Genetic control of growth

Poultry, the second largest contributor to world meat production after pork, has persistently been subjected to improvement through hybridization and effective nutritional management programmes targeted for disease free higher outputs. In poultry, growth rate, mature body weight, egg production and fatness are highly related to circulating growth hormone levels (Anthony et al., Reference ANTHONY, VASILATOS-YOUNKEN, EMMERSON, NESTOR and BACON1990). Myostatin, also known as growth and differentiation factor GDF-8 is expressed at high levels in skeletal muscle where it inhibits myoblast cell proliferation and differentiation and post-natally affects skeletal muscle growth by maintaining the quiescent status of satellite cells (McCroskery et al., Reference MCCROSKERY, THOMAS, MAXWELL, SHARMA and KAMBADUR2003) thus exerting negative regulation on skeletal muscle growth. As far as broiler meat production is concerned, this is the ultimate result of skeletal muscle growth occurring pre-natally by hyperplasia (increase in cell number) and post-natally by hypertrophy (increase in cell size), which affect meat quality traits. Meat quality has been the matter of great concern to producers as well as the research sector. Studying the role of these genes on meat quality traits could be of immense help in developing better meat quality lines.

Growth hormone

Growth hormone (GH) or somatotrophin is synthesized and secreted by the pituitary gland and plays pivotal role in postnatal growth of virtually all the vertebrates. In avian species, growth hormone is reported to reduce lipid accretion along with other physiological functions including appetite control, body composition, egg production; ageing and reproduction. The relationship of GH as a repartitioning agent, allowing relatively more substrate to be made available to muscle over adipose tissue accretion in broiler chicken of 4 to 5 weeks of age, has been reviewed by Vasilatos-Younken (Reference VASILATOS-YOUNKEN1995). Growth hormone strengthens skeletal muscle accretion, particularly in red meat animals, through IGF-1 (GH-GHR-IGF axis) which is a strong stimulator for proliferation and differentiation of satellite cells causing myofibrillar hypertrophy and regeneration of postnatal muscle tissue. Interaction between growth hormone and disease resistance in chicken, through SORF2 (Marek's disease virus protein), has also been reported by Hsiao-Ching Liu et al. (2001).

Growth hormone (GH) and growth hormone receptor (GHR) genes

Avian growth hormone was purified from the adenohyphyseal tissue of ducks, pigeons, turkeys and its chemical nature, as a single chain protein having a molecular weight of approximately 23 Kda, has been determined. In chickens, the growth hormone gene and growth hormone receptor (GHR) gene are located on the tip of the long arm of chromosome 1 (Shaw et al., Reference SHAW, SHOFFNER, FOSTER and GUISE1991) and on the Z chromosome (Burnside et al., Reference BURNSIDE, LIOU and COGHURN1991) respectively. The amino acid sequence was confirmed by Lamb et al.(1988) using cDNA nucleotide sequence analysis. In meat type chickens selected for abdominal fat content, Fotouhi et al. (Reference FOTOUHI, KARATZAS, KUHNLEIN and ZADWOMY1993) identified four DNA polymorphisms in the chicken GH gene (exons and/or introns), one at Sac I and three at Mspl restriction sites; the highly polymorphic nature of GH gene and influence of allelic variations on the fatness/leanness were elucidated. Growth hormone gene polymorphism in Chinese native chickens has been reported by Ip et al. (Reference IP, ZHANG and LUENG2001). Nie et al. (Reference NIE, IP, ZHANG, LEUNG and YANG2002) studied the polymorphism in intron 4 of the chicken growth hormone gene and determined the relationship between RFLPs and certain characteristics of Chinese native chickens. In these experiments, they did not observe any difference in the allelic frequencies at Mspl sites among Chinese native chickens- broilers and layers, but two more alleles were identified only in Chinese native chickens and hybrids. Of these alleles, allele D was found in slow growers, e.g. Taihe silkies and Beijing fatty strains, along with other native breeds. Allele E, characterized by deletion of 50 bp, was identified in the Taihe silkies population. Mspl polymorphism has also been reported in intron IV of GH in Indian broilers (Pramod, 2006).

