Influence of selection on muscle fibre size and

C . Rehfeldt et al. Livestock Production Science 66 2000 177 –188 181 Table 1 a ´ Meat characteristics dependent on the total muscle fibre number of longissimus muscle in Pietrain pigs mean6S.D. 3 Class of muscle fibre number 3 10 Low Middle High 800–1000 . 1000–1200 . 1200–1600 Number of animals 9 9 8 3 Total fibre number 3 10 908656 1112657 13256110 Fibre diameter mm 86.068.2 77.568.6 67.165.4 Lean meat 60.062.3 59.862.4 59.163.4 2 Loin muscle area cm 54.962.0 57.266.0 58.267.5 pH 45 min p.m. 5.9560.36 6.0160.44 6.2060.39 Reflectance 17 h p.m. 4863 4964 4663 Drip loss 4.0162.21 4.3962.85 2.9161.45 a Halothane status as the number of homozygous negative homozygous positive pigs: low, 3 6; middle, 4 5; high, 2 6. ´ Pietrain pigs with the highest number of low-size Larzul et al. 1997, Klont et al. 1998 and Karlsson fibres in the longissimus muscle tended to exhibit the et al. 1999. best meat quality without significant differences in lean meat percentage and loin muscle area. From data reported by Maltin et al. 1997, it may be

4. Influence of selection on muscle fibre size and

suggested that strong muscle fibre hypertrophy con- number tributes to poor tenderness of pig muscle. Similarly, extreme muscle fibre hypertrophy in callipyge lambs 4.1. Genetic variability heritability has been reported to be associated with poor meat quality such as reduced tenderness and juiciness Whether and to what extent a biological trait is Shackelford et al., 1997. inherited and can be changed by selection largely The relationships between fibre number, fibre size depends on its genetic variability, heritability and and meat quality are most obvious when comparing genetic correlation to the criteria used in selection. genotypes or groups of pigs with extremely different As shown in Table 2, about half mouse or two- lean percentage or meat quality, whereas significant thirds pig of the phenotypic standard deviation in within-breed correlations are scarcely presented. muscle fibre number is due to genetic origin. This Also, within normal cattle breeds, no correlations proportion is relatively high as compared with between fibre characteristics and meat quality traits performance traits commonly used in selection of were found in e.g., Wegner et al., 2000. However, farm animals. double-muscled cattle exhibit paler meat and higher Several studies have been conducted to estimate proportions of glycolytic fibres despite higher fibre numbers and similar fibre size compared with other Table 2 cattle breeds e.g., Holmes and Ashmore, 1972; Phenotypic and genetic variation coefficients CV , CV ; of p g a muscle fibre number and size cross-sectional area or diameter Wegner et al., 2000. The results suggest that fibre size and fibre Species muscle Fibre number Fibre size metabolism may be also independently related to CV CV CV CV p g p g ultimate meat quality and that only one small part of Mouse extensor 18.9 9.3 17.8 8.2 its phenotypic variation is due to variations in muscle digitorum longus fibre characteristics. The relationships between meat Pig longissimus 25.9 17.1 13.6 7.6 quality and muscle fibre type composition are not a reviewed here; this subject was discussed in detail by From Rehfeldt et al. 1988, Fiedler and Dietl 1992. 182 C . Rehfeldt et al. Livestock Production Science 66 2000 177 –188 Table 3 2 a Estimates of heritability h for muscle structure traits 2 Species: Heritability h muscle Fibre number area or diameter Fibre size Reference Mouse: extensor digitorum longus 0.23–0.24 0.16–0.21 Rehfeldt et al. 1988 soleus 0.44–0.68 0.07 Nimmo et al. 1985 Chicken: pectoralis superficialis 0.12–0.49 0.00–0.26 Locniskar et al. 1980 Pig: longissimus 0.66–0.88 0.17–0.31 Staun 1968 0.43–0.48 0.30–0.50 Staun 1972 0.28–0.41 0.22–0.34 Fiedler et al. 1991, Dietl et al. 1993 0.22 0.34 Larzul et al. 1997 Cattle: longissimus ND 0.29 Gravert 1963 0.35 0.74 Osterc 1974 ND 0.39 Andersen et al. 1977 a Ranges