Differences in food resource allocation in a long-term selection experiment for litter size in mice. 2. Developmental trends in body weight against food intake
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Differences in food resource allocation in a long-term selection experiment for litter size in mice. 2. Developmental trends in body weight against food intake

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Differences in food resource allocation in a long-term selection experiment for litter size in mice. 2. Developmental trends in body weight against food intake Differences in food resource allocation in a long-term selection experiment for litter size in mice. 2. Developmental trends in body weight against food intake Document Transcript

  • Animal Science 2000, 71: 39-47 © 2000 British Society of Animal Science 1357-7298/00/95810039$20·00 Differences in food resource allocation in a long-term selection experiment for litter size in mice 2. Developmental trends in body weight against food intake W. M. Rauw1, P. Luiting2, M. W. A. Verstegen3, O. Vangen1 and P. W. Knap2 1Department of Animal Science, Agricultural University of Norway, PO Box 5025, 1432 Ås, Norway 2PIC International Group Ltd, Roslin Institute, Roslin, Midlothian EH25 9PS, UK 3Animal Nutrition Group, Wageningen Institute of Animal Science, PO Box 338, 6700 AH Wageningen, The Netherlands Abstract In the accompanying paper, specific genetic factors for body weight and food intake were identified in nonreproductive male and female mice of a line selected for high litter size at birth (average of 22 born per litter) and a non-selected control line (average of 10 born per litter). The existence of these factors are indicated by variation in efficiency parameters such as growth efficiency and maintenance requirements. Residual food intake (RFI) and Parks’ estimates of growth efficiency (AB) and maintenance requirements (MEm) were used to quantify these factors. In the growing period, females had a higher RFI (are less efficient) than males. At maturity, selected mice had higher RFI than control mice and selected females had higher RFI than selected males. AB was higher in selected-line mice than in control-line mice, and higher in males than in females. MEm was higher in selected-line mice than in control-line mice, and higher in females than in males. The results indicate the existence of specific genetic factors for both growth efficiency and maintenance requirements. Selected females may increase RFI in the adult state to anticipate the metabolically stressful periods of pregnancy and lactation, to support a genetically highly increased litter size. Keywords: food intake, growth, litter size, selection. Calculation of residual food intake (RFI) and the method of Parks (1982) to estimate maintenance requirements and growth efficiency are tools to quantify these SGFs. Introduction In the accompanying paper (Rauw et al., 2000) it was shown that growth and food intake curves against time, of non-pregnant individuals resulting from a long-term selection experiment for litter size in mice, differ significantly when standardized by mature body weight. Hence this indicates the presence of line specific genetic factors (SGFs). These results suggest that selection for high litter size has disproportionally changed the resource allocation pattern. The presence of SGFs for both body weight and food intake suggest an interrelationship that may be described by efficiency parameters such as growth efficiency and maintenance requirements. RFI is defined as the part of the food intake that is unexplained by maintenance and production, or in other words, as the difference between the food that is consumed by an animal and its consumption as predicted from a model involving its growth and maintenance requirements. Variation in RFI can be caused by variation in partial efficiencies for maintenance and growth and by variation in metabolic (i.e. food demanding) processes not included in the model, such as physical activity, responses to pathogens and responses to stress (Luiting and Urff, 1991). 39
  • 40 Rauw, Luiting, Verstegen, Vangen and Knap Parks’ (1982) method involves estimation of growth efficiency and maintenance requirements per metabolic kg by fitting curves of body weight against cumulative food intake. The aim of the present study is to investigate the metabolic resource situation in non-reproductive males and females in a long-term selection experiment for litter size in mice. The present paper aims to quantify and explain the line differences in SGFs that have been detected in the accompanying paper (Rauw et al., 2000). Material and methods The experimental set-up, the conditions under which the mice were kept and the measurement methods for body weight and food intake are described in the accompanying paper (Rauw et al., 2000). Briefly, two mouse lines of the Norwegian mouse selection experiment (Vangen, 1993) were used: a line selected for high