The Ay allele at the agouti locus reduces the size and alters the shape of the mandible in mice

Genetic analysis of the mandible morphology in DDD.Cg-Ay/Sgn and C57BL/6J inbred mice

Quantitative trait loci (QTL) mapping analysis was performed for the mandible morphology in DDD.Cg-A^y/Sgn and C57BL/6J inbred mice. The size and shape of the mandible was analysed by landmark-based geometric morphometrics as the centroid size and principal components (PCs), respectively. The A^y allele at the agouti locus significantly reduced the mandible size in DDD/Sgn background, and substantially altered the mandible shape in both strain backgrounds. Single-QTL scans, by including the agouti locus genotype (A^y or non-A^y) as an additive covariate, identified three significant QTL for the centroid size on chromosomes 5, 6 and 17, along with four suggestive QTL on chromosomes 2, 12, 18 and 19. These QTLs explained 46.85% of the centroid size variation in F₂ mice. When the F₂A^y and F₂ non-A^y mice were analysed separately, additional significant QTL were identified on chromosomes 12 and 15 in F₂ non-A^y mice. Single-QTL scans also identified 15 significant QTL for the PC1, PC2 and PC3. When the agouti locus genotype was included as an interactive covariate, nine significant QTLs were identified. Unexpectedly, these agouti-interacting QTLs were identified for relatively minor PCs, for which no significant single-QTL were identified. Therefore, it was suggested that the alteration of the mandible shape in A^y mice was the consequence of interactions between the A^y allele and genes that themselves have relatively small phenotypic effect. Although further in vivo studies are required, we postulated Pkd1 as a possible candidate gene underlying QTL for the centroid size on chromosome 17.

Effect of the Y chromosome on testis weight in mice

We investigated the effect of the Y chromosome on testis weight in (B6.Cg-A^y × Y-consomic mouse strain) F₁ male mice. We obtained the following results: (1) Mice with the Mus musculus domesticus-type Y chromosome had significantly heavier testis than those with the M. m. musculus-type Y chromosome. (2) Variations in Usp9y and the number of CAG repeats in Sry were significantly associated with testes weight. The A^y allele was correlated with a reduced testis weight, and the extent of this reduction was significantly associated with a CAG repeat number polymorphism in Sry. These results suggest that Y chromosome genes not only influence testis weight but also modify the effect of the A^y allele in mediating this phenomenon.

Further characterization of diabetes mellitus and body weight loss in males of the congenic mouse strain DDD.Cg-A(y.)

The A^y allele at the agouti locus causes obesity and promotes linear growth in mice. However, body weight gain stops between 16 and 17 weeks after birth, and then, body weight decreases gradually in DDD.Cg-A^y male mice. Body weight loss is a consequence of diabetes mellitus, which is genetically controlled mainly by a quantitative trait locus (QTL) on chromosome 4. This study aimed to further characterize diabetes mellitus and body weight loss in DDD.Cg-A^y males. The number of β-cells was markedly reduced, and plasma insulin levels were very low in the DDD.Cg-A^y males. Using a backcross progeny of DDD × (B6 × DDD.Cg-A^y) F₁-A^y, we identified one significant QTL for plasma insulin levels on distal chromosome 4, which was coincidental with QTL for hyperglycemia and lower body weight. The DDD allele was associated with decreased plasma insulin levels. When the DDD.Cg-A^y males were housed under three different housing conditions [group housing (4 or 5 DDD.Cg-A^y and DDD males), individual housing (single DDD.Cg-A^y male) and single male housing with females (single DDD.Cg-A^y male with DDD.Cg-A^y or DDD females)], diabetes mellitus and body weight loss were most severely expressed in individually housed mice. Thus, the severity of diabetes and body weight loss in the DDD.Cg-A^y males was strongly influenced by the housing conditions. These results demonstrate that both genetic and nongenetic environmental factors are involved in the development of diabetes mellitus and body weight loss in the DDD.Cg-A^y males.

Genetic background (DDD/Sgn versus C57BL/6J) strongly influences postnatal growth of male mice carrying the A(y) allele at the agouti locus: identification of quantitative trait loci associated with diabetes and body weight loss

Background

Mice carrying the A^y allele at the agouti locus become obese and are heavier than their non-A^y littermates. However, this does not hold true for the genetic background of the DDD mouse strain. At 22 weeks of age, DDD.Cg-A^y females are heavier than DDD females, whereas DDD.Cg-A^y males are lighter than DDD males. This study aimed to determine the possible cause and identify the genes responsible for the lower body weight of DDD.Cg-A^y males.