arise from the application of different methods of heritability estimation. ND, not determined. the heritability of muscle fibre traits by use of 4.2. Differences between breeds different methods. Heritability has been defined in both a broad sense and a narrow sense Falconer, The influence of growth selection on muscle fibre 1981. Estimated by the twin method e.g., Komi and number or size is also apparent from differences Karlsson, 1979, the heritability is the extent to between animals of different breeds or between wild which individual variation of a population is ge- and domestic types of the same species. netically determined. On the other hand, heritability The European domestic pig, which was derived in a narrow sense can be estimated by the use of from the European wild pig, exhibits larger fibres artificial selection experiments e.g., Nakamura et al., Bader, 1983; Szentkuti and Schlegel, 1985; Weiler 1993, that is, the extent to which the individual et al., 1995 but also higher numbers of fibres e.g., variation within a population is passed on to the next semitendinosus muscle, Table 4. Clear differences generation. The coefficients of heritability estimated in muscle fibre number, but not in muscle fibre for muscle fibre number Table 3 range from 0.12 to 0.88, most lying between 0.2 and 0.5. These results Table 4 demonstrate that muscle fibre number is not exclu- Muscle fibre number and muscle fibre cross-sectional area sively determined genetically as has been previously LSMeans6S.E. in the semitendinosus muscle of wild-type WP presumed owing to its relative constancy during and domestic pigs DP at 7 and 20 weeks of age postnatal life. Probably, maternal factors environ- Weeks of age mental and genetic are significant determinants of 7 20 muscle fibre number as the formation of fibres occurs 3 prenatally. Maternal influence on fibre number has Fibre number 3 10 WP 611638 554629 DP 908654 860654 been estimated for mouse Extensor digitorum longus muscle and reported to be about 17 of the pheno- Fibre cross-sectional WP 407636 14406136 2 area mm DP 1082651 38556255 typic variance Rehfeldt et al., 1988. Heritability estimates for muscle fibre size range from low to P , 0.01, P , 0.001 for differences between DP German high, but most lie between 0.2 and 0.3. Landrace and WP. C . Rehfeldt et al. Livestock Production Science 66 2000 177 –188 183 thickness, were reported between Large White and factor GDF-8 that belongs to the transforming miniature pigs of the same age Stickland and growth factor-b TGF-b superfamily and has been Handel, 1986. No marked differences in muscle identified as an important negative regulator of fibre number and size of the longissimus muscle muscle development in a mouse model of gene were apparent between different modern meat-type deletion McPherron et al., 1997. The callipyge pig breeds and crosses in contrast to the ‘older’ fatty condition in sheep as a further example of extreme Saddle Back breed which has a lower fibre number muscular hypertrophy is caused by a mutation of the and size Table 5. When comparing the same callipyge gene located on ovine chromosome 18 muscle of several European pig breeds 20 years ago, Cockett et al., 1994. In contrast to the double- Staun 1963 found differences in muscle fibre size muscled condition in cattle, muscular enlargement in and number. Possibly, in modern meat-type pig callipyge lambs seems mainly to be due to muscle ´ breeds, e.g., Pietrain or Large White, fibre number fibre hypertrophy Carpenter et al., 1996. and size are at the limits of their correlated responses to selection for leanness, and new strategies must be 4.3. Correlated selection responses applied to attain further changes. There are no obvious differences in muscle fibre 4.3.1. Selection experiments number and size in most of the cattle breeds e.g., Genetic relationships between animal performance Osterc, 1974; Wegner et al., 2000. However, an and muscle fibre characteristics can