litter size at birth (S-line) and a non-selected control line (C-line). The experimental population consisted of 10 and 16 animals per sex per line originating from two generations: the 92nd (replicate 1) and the 95th (replicate 2), respectively. Body weight and food intake were measured from 3 to 10 weeks of age, for 5 days/week in replicate 1 and 3 to 4 days/week in replicate 2. Residual food intake The equation used to estimate RFI for each animal was based on the following multiple linear regression of food intake on metabolic body weight and body-weight gain in the control line (within replicate): FCi(C) = b0(C) + (b1(C) ✕ BWi(C)0·75) + (b2(C) ✕ BWGi(C)) + ei(C), where FCi(C) = food consumption of mouse i of the Cline (g/day); BWi(C)0·75 = metabolic body weight of mouse i of the C-line (kg0·75); BWGi(C) = body weight gain of mouse i of the C-line (g/day); b0(C) = C-line population intercept; b1(C), b2(C) = C-line population partial regression coefficients representing maintenance requirements per metabolic kg and requirements for growth, respectively; and ei(C) = the error term, representing RFI of mouse i of the C-line (g/day). Equation (1) was fitted per day from 3 to 10 weeks of age. Subsequently, within replicate, individuals was estimated as: RFI of RFIi(S) = FCi(S) – {b0(C) + (b1(C) ✕ BWi(S)0·75) + (b2(C) ✕ BWGi(S))}, S-line where RFIi(S) = residual food intake of mouse i of the S-line (g/day); FCi(S) = food consumption of mouse i of the S-line (g/day); BWi(S)0·75 = metabolic body weight of mouse i of the S-line (kg0·75); and BWGi(S) = body-weight gain of mouse i of the S-line (g/day); b0(C) , b1(C) and b2(C) are the parameters described in (1). This was done using the estimates from each day of measurement from 3 to 10 weeks of age. The experimental period was subsequently divided, based on daily growth rates presented in the companion paper (Rauw et al., 2000), into a ‘growing period’, i.e. from 3 to 6 weeks of age, and an ‘adult period’, i.e. from 6 to 10 weeks of age. Equation (1) was fitted for the growing period and the adult period from cumulated data on growth and food intake per animal over these periods. Metabolic body weight of the growing period was estimated as the average of metabolic body weights at 3, 4, 5 and 6 weeks of age; metabolic body weight of the adult period was estimated as the average of metabolic body weights at 6, 7, 8, 9 and 10 weeks of age. Finally, equation (1) was fitted for the total period with metabolic body weight estimated as the average of the week values from 3 to 10 weeks of age. RFI for each animal in the C-line equalled its error term ei(C) in (1) and RFI of S-line individuals was estimated as in equation (2). This implies that the average RFI of all C-line mice equalled 0. RFI was consequently estimated per day, for the growing period, for the adult period and for the total period from 3 to 10 weeks of age. Parks’ curves Following Archer and Pitchford (1996), modified Parks’ (1982) curves were fitted with the program SigmaPlot Scientific Graphing System (Jandel, 1992) (1) individual data on body weight against to cumulative food intake from 3 to 10 weeks of age: BWt = A (1 – e–B(CFIt + FI0)), where BWt = body weight of the individual (g) at age t (days from weaning); CFIt = cumulative food intake (g) at age t (days from weaning; at weaning CFI = 0); A = mature (adult, asymptotic) body weight (g); B = rate of maturation of body weight with respect to food intake (per g); FI0 = a translation of equation (3) along the X axis to complete the description of growth (g). A, B and FI0 are parameters to be estimated. The modification of the Parks’ curve involves the inclusion of FI0 to avoid fixing the curve through any point (Archer and Pitchford, 1996). The same package was used to relate individual data on cumulative food intake to age according to a (2) (3)
  • Resource allocation after long-term selection in mice — 2 linear function by Parks (1982; p. 31). To ensure that the food intake represented food intake at maturity, data were restricted to 60 days of age and older (Archer and Pitchford, 1996): CFIt = (t-t5), MFI where CFIt = cumulative food intake of the animal (g) at time t (days from weaning to > 60 days of age); MFI = maximum mature daily food intake (g/day); t = age (days from weaning to > 60 days of age); and t5 = X axis intercept (days from weaning). MFI and t5 are parameters to be estimated. Following Taylor (1980), body weight (kg) and cumulative food intake (kg) of each animal were scaled (i.e. divided) by its mature body weight (A in kg, equation 3), and age was scaled by (t – 3·5)/A0·27, where (t – 3·5) is age in days from 3·5 days after conception (for the justification of why the value of 3·5 was chosen, see Rauw et al. (2000)); gestation length is on average 19 days. Standardized body weight represents ‘degree of maturity’ (u) and standardized age represents ‘metabolic age’ (θ). Scaling of equation (3) gives: BWt/A = 1 – e–(B ✕ A) {(CFIt/A) + (FI0/A)}, which