Results

Growth curves of DDD.Cg-A^y mice were analyzed and compared with those of B6.Cg-A^y mice from 5 to 25 weeks. In DDD.Cg-A^y males, body weight gain stopped between 16 and 17 weeks and the body weight gradually decreased; thus, the lower body weight was a consequence of body weight loss. Quantitative trait locus (QTL) mapping was performed in backcrossed (BC) males of DDD × (B6 × DDD.Cg-A^y) F₁-A^y mice. For the body weight at 25 weeks, significant QTLs were identified on chromosomes 1 and 4. The DDD allele was associated with a lower body weight at both loci. In particular, the QTL on chromosome 4 interacted with the A^y allele. Furthermore, suggestive QTLs for plasma glucose and high molecular weight adiponectin levels were coincidentally mapped to chromosome 4. The DDD allele was associated with increased glucose and decreased adiponectin levels. When the body weight at 25 weeks and plasma glucose levels were considered as dependent and independent variables, respectively, BC A^y males were classified into two groups according to statistical analysis using the partition method. Mice of one group had significantly higher glucose and lower adiponectin levels than those of the other group and exhibited body weight loss as observed with DDD-A^y males.

Conclusions

The lower body weight of DDD.Cg-A^y male mice was a consequence of body weight loss. Diabetes mellitus has been suggested to be a possible contributory factor causing body weight loss. The QTL on distal chromosome 4 contained the major responsible genes. This QTL interacted with the A^y allele, implying the reason why body weight loss occurs in DDD.Cg-A^y but not in DDD males.

Quantitative trait locus mapping of genes associated with vacuolation in the adrenal X-zone of the DDD/Sgn inbred mouse

Background

Adrenal gland of mice contains a transient zone between the adrenal cortex and the adrenal medulla: the X-zone. There are clear strain differences in terms of X-zone morphology. Nulliparous females of the inbred mouse DDD strain develop adrenal X-zones containing exclusively vacuolated cells, whereas females of the inbred mouse B6 strain develop X-zones containing only non-vacuolated cells. The X-zone vacuolation is a physiologic process associated with the X-zone degeneration and is tightly regulated by genetic factors. Identification of the genetic factors controlling such strain differences should help analyze the X-zone function. In this study, a quantitative trait locus (QTL) analysis for the extent of X-zone vacuolation was performed for two types of F₂ female mice: F₂A^y mice (F₂ mice with the A^y allele) and F₂ non-A^y mice (F₂ mice without the A^y allele). These were produced by crossing B6 females and DDD.Cg-A^y males. DDD.Cg-A^y is a congenic mouse strain for the A^y allele at the agouti locus and is used for this study because a close association between the X-zone morphology and the agouti locus genotype has been suggested. The A^y allele is dominant and homozygous lethal; therefore, living A^y mice are invariably heterozygotes.

Results

Single QTL scans identified significant QTLs on chromosomes 1, 2, 6, and X for F₂ non-A^y mice, and on chromosomes 2, 6, and 12 for F₂A^y mice. The QTL on chromosome 2 was considered to be because of the agouti locus, which has been suggested to be associated with X-zone vacuolation. A significant QTL that interacted with the agouti locus was identified on chromosome 8.

Conclusions

The extent of X-zone vacuolation in DDD females was controlled by multiple genes with complex interactions. The murine X-zone is considered analogous structure to the human fetal zone. Therefore, the results of this study will aid in understanding function of not only of the X-zone but also of the human fetal zone. Identifying the genes responsible for the QTLs will be essential for understanding the molecular basis of X-zone function, which is currently unclear.

Quantitative trait locus mapping of genes that control body length and plasma insulin-like growth factor 1 level in mice

BACKGROUND

The Ay allele at the agouti locus causes obesity and promotes linear growth in mice. The effect of the Ay allele on obesity has been extensively investigated, whereas its effect on body length is only poorly analyzed. To gain insight into the genetic control of body length, quantitative trait locus (QTL) analysis was performed in F2 female mice produced by crossing C57BL/6 J females and DDD.Cg-Ay males. A congenic DDD.Cg-Ay strain was established by introgressing the Ay allele from the B6.Cg-Ay strain by backcrossing for 12 generations. DDD.Cg-Ay females were longer than B6.Cg-Ay females; therefore, QTLs that interact with the Ay allele may be identified for body length. In addition, QTL analysis was also performed for plasma insulin-like growth factor 1 (IGF1) levels because IGF1 is known to play essential roles in growth and development. If QTLs for IGF1 levels coincide with those for body length, we can gain endocrinological insight into the QTLs for body length.

RESULTS

Correlations between body length and IGF1 levels were statistically significant in F2 populations. For body length, two significant QTLs were identified on chromosomes 15 and 17. For IGF1 levels, three significant QTLs were identified on chromosomes 10, 12, and 19. QTLs on chromosomes 12 and 19 appeared to be novel, and the latter interacted with the Ay allele.

CONCLUSION

QTLs for body length and IGF1 levels contained candidate genes that were components of the growth hormone/insulin-like growth factor axis. However, there was no overlap between QTLs for these two traits. Contrary to our expectations, QTLs that interacted with the Ay allele were identified not for body length but for IGF1 levels. Body length and IGF1 levels were, thus, controlled by different sets of genes.