-r (Bailit et al., Hum Biol 42:626-638, 1970). One striking result of the antimere correlations is the great asymmetry of the buccolingual diameters of the lower incisors. The other deciduous and permanent teeth (aside from the third molars) do not exhibit such extreme asymmetry (unpublished data). Although this finding is suggestive, its true significance cannot be assessed without additional data from other populations. Another surprising result is that the asymmetry of the mesiodistal crown diameters is greater for the Michigan whites in nine of ten cases than that reported by Moorrees and Reed for 184 whites. The only exception is the mandibular lateral incisor which is slightly less asymmetric in the