be derived from exception are double-muscled cattle which exhibit both selection experiments and from genetic correla- almost double the number of muscle fibres compared tion coefficients. with other cattle breeds, whereas no differences in Differences in muscle mass obtained by breeding fibre size are apparent Ouhayoun and Beaumont, and selection are due to changes in both muscle fibre 1968; Holmes and Ashmore, 1972; Wegner et al., number and muscle fibre size. This can be concluded 2000. Prenatal studies with double-muscled cattle from a series of selection experiments for large body suggest that the higher number of muscle fibres is a size or rapid growth rate with several species includ- consequence of delayed differentiation and extended ing the mouse Luff and Goldspink, 1967; Hanrahan myoblast proliferation Picard et al., 1995, and et al., 1973; Aberle and Doolittle, 1976; Penney et serum from double-muscled foetuses induced higher al., 1983; Rehfeldt and Otto, 1985; Timson et al., ¨ proliferative responses in L myoblasts compared 1985; Rehfeldt and Bunger, 1990; Brown and Stick- 6 with serum of normal cattle foetuses Gerrard and land, 1994; Summers and Medrano, 1994, pig Judge, 1993. The double-muscled phenotype arises Wicke, 1989; Wicke et al., 1991; Brocks et al., from mutations in the myostatin gene Grobet et al., 1998, chicken Smith, 1963; Mizuno and Hikami, 1997; Kambadur et al., 1997; McPherron and Lee, 1971; Aberle and Stewart, 1983; Remignon et al., 1997. Myostatin is a growth and differentiating 1994, 1995, quail Fowler et al., 1980 and turkey Cherel et al., 1994. Growth selection leads to increases in myoblast and or satellite cell proliferation rates as indicated Table 5 by higher myonuclear numbers e.g., Knizetova et Muscle fibre number and diameter mean6S.D. in the longis- al., 1972; Penney et al., 1983; Brown and Stickland, simus muscle of different pig breeds Fiedler et al., 1989; Kuhn et 1994, higher DNA synthesis rate Knizetova et al., al., 1998 1972 and higher total muscle DNA content e.g., Pig breed n Fibre number Fibre diameter 6 Knizetova et al., 1972; Martin and White, 1979; 3 10 mm Fowler et al., 1980; Campion et al., 1982; Jones et German Landrace 694 1.04160.280 68.969.5 al., 1986; Mitchell and Burke, 1995. We also German Large White 137 1.01660.251 70.068.4 recently demonstrated this by the in vitro growth of Leicoma 1052 1.06160.275 68.669.4 Schwerfurter 77 1.10960.309 68.9610.7 satellite cells derived from differently selected mouse ´ Pietrain 26 1.10760.178 71.368.8 lines Fig. 4. According to the principles of skeletal Saddle Back 17 0.90960.178 67.167.8 muscle growth, higher proliferation rates contribute 184 C . Rehfeldt et al. Livestock Production Science 66 2000 177 –188 Burke, 1995, but no increases in IGF-I concen- trations were found. The correlated responses of these and other hormones and growth factors to growth selection need to be further investigated. 4.3.2. Genetic correlation coefficients Changes that are to be expected in response to selection can be predicted from genetic correlation coefficients. It has been found consistently that the antagonism between fibre size and number men- tioned above is based on a genetic relationship. The genetic correlation coefficients between fibre size and number estimated for the mouse, chicken and Fig. 4. DNA accumulation in cultures of satellite cells isolated pig were found to range from 2 0.4 to 2 0.8 Staun, from mouse lines selected for 6-week protein accretion DU-6P, body weight DU-6 or an index from body weight and treadmill 1972; Locniskar et al., 1980; Rehfeldt et al., 1988; performance DU-6 1 LB over 71 generations, and from a Fiedler et al., 1997; Larzul et al., 1997. Conse- control DU-Ks. FBS, foetal bovine serum Walther, 1999. quently, selection of animals with high muscle mass due mainly to large fibres will in turn produce offspring with low fibre number. to the formation of higher muscle fibre numbers and, In pigs, positive