A restriction fragment length polymorphism (RFLP), resulting in the expression of aberrant GHR protein, has been reported in dwarf (dwdw) chicken, which reduces the body weight by 30-40% than normal birds (Burnside et al., Reference BURNSIDE, LIOU and COGHURN1991 and 1992). Studies revealed that, in sex linked dwarf chickens, stunted growth and reduced immune responsiveness could be due to mutations in the growth hormone receptor gene (Agarwal et al., Reference AGARWAL, COGBURN and BURNSIDE1994; Pinard and Monvoisin, Reference PINARD and MONVOISIN1994). Deletion in the intracellular domain of GHR in dwarf broilers was found to decrease muscle cell proliferation in chicken (Goddard et al., Reference GODDARD, JOHNSON, GILHOOLEY, GARDNER, GRAY, WILKIE and BUTTERWITH1996). Feng et al. (Reference FENG, KUHNLEIN, FAIRFULL, AGGREY, YAO and ZADWOMY D.1998) identified Hind III RFLPs in growth hormone receptor gene with a higher frequency of the Hind III + allele in the lean compared to the fat line of meat type chickens. Miao et al. (Reference MIAO, MORRIS, SHARP and DUNN1999) detected and assayed polymorphism in the GHR locus, in a commercial broiler breeder population. Lau and Leung (Reference LAU and LEUNG2002) studied the polymorphisms in chicken GHR gene in three different lines; they detected a new RFLP in exonic region of this gene in Chinese native chickens upon Alul restriction enzyme digestion. Cogburn et al.(2007) inferred that marker-assisted selection, transgenics, and RNA interference are able to modify the phenotype of the chicken to render higher production yields.

Insulin like growth factor system (IGF)

IGFs are critical for the growth and survival of individuals and serve as endocrine, autocrine and paracrine stimulators of mitogenesis and cellular transformation. The IGF system controlling the prenatal muscle development and postnatal muscle growth is composed of IGF-I, IGF-II, the IGF receptors, and the IGF binding proteins (IGFBPs). IGF-I is a single chain protein (7.5kDa), mainly produced by the liver and released in circulation upon interaction of GH with hepatic GH-receptors. A decrease in fractional protein synthesis rate (Ks) in the breast muscle of 1 or 2 weeks old chicks with concomitant decrease in hepatic IGF-I mRNA expression during fasting was reported by Kita et al.(1998). Tomas et al. (Reference TOMAS, PYM, MCMURTRY and FRANCIS1998) demonstrated that continuous infusion of IGF-I at 300 µg/kg body wt per day could improve the weight gain in female broilers and other strains of chickens with weights with bodyweights between 600 and 700g. The relationship of IGF-1 on avian muscle protein synthesis in Leghorn chickens has been studied by Conlon and Kita (Reference CONLON and KITA2002). Amills et al. (Reference AMILLS, JIMENEZ, VILLALBA, TOR, MOLINA, CUBILO, MARCOS, FRANCESCH, SANCHEZ and ESTANY J.2003) reported three Single nucleotide polymorphisms (SNPs) in chicken IGFs and studied their association with growth and feeding traits. In vivo and in vitro investigations have revealed that IGF-I and IGF-II stimulate proliferation and differentiation of myoblasts and satellite cells both in a time and concentration-dependant manner, via interaction with type I IGF receptors (Oksbjerg et al., Reference OKSBJERG, GONDRET and VESTERGAARD2004). The chicken IGF-I gene maps to 165.95 cM on chromosome 1 (Nie et al., Reference NIE, LEI, OUYANG, ZENG, YANG and ZHANG2005).

