Genetic analysis of a selection experiment on the growth curve of chickens.

A selection experiment on the shape of the growth curve was performed on meat-type chickens through combined selection on juvenile and adult BW. Line X-+ was selected for low BW at 8 wk (BW8) and high BW at 36 wk (BW36). Line X+- was selected for high BW8 and low BW36. Line X++ was selected for high BW8 and BW36, and X-- was selected for low BW8 and BW36. Line X00 was maintained as an unselected control. Data on the first 14 generations (i.e., 38,693 birds) were used. The growth curve was modeled using a Gompertz function on 7,143 birds that were weighed regularly. Selection for higher BW8 increased BW from 4 to 16 wk, initial specific growth rate, and maturation rate and decreased age at inflection. Selection for higher BW36 resulted in increased BW36, asymptotic BW, and estimated BW at hatching. Body weights were more modified in Lines X++ and X--, but the growth curve parameters changed more in Lines X-+ and X-- than in Lines X++ and X+-.

[The relationship between heart rate and type of growth in the domestic fowl]. / Note sur les relations entre le rythme cardiaque et le type de croissance chez la poule domestique.

The heart rate at 6 and 24 weeks of age and the relative weight of the heart at 8 and 24 weeks were studied in four strains of domestic fowl, differing in growth and, in this study, in body weight. Heart rate at 6 weeks of age was not related to sex, body weight, absolute or relative heart weight. Heart rate at 24 weeks of age was related to relative heart weight at 24 weeks, to absolute heart weight at 6 and 24 weeks, and to body weight at 6 weeks. There was a marked sex effect at 24 weeks. No within-species relationship between heart rate and body weight was found, but a negative correlation existed between heart rate and relative heart weight.

Effects of selection for high and low plasma glucose concentration in chickens.

1. Two lines of broilers exhibiting low (LG) or high (HG) plasma glucose concentrations were selected from a pure line of White Rock chickens. 2. Realised heritabilities were close to 0.25 in both lines. 3. The LG line was significantly fatter than the HG line; this difference was more pronounced in females than in males. 4. Food was utilised less efficiently by the LG line than by the HG one.

Effects of divergent selection for body weight on three skeletal muscles characteristics in the chicken.

1. Histochemical (fibre type distribution and areas) and biochemical (myosin isoforms) characteristics of three muscles, M. anterior latissimus dorsi, M. pectoralis major and M. sartorius, were compared among male chickens of two lines at 11 and 55 weeks of age. 2. The lines were derived from a divergent selection based on growth rate. Cockerels from the Fast Growing Line (FGL) were 2.3 times heavier than those from the Slow Growing Line (SGL) when 11 weeks old and 1.7 times at 55 weeks of age. The latter age was chosen as representative of the adult stage and the 11-week age because, at this time, FGL cocks weighed as much as SGL cockerels at 55 weeks. 3. At both ages, the two lines showed similar fibre type distributions, but the total number in the ALD muscle, and the size (cross-sectional areas) of fibres in each muscle were higher in the FGL compared with the SGL (14.6% and 33% more at 11 and 55 weeks of age respectively in favour of the FGL birds). 4. The two lines displayed similar myosin isoform patterns when adult muscles were compared (55 weeks). They differed slightly at 11 weeks of age, muscle differentiation being completed only in the FGL. 5. Comparisons of the two lines at the same live weight (i.e. FGL cockerels at 11 weeks of age and SGL cockerels at 55 weeks) showed larger muscle fibres in the SGL and no difference in the isomyosin patterns.

Selection for rapid growth increases the number and the size of muscle fibres without changing their typing in chickens.

Quantitative (muscle fibre number and cross-sectional areas) and qualitative (myosin isoforms and metabolic enzyme activities) characteristics of two muscles, M. pectoralis major and M. anterior latissimus dorsi, were compared among male chickens of two lines during growth from hatching to adulthood. The lines were derived from a divergent selection based on growth rate. The two muscles were chosen on the basis of their histochemical profile. Pectoralis major muscle contains only fast contracting muscle fibres whereas anterior latissimus dorsi muscle is almost entirely made up with slow contracting fibres. At both ages, the two lines showed similar fibre type distributions. At hatching, fibre cross-sectional areas were equivalent in the two lines, but after the first week, animals from the fast growing line exhibited wider fibre areas, whatever the muscle, than animals from the slow growing line. The total number of fibres in a muscle was found greater in the fast growing line, irrespective of whether it was exactly determined (anterior latissimus dorsi muscle, + 20%) or only estimated (pectoralis major muscle). This number remains constant in the two lines throughout the growth. Myosin isoform profiles and metabolic enzyme activities were similar in the two lines, at both ages, and were in good agreement with the histochemical muscle fibre profiles.

