A search for quantitative trait loci exhibiting imprinting effects on mouse mandible size and shape.

Genomic imprinting refers to the pattern of monoallelic parent-of-origin-dependent gene expression where one of the two alleles at a locus is expressed and the other silenced. Although some genes in mice are known to be imprinted, the true scope of imprinting and its impact on the genetic architecture of a wide range of morphometric traits is mostly unknown. We therefore searched for quantitative trait loci (QTL) exhibiting imprinting effects on mandible size and shape traits in a large F(3) population of mice originating from an intercross of the LG/J (Large) and SM/J (Small) inbred strains. We discovered a total of 51 QTL affecting mandible size and shape, 6 of which exhibited differences between reciprocal heterozygotes, the usual signature of imprinting effects. However, our analysis showed that only one of these QTL (affecting mandible size) exhibited a pattern consistent with true imprinting effects, whereas reciprocal heterozygote differences in the other five all were due to maternal genetic effects. We concluded that genomic imprinting has a negligible effect on these specific morphometric traits, and that maternal genetic effects may account for many of the previously reported instances of apparent genomic imprinting.

2,3,7,8-Tetrachlorodibenzo-p-dioxin affects fluctuating asymmetry of molar shape in mice, and an epistatic interaction of two genes for molar size.

Fluctuating asymmetry (FA), random variation between left and right sides in a bilaterally symmetrical character, is a commonly used measure of developmental instability that is expected to increase with increasing environmental stress. One potential stressor is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a powerful toxicant known to disturb tooth development. In this study, mice in the F(2) generation produced from an intercross between two inbred strains (C57BL/6J and AKR/J) were exposed in utero to TCDD. We hypothesized that TCDD would increase FA in the molars of exposed mice over that of the control mice. In addition, we hypothesized that we would discover genes for molar size, shape or asymmetry whose expression would be affected by TCDD. We detected a very small, but significant, increase in FA of molar shape (but not size) in the TCDD-exposed mice compared to the control mice, although molar size and shape did not differ between these groups. Although we did not uncover any genes that acted differently in the TCDD exposed and control groups, we did identify two genes whose dominance by additive epistatic effect on molar size was affected by TCDD. We concluded that although TCDD may be affecting the expression of some genes governing the development of molars in our population of mice, FA of molar size and shape is not a particularly sensitive indicator of this effect.

Qualitative effects of dioxin on molars vary among inbred mouse strains.

OBJECTIVE: We evaluated the effects of different levels of the potent environmental toxicant and teratogen, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), on molar development in mice in six inbred strains, all with TCDD responsive Ahr alleles. DESIGN: Pregnant females were exposed on gestation day 13 to 4 different levels of TCDD (control, 0.01, 0.1 and 1.0 microg/kg) and their offspring were examined for the frequency of missing third molars (M3s) and for differences in first mandibular molar (M1) cuspal morphology. RESULTS: Missing M3s were prevalent only in mice in two strains, C3H/HeJ and CBA/J, and their frequency significantly increased with increasing TCDD exposure. The frequency of the M1 variant was high in mice in only one strain, C57BL/10J, and was significantly higher in the treated compared with the control group. CONCLUSIONS: Inbred mice strains exhibited differential responses to TCDD suggesting that there is a genetic component, beyond Ahr differences, mediating the effects of TCDD on molar development.

An epistatic genetic basis for fluctuating asymmetry of tooth size and shape in mice.

Although there typically is little additive genetic variation for fluctuating asymmetry (FA), or variation in nondirectional differences between left and right sides of bilateral characters, several investigators have hypothesized that FA may have an epistatic genetic basis. We tested this hypothesis by conducting a whole genome scan of FA of size and shape of the mandibular molars in house mice from an F2 intercross population generated from crossing the Large (LG/J) and Small (SM/J) inbred strains. Although no individual genes (QTLs=quantitative trait loci) on any of the 19 autosomes significantly affected FA for centroid size, and only two affected shape FA, a number of pairwise combinations of QTLs exhibited significant epistasis for FA in both molar size and shape. The QTLs involved in these interactions differed for FA in molar size versus FA in molar shape, but their epistatic contributions to the total variance was nearly the same (about 20%) for FA in both molar characters. It was noted that the genetic architecture of FA in the molar characters, consisting of little or no additive genetic variance but an abundance of epistatic genetic variance, is consistent with that of other typical fitness components such as litter size.

The influence of hybridization between African and European honeybees, Apis mellifera, on asymmetries in wing size and shape.