In an expression analysis of IGF systems in Korean Native Ogol chickens (KNOC), Yun et al. (Reference YUN, SEO, KIM and KO2005) concluded that the endocrine function of IGF-I was found to be more important than auto/paracrine action in juvenile body weight growth, and expression of IGF-II was involved in body growth. Zhou et al. (2005) reported the association of an already reported SNP (A→C) with growth, body composition, skeletal integrity, hormonal and metabolic factors in chicken. This SNP has also been reported in Indian Coloured Synthetic Male line broilers (Nandedkar et al., Reference NANDEDKAR, SAXENA, SAXENA, AHMED, SUNDARESAN, KUMAR, SINGH, JAIN and NEHETE2008).

Transforming growth factor beta (TGF-β)

Transforming growth factors β (TGF-β) are part of a group of multifunctional growth factors regulating a broad spectrum of biological activities related to morphogenesis, development, differentiation and immune function. TGF-β1, inhibits differentiation of myogenic cells (fusion, elevation of creatine- kinase activity, appearance of acetylcholine receptors, transition from beta and gamma to alpha actin, and expression of other muscle specific mRNAs and the corresponding proteins). It also suppresses expression of myogenic regulatory MyoD1 (Vaidya et al., Reference VAIDYA, RHODES, TAPAROWSKY and KONIECZNY1989) and blocks expression of myogenic differentiation genes such as myogenin (Massague at al., 1991). Remignon et al. (Reference REMIGNON, GARDAHAUT, MARCHE and RICHARD1995) observed higher numbers of muscle fibres in chicks selected for fast muscle growth than in unselected breeds. Duclos et al. (Reference DUCLOS, CHEVALIER, REMIGNON, RICHARD, GODDARD and SIMON1996) observed that higher growth rate is based on better post natal growth of muscle fibres due to higher satellite cell proliferation, dependent on their sensitivity to growth factors. The TGF-β3 ‘super family’ encompasses a large group of selected growth and differentiation factors that play important roles in regulating development and tissue homeostasis (McPherron and Lee, Reference MCPHERRON and LEE1996). As per Kagami et al. (Reference KAGAMI, KUHARA, YASUTOMO, LESTER, REUTTER and KURODA1996), TGF-β3 regulates integrin expression and assembly and inhibits muscle differentiation and expression of myogenin, a muscle regulatory transcription factor. Modified integrin expression would disrupt normal cell matrix interactions and cell signal transduction pathways, resulting in abnormal sarcomere structure. Piek et al. (Reference PIEK, HELDIN and TEN DIJKE1999) reported that TGF-β super family members, including activins, inhibins, TGF-β1 and TGF-β2, are multifunctional cell signalling proteins and play pivotal role in tissue homeostasis and development. In avian species, the muscle regulatory factor (MRF) genes control the muscle fibre formation at hatching, and their number is determined by prenatal events, influenced by breed, genotype, maternal nutrition, bioactive agents and abiotic factors (Tepas and Soumillion, Reference TEPAS and SOUMILLION2001). The role of TGF-β2 in embryonic development and organogenesis in chicken has been studied by Saxena et al. (Reference SAXENA, SUNDARESAN, MALIK, AHMED, SAXENA, KUMAR, NANDEDKAR and SINGH2007). As the satellite cell proliferation affects muscle protein turn over, consequently, the eating quality of the meat is also affected. Larger numbers of small muscle fibres impart better meat quality, as compared to that from lesser numbers of larger muscle fibres. This is because smaller muscle fibres are able to cope better with stress factors, leading to reduced stress induced meat deterioration, e.g. water holding capacity. So, selection for higher muscle fibre numbers of moderate size is more advantageous in rendering both high meat content and good meat quality (as determined by water holding capacity).