genetic correlations were ob- postnatally, to the accumulation of more myofibre served for muscle fibre number or size with lean nuclei. There is some evidence that selection for meat percentage Table 6, which is consistent with growth or body weight mainly stimulates myoblast results reported previously by Dietl et al. 1993 and proliferation and muscle fibre formation without a Larzul et al. 1997. We did not find significant change in muscle DNA:protein ratio Martin and correlations for both fibre characteristics with backfat White, 1979; Fowler et al., 1980; Campion et al., thickness Table 6. In contrast, Larzul et al. 1997 1982; Penney et al., 1983. In contrast, for modern reported a negative correlation 20.26 of fibre size meat-type chickens Knizetova et al., 1972; Jones et with backfat thickness. The genetic correlations of al., 1986; Mitchell and Burke, 1995, pigs selected fibre size and number with average daily gain seem for meat content Nøstvold et al., 1984 and mice to be contradictory as the estimated coefficients selected long-term for protein content Rehfeldt and range from 2 0.49 to 1 0.46 for fibre number and ¨ Bunger, 1990, decreased muscle DNA:protein or from 0.03 to 0.74 for fibre size Table 6; see also, nuclear:cytoplasm ratios have been reported. There are suggestions that the extent of hyper- trophic and or proliferative response depends on Table 6 Genetic correlations of longissimus muscle fibre number and fibre how the applied selection leads to changes in the cross-sectional area r 6S.E. with traits of growth and pork g hormonal system, especially in the growth hormone quality estimated from data of half- and full-sib groups n 5 1997 GH insulin-like growth factor-I IGF-I axis. IGF- from 514 sires and 1078 dams of four German pig genotypes I is an important growth factor which stimulates REML procedure myoblast and satellite cell proliferation White and Trait Fibre number Fibre cross- Esser, 1989; Florini et al., 1991. Plasma concen- sectional area trations of GH were significantly decreased and a Average daily gain g day 0.4660.15 0.0360.19 plasma IGF-I was increased in response to growth Backfat thickness mm 2 0.0560.11 2 0.1260.18 selection in mice Medrano et al., 1991; Moride and Lean meat percentage 0.3860.12 0.5260.08 Hayes, 1993; Schadereit et al., 1998. Similarly, Drip loss 2 0.0560.19 0.6460.25 Reflectance 17 h p.m. 2 0.0560.14 0.3260.14 plasma GH concentrations were lower in broiler pH 45 min p.m. 0.1360.14 2 0.3760.19 chickens highly selected for muscle mass compared a with slow-growing Bantam chickens Mitchell and Related to carcass weight. C . Rehfeldt et al. Livestock Production Science 66 2000 177 –188 185 Staun, 1972; Dietl et al., 1993; Larzul et al., 1997. low line and the proportion of stress-susceptible As shown by Larzul et al. 1997, lean tissue growth halothane-positive pigs shifted from about 50 to rate correlates with fibre size r 5 0.47 but not with zero in the low line and to 70 in the high line. g fibre number r 5 0.08. Fibre size in turn correlates Moreover, it has also been demonstrated by simu- g negatively with good pork quality, as exemplified by lated selection with mouse and pig data that, if genetic correlations with different quality charac- muscle structure traits were included in selection teristics, such as drip loss, lightness and pH value indices, selection responses in commonly used per- Table 6; see also, Staun, 1972; Larzul et al., 1997, formance traits could be markedly improved Re- and the genetic correlations are numerically higher hfeldt et al., 1989; Dietl et al., 1993. This is of than the phenotypic ones. The genetic correlation importance with regard to a possible use of muscle between fibre number and meat quality is less clear, structure characteristics in farm animal selection. but the individual coefficients bear the opposite sign which appears logically consistent due to the antago- nistic relationship of fibre number and size.

5. Conclusions