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To elucidate the involvement of growth hormone (GH) in the genetic change produced by long-term selection in growth and fatness, a ‘GH knock-out study’ on over 900 mice was undertaken. Lines used had been selected for more than 50 generations for high (PH) and low (PL) body weight (initially protein mass) at 70 d(ays) and for high (F) and low fat content (L) at 98 d, producing a 3-fold difference in body weight and a 5-fold difference in fat content. GH deficiency was achieved by repeated backcrossing into each line a recessive mutant gene (lit) which has a defective GH releasing factor receptor. In the absence of GH, the P lines still differ in body weight (21 d to 98 d): e.g. at 98 d homozygous lit/lit: PH = 24·2 g, PL = 10·0 g; wild-type (wt): PH = 57·4 g, PL = 18·7 g. The effect of the GH deficiency on body weight (untransformed) was very much larger in the PH than in the PL line, but the interaction was much smaller, although still significant, on the log scale. This indicates that changes in the GH system contribute only a small part of the selection response in growth. GH deficiency increased fat percentage in all lines (including P), especially in males (99 d, males lit/lit: F = 26·4%, L = 6·9%; wt: F = 22·0%, L = 4·8%; females: 20·2%, 5·2%, 20·7%, 3·0%) with significant genotype×line and genotype×sex interactions. The interactions between the effects of the lit gene and the genetic background were, however, relatively small compared with these main effects and again indicate that other systems contributed most of the selection response.

Based on the outbred mouse strain Fzt: Du, which has been obtained by systematic crossing of four inbred and four outbred lines, a long-term selection experiment was carried out for total protein amount (PA) in the carcass, starting in 1975. An unselected control line (CO) was kept under the same management but without continuous protein analysis. The protein amount of male carcasses at 42 days of age (P42) increased from 2·9 g in generation 0 to 5·2 g at generation 70, representing 97% of a theoretical selection limit. The total selection response amounts to 2·3 g, which is about 80% above the initial value and corresponds to 9σp or 12σA . The estimated realized heritability of protein amount decreased from 0·56 to 0·03 at generation 70, which was due to an increase in phenotypic variance from 0·065 to 0·24 g2 and a reduction in genetic variance from 0·04 to 0·01 g2. Half the selection response was obtained after about 18 to 23 generations, a half-life of 0·25 to 0·3 Ne. The maximum selection response was 0·094 g/generation and the response was 0·01 g/generation at generation 70. The measurements of body weights at 0, 10, 21, 42 and 63 days throughout the experiment showed a strong correlated effect for all weights. The PA mice are one of the heaviest lines of mice ever reported, and do not differ significantly in their body composition from control mice at 42 days. The direct selection response was due primarily to increased general growth. Body weight and protein amount are phenotypically and genetically highly correlated (rp=0·82, rA≈1); however, selection for body weight led to fatter animals, whereas selection for protein opposed increased fatness (at least until selection age). This may be of general importance in animal breeding. The comparatively high selection response in this experiment seems due to the heterogeneity of the base population, the relatively high effective population size, and the duration of the experiment.

A study was undertaken to test whether the elimination of metabolic pathways strongly involved in growth and fatness, comprising thyroid hormones (TH) and growth hormone (GH), is responsible for a substantial part of the genetic change produced by selection. Lines used in this study have been selected for about 50 generations for high (PH) and low (PL) body weight at 10 weeks and for high (F) and low fat content (L) at 14 weeks, producing a 3-fold difference in body weights and a 5-fold difference in fat content. Thyroid ablation was achieved by repeated backcrossing into the four selection lines of a transgene comprising the HSV1-tk gene coupled to the promoter of the thyroglobulin gene. Hemizygous pregnant dams were treated with ganciclovir leading to thyroid-ablated dams and offspring and therefore to a lack of TH and subsequently of GH. In the absence of TH and GH, lines still differ in body weight over the period studied (10 d to about 100 d; e.g. at the end PH=32·1 g vs PL=10·2 g) and in fat content (F=16·2% vs L=3·8%) ; the corresponding values for the wild-type controls were PH=49·9 g vs PL=17·4 g and F=27·5% vs L=4·8%. The effect of the transgene depended on the genetic background for body weights at most ages and for relative gonadal fat pad weights, but less for fat content. The L line showed the lowest growth depression. The lit gene, which causes GH but not TH deficiency, was also transferred by repeated backcrosses into three of these lines (PH, PL, F). The combined deficiency of TH and GH had bigger effects on body weights at earlier ages than did GH deprivation. The data show that changes in the TH- and GH-systems are not the only cause of line differences in growth and fatness resulting from long-term selection, but both are involved to a significant extent. The interactions between the effects of the transgene and of the lit gene and the genetic background were, nevertheless, relatively small and therefore these results support a polygenic model of selection response.