Extra cellular matrix (ECM) and TGFβs

Fibrillogenesis governing the rate and extent of collagen is affected by the interaction between the core protein of decorin (proteoglycan) and collagen (Scott, Reference SCOTT1988). With the increase in decorin core protein synthesis, the length of the glycosaminoglycan chain increases (Bassols and Massague, Reference BASSOLS and MASSAGUE1988), which may modify binding of decorin to collagen. This alters ECM architecture through changes in organization of collagen fibrils which ultimately affects tissue structure and function.

Increases in TGF-β3 regulates the synthesis of decorin (Bassols and Massague, Reference BASSOLS and MASSAGUE1988), collagen and integrin (Heino et al., Reference HEINO, IGNOTZ, HEMLER, CROUSE and MASSAGUE1989; Heino and Massague, Reference HEINO and MASSAGUE1990). Yamaguchi et al. (1990) reported suppression of TGF-β3 activity on binding with decorin. ECM components (glycoproteins, collagen and proteoglycans) are also important in regulating muscle differentiation (McLennan, Reference MCLENNAN1993). The in ovo detection of TGF- β 1 and the expression of TGF-β2 and β4 (at days 1.5 and day 4 of incubation) evinced the importance of this growth factor in early development of avian species (Jakowlew et al., Reference JAKOWLEW, CIMENT, TUAN, SPORN and ROBERTS1994). Findings of Melo et al. (1996) confirmed the necessity of proteoglycans for skeletal muscle differentiation. Velleman and Coy (Reference VELLEMAN and COY1998) reported the expression of TGF-β1 and TGF-β2 in post-hatch chickens and cultured chicken satellite cells. In avian species low score normal muscle weakness (LSN) has been attributed (Velleman, Reference VELLEMAN1999) to interactions between proteoglycan decorin with TGF-β3 and regulates collagen fibrillogenesis and cellular growth properties.

Myostatin

Myostatin, also known as growth and differentiation factor-8 (GDF-8), is a new member of TGF- β3 super family. It is expressed at high levels in skeletal muscles and at low levels in tissues including brain, spleen and intestine. Myostatin works as a negative regulator of skeletal muscle growth. In chickens, as seen for many other TGF-β3 factors, myostatin appears to be highly conserved. McPherron and Lee, Reference MCPHERRON and LEE1997) showed that mice deprived of myostatin exhibited a two to three fold increase in skeletal muscle mass due to hyperplasia and hypertrophy. According to Thomas et al. (Reference THOMAS, LANGLEY, BERRY, SHARMA, KIRK, BASS and KAMBADUR2000), C2C12 myoblasts incubated with myostatin, showed an increase in p21 expression, and a decrease in cdk2 protein, with an accumulation of hypophosphorylated Rb protein, leading to the arrest of myoblasts in G1 phase of cell cycle, ultimately deregulating the proliferation of myoblasts.