can be written as u = 1 – e–(B ✕ A) {(CFIt/A) + (FI0/A) (5). The two parameters B and FI0 are now scaled to B ✕ A and FI0/A; Parks (1982) defined A ✕ B as the growth efficiency parameter (AB). Equation (4) was scaled in the same way as equation (2a) in the accompanying paper (Rauw et al., 2000): CFIt/A = (MFI/A0·73) {(t/A0·27) – (t5/A0·27)}, which can be written as CFIt/A = (MFI/A0·73) (θ – θ5) The two parameters MFI and t5 are now scaled to MFI/A0·73 and θ5; when MFI is expressed in kJ metabolic energy by multiplying it with the metabolizable energy content of the food (12·6 kJ ME per g), MFI/A0·73 represents the adult maintenance requirements per metabolic kg (MEm). Data analyses The SAS program (Statistical Analysis Systems Institute, 1985) was used for statistical analyses of RFI and all estimated parameters. The model used to describe the individual data was: 41 Yijkl = µ + Ri + Lj + Gk + (RL)ij + (RG)ik + (LG)jk + (RLG)ijk + eijkl, where Yijkl = individual daily RFI, RFI in the growing period, the adult period and the total period and all (4) unscaled and scaled parameters estimated from equations (3) and (4); µ = overall mean; Ri = effect of replicate i (replicate 1, replicate 2); Lj = effect of line j (control, selected); Gk = effect of sex k (female, male); (RL)ij = interaction effect of replicate i by line j; (RG)ik = interaction effect of replicate i by sex k; (LG)jk = interaction effect of line j by sex k; (RLG)ijk = interaction effect of replicate i by line j by sex k; eijkl = error term of animal l; eijkl NID(0, σ2e). Phenotypic correlations between RFI in the growing period, the adult period and the total period, and A, MFI, AB and MEm were estimated adjusted for replicate, line and sex effects and their interactions. Results Residual food intake Figure 1 shows the average daily RFI for each sex in each line from 3 to 10 weeks of age. Results are presented for each replicate. Trends are described by linear lines for C-line mice and logarithmic lines for S-line mice. R2 values of the multiple regressions according to equation (1) per day were in the range of 42% to 86% in replicate 1 and 47% to 83% in replicate 2. Least-squares means of RFI in the growing period, the adult period and the total period for each sex in each line (adjusted for effect of replicate) are presented in Table 1. R2 values of the multiple regressions according to equation (1) per period in replicate 1 and replicate 2 were 88% and 81% for the growing period, 74% and 66% for the adult period, and 77% and 75% for the total period, respectively. Figure 1 shows that in C-line animals a trend in RFI from 3 to 10 weeks of age was absent: average daily RFI in C-line animals hovered around 0. The reason for this is that equation (1) was based on all C-line animals. Hence the average RFI of the C-line (6). population was 0 and therefore the average values of males and females of the C-line were symmetrical around 0. Figure 1 shows an increasing trend in Sline animals: in general RFI of S-line mice was lower than RFI of C-line mice during the first days after weaning and higher during the last weeks. This shift occurred somewhat sooner in replicate 2 (around 4·5 weeks of age) than in replicate 1 (around 5·5 weeks of age). Therefore, in the growing period from 3 to 6 weeks of age, S-line mice had a lower RFI than C-line mice in replicate 1, although this was not significant, and a significantly higher RFI in replicate 2. In the adult period and over the total period, S-line
  • 42 Rauw, Luiting, Verstegen, Vangen and Knap Replicate 1 Replicate 2 2·0 Residual food intake (g) 1·5 1·0 0·5 –0·5 –1·0 –1·5 21 26 31 36 41 46 51 56 61 66 71 21 26 31 36 41 46 51 56 61 66 71 Age (days) Figure 1 Average daily residual food intake from 3 to 10 weeks of age for each sex in each replicate of each line. The period before the vertical line (drawn at 6 weeks of age) is the growing period; after the line is the adult period (no. = 10 in replicate 1, no. = 16 in replicate 2). C = control-line (circle), S = selection-line (square), F = female (open symbol), M = male (closed symbol). Linear trend lines are fitted to data on C-line mice; logarithmic trend lines are fitted to data on S-line mice. mice had significantly higher RFI than C-line mice, i.e. they were less food efficient. Figure 1 shows furthermore that RFI was consistently higher in females than in males. In the growing period, a significant interaction between line and sex (P < 0·05) indicates that this sex difference was significantly greater in the S-line than in the C-line. In the adult period, a significant interaction between line and sex (P < 0·01) indicates that this sex difference was significant in S-line mice only. A significant interaction between sex and replicate (P < 0·05) indicates that the difference in RFI between males and females was significantly greater Table 1 Least-squares means per sex per line† (adjusted for effect of replicate) and standard errors of the least-squares means, of daily