Divergent selection for high or low growth rate modifies the response of muscle cells to serum or insulin-like growth factor-I in vitro.

Genetic differences in growth potential could result from changes in the levels of growth stimulatory factors or in the response of target tissues. The latter possibility was tested in adult myoblasts prepared from chickens selected for high (HG) or low growth rate (LG). Stimulation of [3H]-thymidine incorporation into DNA by serum was of higher amplitude in HG than LG muscle cells irrespective of whether the cell preparations were enriched in myoblasts or fibroblasts. HG myoblasts were also more responsive to insulin-like growth factor-I (IGF-I) in terms of [3H]-thymidine incorporation. IGF analogues with a reduced affinity for IGF binding proteins gave similar results suggesting that activity of binding proteins could not explain the difference between cells from the HG and LG lines. This difference was restricted to the proliferative stage because in myotubes, basal or IGF-I stimulated glucose and amino acid transports, tyrosine incorporation and protein degradation were not different.

Plasma glucose-insulin relationship in chicken lines selected for high or low fasting glycaemia.

1. Selected Fat or Lean chickens differ in their plasma glucose insulin relationship: in the fed or fasted state, Fat chickens have a lower glycaemia associated with normal or higher insulinaemia, depending upon the difference in glycaemia. 2. Conversely, chickens selected for low fasting glycaemia (LG) are fattier than their counterparts selected for high fasting glycaemia (HG), although the divergence in fat content is lower than in the Fat-Lean model. 3. The plasma glucose insulin relationship has been investigated in males of the HG and LG lines in the F4 and F5 generations. 4. A difference in glycaemia is suggested during embryonic development and was present at hatching and later on in the fasted or the fed state; insulinaemia did not differ. 5. During refeeding after an overnight fast, glycaemia differed between lines (except at intermediate times); cumulative food intake and insulinaemia were similar. 6. During a glucose tolerance test, glucose disposal rate and insulinaemia were rather similar. 7. Exogenous insulin exerted a very similar hypoglycaemic effect in both lines. 8. Other variables (body temperature, plasma concentrations of potassium and alpha NH2-non protein nitrogen) did not differ between HG and LG chickens. 9. In conclusion, HG and LG chickens do not exhibit any differences in glucose disposal rate, insulinaemia (in various nutritional conditions) or sensitivity to exogenous insulin, which contrasts with Fat or Lean chickens and may explain why HG and LG chickens have diverged to a lesser extent in fat content.

Genetic parameters of growth curve parameters in male and female chickens.

1. Individual growth curves of 7143 chickens selected for the form of the growth curve were fitted using the Laird form of the Gompertz function, BW4=BW0xe(L/K)(1-e-Kt) where BWt is the body weight at age t, BW0 the estimated hatching weight, L the initial specific growth rate and K the maturation rate. 2. Line and sex effects were significant for each parameter of the growth curve. In males, L, BW0, age and body weight at inflection (T(I)and BWI) were higher whereas K was lower than in females. Lines selected for high adult body weight had higher BW0 and BW(I) whereas lines selected for high juvenile body weight had larger estimates of L and lower estimates of T(I). 3. Data from 38,474 animals were included in order to estimate the genetic parameters of growth curve parameters in males and females, considering them as sex-limited traits. Genetic parameters were estimated with REML (REstricted Maximum Likelihood) and an animal model. Maternal genetic effects were also included. 4. Heritabilities of the growth curve parameters were moderate to high and ranged between 0.31 and 0.54, L, BW0 in both sexes and BW(I) in males exhibited significant maternal heritability. Heritabilities differed between males and females for BWI and T(I). Genetic correlations between sexes differed significantly from one for all parameters. L, K and T(I) were highly correlated but correlations involving BW0 and BW(I) were low to moderate. 5. Sexual dimorphism of body weight at 8 and 36 weeks and of L, K and T(I) was moderately heritable. Selection on growth curve parameters could modify the difference between sexes in precocity and thus in body weight at a given age.