We examined the possible role of hybridization in the invasion process of the African honeybee by testing two hypotheses regarding fluctuating asymmetry (FA), a measure of developmental stability, in wing characteristics: (1) FA should be higher in hybrid versus parental genotypes of African and European races; (2) FA should be lower in African bees compared to hybrid and European workers. Parental and reciprocal hybrid worker genotypes were cross fostered in common-hive rearing environments. We did not find greater FA for wing size and shape in the hybrids compared to both parental types. However, we did find significantly lower FA of shape in the African workers compared to the European and hybrid workers, suggesting that European bees and their hybrids may have compromised fitness relative to African bees. We also found that the two hybrid genotypes significantly differed in overall wing size and shape. If these differences affect wing aerodynamics, then the paternity of hybrids may influence worker performance and could potentially contribute to the loss of European matrilines. Hybridization had few consistent effects on directional asymmetry for wing size and shape. Genotypic factors played a far greater role in determining the effect of hybridization on wing morphology than did differences in rearing environment. Thus, African bees may have lower FA for wing shape (and by inference greater developmental stability) relative to European and hybrid workers, which may contribute to the ability of African bees to displace European honeybee races in invaded regions.

2,3,7,8-tetrachlorodibenzo-p-dioxin affects size and shape, but not asymmetry, of mandibles in mice.

Fluctuating asymmetry (FA), the random differences between the left and right sides of a bilaterally symmetrical character, is often purported to be a sensitive measure of developmental instability particularly in populations exposed to environmental stressors. As the level of developmental instability increases, often too does the level of FA. In this study we tested the hypothesis that exposure of pregnant mice to low doses of 2,3,7,8-tetrachlordibenzo-p-dioxin (TCDD) would increase the level of FA in the mandibles of their offspring. We used ten landmark coordinates around the mandible to create a single size variable (centroid size) and 20 Procrustes shape variables. These were used to test for effects of dioxin on mandible size and shape and their asymmetries. We found no detectable effect of TCDD on levels of FA in either size or shape of the mandible, but TCDD did produce a significant decrease in mandible size, and a significant effect on the overall shape.

Genetic architecture of mandible shape in mice: effects of quantitative trait loci analyzed by geometric morphometrics.

This study introduces a new multivariate approach for analyzing the effects of quantitative trait loci (QTL) on shape and demonstrates this method for the mouse mandible. We quantified size and shape with the methods of geometric morphometrics, based on Procrustes superimposition of five morphological landmarks recorded on each mandible. Interval mapping for F(2) mice originating from an intercross of the LG/J and SM/J inbred strains revealed 12 QTL for size, 25 QTL for shape, and 5 QTL for left-right asymmetry. Multivariate ordination of QTL effects by principal component analysis identified two recurrent features of shape variation, which involved the positions of the coronoid and angular processes relative to each other and to the rest of the mandible. These patterns are reminiscent of the knockout phenotypes of a number of genes involved in mandible development, although only a few of these are possible candidates for QTL in our study. The variation of shape effects among the QTL showed no evidence of clustering into distinct groups, as would be expected from theories of morphological integration. Further, for most QTL, additive and dominance effects on shape were markedly different, implying overdominance for specific features of shape. We conclude that geometric morphometrics offers a promising new approach to address problems at the interface of evolutionary and developmental genetics.

Size and fluctuating asymmetry of morphometric characters in mice: their associations with inbreeding and t-haplotype.

Fluctuating asymmetry (FA), a ubiquitous type of asymmetry of bilateral characters, often has been used as a measure of developmental instability in populations. FA is expected to increase in populations subjected to genetic stressors such as inbreeding or environmental stressors such as toxins or parasites, although results have not always been consistent. We tested whether FA in four skeletal size characters and mandible shape was greater in a population of wild-derived mice reared in the laboratory and subjected to one generation of inbreeding (F = 0.25) versus that in an outbred group (F = 0.00). FA did not significantly differ between the inbred and outbred groups, despite the fact that these two groups differed dramatically in fitness under seminatural population conditions. As far as we know, this is the first study to evaluate the relationship between FA and inbreeding in wild house mice, and our general conclusion is opposite that of earlier work on laboratory inbred strains of mice and their hybrids. Size for two of the characters was significantly less in inbreds than in outbreds, however, and there was a significant difference between inbreds and outbreds in the signed differences of right and left sides in one character (humerus length). Some of the mice in both groups also were heterozygous or homozygous carriers of the t-complex. Because mice carrying this chromosome 17 variant are known to have reduced fitness, we also tested whether they had greater FA than mice carrying non-t-haplotypes. The overall level of a composite FA index calculated from all four characters was in fact significantly higher in the t-bearing mice. These combined results suggest that FA is not a generally sensitive proxy measure for fitness, but can be associated with fitness reductions for certain genetic stressors.

Quantitative genetics of geometric shape in the mouse mandible.