residual food intake (RFI) in the growing period, the adult period and the total period, Parks’ parameter estimates of mature body weight (A), rate of maturation of body weight with respect to food intake (B), translation parameter of equation (3) (FI0), mature food intake (MFI), X-axis intercept of equation (4) (t5), scaled B (growth efficiency, AB), scaled FI0, scaled MFI multiplied by the metabolizable energy content of the food (maintenance requirements per metabolic kg, MEm), and scaled t5 (θ5) CF RFI growing period (g) RFI adult period (g) RFI total period (g) A (g) B (per g) FI0 (g) MFI (g/day) t5 (days) AB Scaled FI0 MEm (kJ/kg) θ5 CM SF SM s.e. 0·0859a 0·0822a 0·0827a 30·5a 0·0107ab 57·2a 4·93a 22·5a 0·321a 1·86a 857a 100a –0·0859b –0·0822a –0·0827b 37·8b 0·0104ab 51·2ab 5·22b 21·3b 0·381b 1·35ab 774b 91·6b 0·212a 0·811b 0·550c 41·9c 0·0098a 67·2c 6·78c 23·9c 0·408b 1·60c 923b 95·2c –0·180b 0·271c 0·195a 50·5d 0·0115b 46·4b 6·87c 21·1b 0·577c 0·926b 814c 84·2d 0·0553 0·0664 0·0544 0·748 0·000544 3·02 0·0924 0·438 0·0174 0·0683 13·0 1·10 † C = control line, S = selection line, F = females, M = males. a,b,c,d Values with different superscripts are significantly different (P < 0·05).
  • Resource allocation after long-term selection in mice — 2 in replicate 1 in the growing period and in the total period. Parks’ curves Average curves for each sex in each line (adjusted for replicate effect) fitting equations (3) and (5) are given in Figure 2a and 2b, respectively. R2 values of individual curves were 90% to nearly 100%; all R2 values of individual linear regressions fitting equation (4) were nearly 100%. Least-squares means of A, B, FI0 (equation 3), MFI, t5 (equation 4), scaled B (AB), scaled FI0 (equation 5), scaled MFI multiplied by the energy content of the food (MEm) and scaled t5 (equation 6) for each sex in each line (adjusted for replicate effect), are presented in Table 1. Parks’ growth curves fitted the data very well, as indicated by the R2 values. Therefore, the curve parameters could be estimated with reasonably high accuracies. The coefficients of variation (i.e. the standard errors as a proportion of Body weight (g) 55 (a) 50 45 40 35 30 25 20 15 10 50 0 100 150 200 250 300 350 Cumulative food intake (g) Degree of maturity (u) 1·0 (b) 0·9 0·8 43 the associated estimate) among individual animals ranged from 1·48 to 2·45 for A, from 4·73 to 5·55 for B, and from 4·49 to 6·51 for FI0. S-line individuals had a significantly higher mature body weight (A) than C-line mice and A was significantly higher in males than in females. These estimates of A were, as expected, very similar to the estimates of A according to equation (1a) in the accompanying paper (Rauw et al., 2000). S-line males had a significantly higher maturation rate of body weight with respect to food intake (B) than S-line females. Estimates of B were very similar for C-line males and females and lay in between the estimates of S-line males and females. A significant interaction between line and sex (P < 0·05) indicates that the time origin of equation (3) (FI0) was later in females than in males but this difference was significant in the S-line only. Estimates of mature food intake (MFI) were significantly higher in S-line mice than in C-line mice. MFI was significantly higher in males than in females, though this was significant in the C-line only. Furthermore, MFI was significantly higher in replicate 2 than in replicate 1 (P < 0·001). The estimates according to equation (4) were very similar to the estimates according to equation (2a) in the accompanying paper (Rauw et al., 2000), although in the present experiment, the linear function related cumulative food intake to age greater than 60 days. Differences between males and females in MFI according to equation (4) were smaller than in MFI according to equation (2a) in the accompanying paper (Rauw et al., 2000); differences between S-line males and females were no longer significant. A significant interaction between line and replicate (P < 0·05) indicates that the X axis intercept of equation (4) (t5) was significantly later in S-line animals than in C-line animals in replicate 1 but no significant line difference existed in replicate 2. The t5 was significantly later in females than in males. 0·7 0·6 0·5 0·4 0·3 0 2 4 6 8 Cumulative food intake/A Figure 2 Average curves fitting equation 3 (a) and 5 (b) for each sex in each line. All curves are based on least-squares means of curve parameters for each sex in each line, adjusted for effect of replicate (no. = 26): control-line (dashed line), selection-line (solid line), female (light), male (bold). Estimates of scaled rate of maturation with respect to food intake (growth efficiency, AB) were significantly higher in mice of the S-line than in mice of the C-line. Males had higher AB than females; a significant interaction between line and sex (P < 0·01) indicates that this difference was significantly greater in the S-line than in the C-line. Scaled FI0 was significantly later in C-line animals than in S-line animals and significantly later in females than in males. S-line mice had significantly higher scaled MFI multiplied by the ME content of the food (maintenance requirements per metabolic kg, MEm) than C-line mice; a significant interaction between