Myostatin is a negative regulator of muscle growth and mutations in its gene lead to muscular hypertrophy and reduced fat. In cattle, this is seen in ‘double muscled’ breeds. We have used marker-assisted introgression to introduce a murine myostatin mutation, MstnCmpt-dl1Abc [Compact (C)], into an inbred line of mice (DUHi) that had been selected on body weight and had exceptional growth. Compared with homozygous wild-type mice, homozygous (C/C) mice of this line were ~4–5% lighter, had ~7–8% shorter tails, substantially increased muscle weights (e.g. quadriceps muscle in males was 59% heavier) and an increased ‘dressing percentage’ (~49% vs 39%), an indicator of overall muscularity. The weights of several organs (e.g. liver, kidney, heart and digestive tract) were significantly reduced, by 12–20%. Myostatin deficiency also resulted in drastic reductions of total body fat and of various fat depots, total body fat proportion falling from ~17·5% in wild-type animals of both sexes to 9·5% and 11·6% in homozygous (C/C) females and males, respectively. Males with a deficiency in myostatin had higher gains in muscle traits than females. Additionally, there was a strong distortion of the segregation ratio on the DUHi background. Of 838 genotyped pups from inter se matings 29%, 63% and 8% were homozygous wild type (+/+), heterozygous (C/+) and homozygous (C/C), respectively, showing that MstnCmpt-dl1Abc has lower fitness on this background. This line, when congenic, will be a useful resource in gene expression studies and for finding modifying genes.

The mouse lines were developed by long-term selection for fatness, after which the fat line (F) had about a 5-fold (23% vs 4%) higher fat percentage than the lean (L) line at 14 weeks; but the lines differed little in fat-free body weight. To assess the contribution of genetic changes in leptin hormone level to the selection response, plasma leptin levels were assayed in these lines in generation 60 and in an unselected control (C) from the same base population. With access to food prior to assay, the F, C and L lines had 16·5, 0·91 and 0·26 ng/ml leptin, respectively. In fasted animals these levels were much lower: 2·98, 0·171 and 0·0087 ng/ml, respectively. Thus the leptin levels differ greatly between the lines, with the fattest mice showing the highest level: almost 20 times higher than the control and 60–300 times higher than the L line. These correlated selection effects are an order of magnitude greater than the direct selection response, and believed to be much larger than seen for any hormonal or other trait. Correlations between leptin level and fat amount were high (over 0·86) in fed or fasted animals of the F line, indicative of leptin resistance.

To assess the role of genetic changes in sensitivity to leptin hormone in contributing to responses to long-term selection for fatness, leptin was administered to a long-term fat selected (F) and a control line (C) of mice. These lines differ almost three fold in their percentage of fat (fat%) at about 15 weeks of age. Treated (T) animals received twice-daily intraperitoneal injections of 5 mg/kg leptin from 91 to 105 days of age; untreated (U) animals received equivolume injections of phosphate-buffered saline. Treated compared with untreated animals in both lines had significantly (PP>0·05), however. There was a very wide range of fatness (estimated from dry matter content) among FT animals (3–29%), much higher than in FU (15–31%), CT (0·7–6·4%) and CU (2–15%) animals. While sensitivity to leptin remains in the fat line, response appears to vary among animals at the dose level used.