We combine the methods of geometric morphometrics and multivariate quantitative genetics to study the patterns of phenotypic and genetic variation of mandible shape in random-bred mice. The data are the positions of 11 landmarks on the mandibles of 1,241 mice from a parent-offspring breeding design. We use Procrustes superimposition to extract shape variation and restricted maximum likelihood to estimate the additive genetic and environmental components of variance and covariance. Matrix permutation tests showed that the genetic and phenotypic as well as the genetic and environmental covariance matrices were similar, but not identical. Likewise, principal component analyses revealed correspondence in the patterns of phenotypic and genetic variation. Patterns revealed in these analyses also showed similarities to features previously found in the effects of quantitative trait loci and in the phenotypes generated in gene knockout experiments. We used the multivariate version of the breeders' equation to explore the potential for short-term response to selection on shape. In general, the correlated response is substantial and regularly exceeds the direct response: Selection applied locally to one landmark usually produces a response in other parts of the mandible as well. Moreover, even selection for shifts of the same landmark in different directions can yield dramatically different responses. These results demonstrate the role of the geometry and anatomical structure of the mandible, which are key determinants of the patterns of the genetic and phenotypic covariance matrices, in molding the potential for adaptive evolution.

Quantitative trait loci for directional but not fluctuating asymmetry of mandible characters in mice.

Non-directional variation in right minus left differences in bilateral characters, referred to as fluctuating asymmetry (FA), often has been assumed to be largely or entirely environmental in origin. FA increasingly has been used as a measure of developmental stability, and its presumed environmental origin has facilitated the comparisons of populations believed to differ in their levels of stability. Directional asymmetry (DA), in which one side is consistently larger than the other, has been assumed to be at least partially heritable. Both these assumptions were tested with interval mapping techniques designed to detect any quantitative trait loci (QTLs) affecting FA or DA in 15 bilateral mandible characters in house mice resulting from a cross of the F1 between CAST/Ei (wild strain) and M16i (selected for rapid growth rate) back to M16i. For purposes of the analysis, all mandibles were triply measured and 92 microsatellite markers were scored in a total of 350 mice. No significant QTLs were found for FA, but three QTLs significantly affected DA in several characters, confirming both assumptions. The QTLs for DA were similar in location to those affecting the size of several of the mandible characters, although they accounted for an average of only 1% of the total phenotypic variation in DA.

Measurement precision of body composition variables using the lunar DPX-L densitometer.

The objective of the study was to determine the precision of total- and regional-body composition measurements from a total-body scan using dual-energy X-ray absorptiometry (DXA). This is critical information necessary to determine the smallest change from baseline that could be detected with statistical significance when conducting longitudinal measurements of body composition variables in an individual. Twenty volunteers were scanned once each day for 4 consecutive days using a Lunar DPX-L densitometer and manufacturer-supplied software (version 1.3z). Coefficients of variation (CV, %) derived from data using the (preferred) extended research mode of analysis were 0.62, 1.89, 0.63, 2.0, 1.11, 1.10, and 1.09% for total-body bone mineral density (BMD), total percentage fat, total body tissue mass, fat mass, lean mass, bone mineral content (BMC), and total bone calcium, respectively. Regional measurements (arm, leg, trunk, pelvis, and spine) were less precise than total body measurements, with CVs in the range of 1% to 3% (but fat mass for arms was 4.26%, trunk 3.08%, BMC 3.65%). Small but statistically significant differences in mean values for most body composition variables were found when data were compared between extended and standard modes of analysis. Inconsistent use of analysis mode in a cohort or when following a patient longitudinally may negatively affect precision. We conclude that the measurement precision of total and regional body composition variables was generally comparable to the precision limits typically associated with lumbar spine and proximal femur BMD data.

Quantitative trait loci for fluctuating asymmetry of discrete skeletal characters in mice.

Levels of fluctuating asymmetry (FA) are often taken as indicators of the degree to which genotypes differ in their ability to buffer genetic and environmental sources of variation. Interval mapping techniques were used to search for quantitative trait loci (QTLs) affecting directional asymmetry (DA) and fluctuating asymmetry (FA) in six bilateral discrete skeletal traits in house mice. These six characters as well as 76 microsatellite markers were scored in over 500 mice that resulted from crosses of F1 mice originally produced from matings of the Large (LG/J) and Small (SM/J) inbred strains. The number of QTLs affecting DA in each of the characters was no more than expected by chance alone so it was concluded that there was little evidence for individual genes affecting DA. There appeared to be a genetical basis for FA in these characters, however, because the number of QTLs significantly affecting FA (10 at the 5% level, three at the 1% level) was greater than expected by chance alone. The 10 QTLs significantly affecting FA in any given character were located on eight different chromosomes, mostly at locations for QTLs affecting other characters or DA in other characters. Their cumulative contribution to the total phenotypic variance was small, averaging only 3.9% per locus. Dominance genotypic values for these QTLs were more extreme than additive genotypic values, suggesting that heterozygotes at many loci are better buffered than homozygotes and that allelic interactions (dominance) may play an important role in the production of FA.