  • 44 Rauw, Luiting, Verstegen, Vangen and Knap Table 2 Phenotypic correlations between residual food intake (RFI) in the growing period, the adult period and the total period, and Parks’ parameter estimates of mature body weight (A), mature food intake (MFI), growth efficiency (AB) and maintenance requirements per metabolic kg (MEm), adjusted for replicate, line and sex RFI growing period A MFI AB MEm RFI adult period RFI total period –0·01 0·41*** 0·22* 0·41*** 0·17 0·82*** –0·11 0·43*** 0·05 0·78*** 0·21* 0·72*** line and replicate (P < 0·01) indicates that this was mainly caused by replicate 1. MEm was significantly higher in females than in males. Scaled t5 (θ5) was later in C-line mice than in S-line mice, though a significant interaction between line and replicate (P < 0·01) indicates that this was significant in replicate 2 only. θ5 was later in females than in males. Table 2 presents phenotypic correlations (adjusted for replicate, line and sex) between RFI in the growing period, the adult period and the total period, with A, MFI, AB and MEm. The correlations between RFI in the growing period and RFI in the adult period (r = 0·47), between RFI in the growing period and RFI in the total period (r = 0·77) and between RFI in the adult period and RFI in the total period (r = 0·87) were positive and highly significant (P < 0·001). RFI in none of the periods was significantly correlated with A. RFI in the growing period and in the total period but not in the adult period, was significantly positively correlated with AB. RFI in all periods was significantly positively correlated with both MFI and MEm. The correlation between RFI and MFI was highest in the adult period and lowest in the growing period. The correlation between RFI and MEm was highest in the total period. Discussion In the accompanying paper, significant differences in SGFs for both body weight and food intake were identified in non-reproductive male and female mice between a line selected for high litter size at birth and a non-selected control line (Rauw et al., 2000). These results lead to the question: is the genetic change in the body weight curve against time caused by the genetic change in the food intake curve against time; or in other words, has selection for high litter size disproportionally changed the resource allocation pattern. Furthermore, scaling of time variables with A0·27 (Rauw et al., 2000) is not very well verified within species. Taylor (1985) suggested, therefore, the use of cumulative food intake as a time-scale for growth instead of chronological age, and the subsequent scaling of cumulative food intake with A. For these reasons, in the present paper, RFI and Parks' (1982) curves of body weight against cumulative food intake were studied in the same lines. Both tools describe the relationship between body weight and food intake during growth or at maturity with a single parameter. RFI must be interpreted as a conversion parameter (i.e. output/input) and is estimated during growth and at maturity. Parks' estimate of growth efficiency (AB) is an efficiency parameter (i.e. input/output) and is estimated during growth. Parks' estimate of maintenance requirements per metabolic kg (MEm) is a conversion parameter and is estimated at maturity. The higher maturation rate independent of mature body weight in the selection line, as presented in the accompanying paper (Rauw et al., 2000), suggests a higher growth efficiency in the selection line. Indeed, the Parks' estimate of AB was significantly higher in the selection line, especially in males. Parks' estimate of AB represents the amount of food required for the first unit of body-weight gain when there is no body weight to maintain and all food is directed towards growth. As such it should be most closely related to the RFI at the 1st day of measurement (which is not the 1st day of growth but as close as we can get), albeit an efficiency parameter versus a conversion parameter. Indeed, RFI was significantly lower in the selection line at the first day (and more strongly so than on subsequent days), which corresponds with the higher AB in the selection line. Furthermore, AB represents the constant rate of decrease in growth efficiency with increasing degree of maturity (Parks, 1982). This is caused by an increase in food requirements for maintenance relative to the food requirements for growth, and by an increase in the food requirements per unit growth caused by an increase in lipid-to-protein ratio, when the animal matures. Because AB was higher in the selection line, its maintenance requirements per metabolic kg must have been higher and/or the lipid-to-protein ratio in its body gain must have increased faster than in the control line. The former corresponds to the higher MEm estimated in the selection line (especially in females), although this was an estimate at maturity which could be different from that at an immature stage. The phenotypic
  • Resource allocation after long-term selection in mice — 2 food requirements for maintenance are underestimated: MEm is higher in S-line animals than in C-line animals. The change in RFI in S-line animals from negative to positive occurs when the higher MEm dominates the higher AB. Because MEm in S-line females is much higher than in S-line males, and AB in S-line females is lower than in S-line males, this change occurs much earlier in the females than in the males, leading to a higher RFI overall in the growing period. 0·40 0·35 0·30 AB ✕ (1 – u) 45 0·25 0·20 0·15 0·10 0·05 0·00 –0·05 20 30 40 50 Age (days) 60 70 Figure 3 Decrease in growth efficiency with increasing degree of maturity (AB ✕ (1–u)) for each sex in each line adjusted for replicate effect (no. = 26). u = body weight/A, control-line (circle), selection-line (square), female (open symbol), male (closed symbol). correlation between AB and MEm (adjusted for line, sex and replicate) points in the same direction (r = 0·39, P < 0·01). Differences in growth composition between these lines will be investigated in forthcoming experiments. Figure 3 shows the decrease in growth efficiency with increasing degree of maturity (AB ✕ (1 – u), where u = body weight/A (Rauw et al., 2000)) for the two lines and sexes (adjusted for replicate effect). It shows a similar pattern to that found for RFI during growth (Figure 1): S-line males are somewhat more efficient during growth and show a somewhat lower RFI (although not significant) than the other three line-by-sex combinations. Also within line, sex and replicate, the pattern of variation in AB × (1 – u) and RFI are similar: phenotypic correlations between AB × (1 – u) and RFI, adjusted for line, sex and replicate, range, over time, between 0 and –0·28 (becoming significant at –0·20 for P < 0·05). The fact that RFI during growth in the females of the selection line is positive is caused by the early change in daily RFI from negative to positive. A negative RFI in S-line animals is caused by an overestimation of food requirements by the model, i.e. as expected from food requirements of the control population on which the model is formed. Correspondingly, a positive RFI in S-line animals is caused by an underestimation of food requirements by the model. Overestimation of food requirements for S-line animals during the first days after weaning is caused by overestimation of food required for growth: S-line animals grow more efficiently than C-line animals. The subsequent underestimation of food requirements for S-line animals occurs when Presumably, this also explains the positive sign of the phenotypic correlations of RFI in the growing period with AB and MEm. Archer and Pitchford (1996) found a negative phenotypic correlation between AB and RFI in the first 2 weeks after weaning and no significant correlation thereafter. They also found no significant correlation between MEm and RFI in the first 2 weeks after weaning and a positive correlation thereafter. This could indicate a difference in the point where the dominance of AB changes into the dominance of MEm (namely a bit later). The latter is quite possible, considering the large environmental effect on MEm. In the adult period, S-line mice had a higher RFI than C-line mice and S-line females had a higher RFI than S-line males. Since growth is virtually absent at maturity, the differences in RFI are largely caused by differences in maintenance requirements per metabolic kg. This corresponds very well with the higher MEm estimate in the selection line, especially in the females. Furthermore, RFI in the adult period also has a strong positive phenotypic correlation with MEm within line, sex and replicate (and, obviously, there is no significant correlation between RFI in the adult period and AB). Since tissues with high protein or high lipid levels have different maintenance requirements, line differences in body composition may explain part of the line difference in adult RFI. Differences in adult body composition between the lines will be investigated in forthcoming experiments. However, Archer and Pitchford (1996) found that variation in body composition in adult mice accounted for only a very small part (less than 9%) of variation in RFI. The connection between Taylor's (1980) genetic size scaling rules and RFI in adult animals is described by Luiting et al. (1997): adjustment for adult size in equation (1) is accomplished by the inclusion of metabolic body weight (BW0·75) as a covariate in the model, which is similar to the inclusion of A0·73 in the case of adult animals. Consequently, both the estimates of adult RFI (i.e. mature food intake in g/day adjusted linearly for mature metabolic weight) and Parks' mature food intake scaled
  • 46 Rauw, Luiting, Verstegen, Vangen and Knap and our own data (without the S-line males) as given in Figure 4: they have a high AB because of their very high A in comparison with the other populations. The linear relation between AB and A shows furthermore that variation in this scaled efficiency parameter can still be explained by variation in mature size: AB is not independent of A. Growth efficiency 60 55 50 45 40 35 30 25 25 35 45 55 A Figure 4 Estimates of growth efficiency (AB) in relation to estimates of mature body weight (A): triangle: Archer and Pitchford (1996); circles: Parks (1982); squares: present study; open: females; solid: males. A linear regression line is fitted through all data excluding selection-line males of the present study (AB = 6·47 + 0·92A, R2 = 0·60). according to Taylor (1980) as presented in equation (6) (i.e. mature food intake in g/day divided by mature metabolic weight), are estimates of mature food intake independent of mature size. With RFI, adjustment for mature size is accomplished by linear regression rather than by division, which is statistically a better approach (Packard and Bordman, 1988). In growing animals, however, the linear adjustment with the covariate BW0·75 is not the same as adjustment for adult size A0·73, and comparisons are made at the same chronological age. Hence, some adjustments for mature size and physiological age would be needed. The first could be taken care of by including A0·73 as a covariate in the model, and the latter by measuring food intake and weight over the entire growth period up to the stage when mature size is reached or by including some measurement of stage of development into the model (given the terms already present in the model, it is not clear what this measurement should be). However, the correlations between RFI and mature size were very close to zero and non-significant in our data (Table 2) and the same holds for the correlations between RFI and stage of development (calculated as W/A) in Archer and Pitchford (1996). This suggests that the phenotypic adjustments for size and stage of development will not be very crucial but they may be when the variation between animals in these traits is larger. Our estimate of growth efficiency AB in the S-line males is particularly high compared with the ones presented by Parks (1982) and Archer and Pitchford (1996) in mice (see Figure 4). However, AB of the S-line males is predicted very well by the linear regression of AB on A based on these literature data Replicate 1 was conducted from June to August 1997 while replicate 2 was conducted from March to May 1998. There has not been any climate regulation in either of the replicates but temperature and humidity have been registered in replicate 2: temperature was on average 20·3 (s.e. 0·63) ºC and average humidity was 0·616 (s.e. 0·048). It is likely that the temperature was higher and humidity lower during the summer months. Indeed, daily mature food intake and MEm were lower in replicate 1. This could be the reason for the later appearance of the point where the dominance of AB changes into the dominance of MEm as can be seen from the change in sign of RFI (hence a lower RFI in the growing period in replicate 1 than in replicate 2). This seems probable because of the large effect of MEm on food efficiency both during growth and at maturity. Because many maintenance processes are very dependent on environmental factors and are sometimes only expressed under certain circumstances, line-byreplicate interactions could be expected. From the point of view of resource allocation, animals with low RFI may be particularly short in resources left to respond adequately to unexpected stresses and diseases. Laying hens selected for high RFI were better adapted to stressors such as relocation, high temperatures and ACTH than hens selected for low RFI (Luiting et al., 1994; Bordas and Minvielle, 1997). It is interesting that particularly adult females of the selection line have a very high RFI, since it is these animals that can express the trait their genotype has been selected for: a high litter size at birth. This higher RFI may therefore anticipate the metabolically stressful periods of pregnancy and especially lactation. These periods may be expected to considerably change the physiological resource demand, since the S-line females have to support a genetically highly increased litter size. The question is whether this can be supported by an increase in food intake during these periods or whether the RFI will drop considerably. Forthcoming research will aim to further investigate this. Acknowledgements This study has been supported by a grant from the Norwegian Research Council, project number 114258/111, ‘Consequences of selection for high litter size for the ability to cope in a stressful environment’. Kari Kjus is gratefully acknowledged for carrying out the Norwegian mouse
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