Recombinant myostatin proteins inhibit cell proliferation and DNA synthesis in C2C12 muscle cells, suggesting that myostatin may control muscle mass by inhibiting muscle growth and regeneration (Taylor et al., Reference TAYLOR, BHASIN, ARTAZA, BYHOWER, AZAM, WILLARD, KULL and GONZALEZ-GADAVID2001). Myostatin, also inhibits MyoD activity and expression via Smad3 resulting in failure of the myoblasts to differentiate into myotubes (Langley et al., Reference LANGLEY, THOMAS, BISHOP, SHARMA, GILMOUR and KAMBADUR2002). Joulia et al. (Reference JOULIA, BERNARDI, GARANDEL, RABENOELINA, VEMUS and CABELLO2003) confirmed that myostatin inhibits both myoblast cell proliferation and differentiation by mainly targeting myogenin and p21. Myostatin stimulates satellite cell quiescence and negatively affects the renewal of satellite cells through regulation of p21 (a cdk inhibitor), and decreasing the levels and activity of cdk2 protein in satellite cells. It also negatively regulates the G1 to S progression, and thus maintains the quiescent status of satellite cells (McCroskery et al., Reference MCCROSKERY, THOMAS, MAXWELL, SHARMA and KAMBADUR2003). Post-natally, myostatin acts as a negative regulator of skeletal muscle growth and myostatin inhibitors could be expected to provide a therapeutic benefit in diseases where muscle mass is a limiting factor (Whittemore et al., Reference WHITTEMORE, SONG, K, LI, AGHAJANIAN, DAVIES, GIRGENRATH, HILL, JALENAK, KELLEY, KNIGHT, MAYLOR, R, O'HARA, PEARSON, QUAZI, A, RYERSON, TAN, TOMKINSON, VELDMAN, WIDOM, A, WRIGHT, WUDYKA, ZHAO and WOLFMAN2003). Expression profile of the myostatin gene during embryonic organogenesis has been elucidated in Indian broiler (Sundaresan et al., Reference SUNDARESAN, SAXENA, SINGH, JAIN, P, SINGH, ANISH, NISCHAL, SAXENA and AHMED2008). In vitro silencing of myostatin gene in chicken has also been reported (Sato et. al., 2006 and Nandedkar et al., Reference NANDEDKAR, SAXENA, SAXENA, AHMED, SUNDARESAN, KUMAR, SINGH, JAIN and NEHETE2008).

Myostatin gene polymorphism

Chicken myostatin gene is composed of three exons and two introns (Baron et al., Reference BARON, WENCESLAU, ALVARES, NONES, RUY, SCHMIDR, ZANELLA, COUTINHO and LEDUR2002). High levels of polymorphism in myostatin sequences (exon 1, 2 and intron 1) in two different chicken lines, selected for body weight and egg production, have been reported by Baron et al. (Reference BARON, WENCESLAU, ALVARES, NONES, RUY, SCHMIDR, ZANELLA, COUTINHO and LEDUR2002). In these experiments, exon 2 sequences revealed different alleles in the layer and broiler lines, which could be used for studying economically important traits in poultry.

Mott and Ivarie (Reference MOTT and IVARIE2002) analyzed the sequence and expression patterns of myostatin gene in growth selected lines of chicken and quail and did not find significant polymorphism or differences in RNA expression levels in the embryonic muscles of experimental Japanese quail. Li et al. (Reference LI, DEEB, ZHOU, MITCHELL, ASHWELL and LAMONT2003) observed that TGF β 2 and TGF- β4 polymorphisms were related to skeletal traits, and TGF β3 polymorphism was associated with growth and body composition traits. They concluded that TGF- β2, β4 and β3 may be good candidate genes of QTL for increasing leg bone strength, length and improving breast muscle weight. Pramod (2006) reported Msp I and Pst I polymorphism in the myostatin gene (I and II exons and intron 1) in Indian broilers.

Effect of growth on muscle fibre characteristics

The proliferation of cells in chicken embryos of heavy breeds is faster than in light breeds. Skeletal muscles constitute nearly 50% of the total body weight and edible meat yield in the chicken. While studying musculo-skeletal development in egg production (E line) and meat production lines(M line) in turkey, Swatland (Reference SWATLAND1989) did not find differences between lines for muscle fibre numbers and fibre cross sectional area, irrespective of the fact that breast and keel length and depth were greater in M-line than the E-line. Bastiaens et al. (1991) reported higher muscle collagen content in slow growing broilers than the fast growers. Meat quality is net outcome of muscle morphology, biochemistry and muscle fibre configuration. Geese selected for meat yield have larger fibres and greater diameter than those in layer birds (Klosowka et al., 1993; Remignon et al., Reference REMIGNON, MARCHE and RICHARD1993). According to Dransfield and Sosnicki (Reference DRANSFIELD and SOSNICKI1999) fibre diameters are responsible for determining meat toughness and improved growth rate rendered larger diameter muscle fibres, higher number of glycolytic fibres with lower proteolytic potential (causing more toughness and reduced WHC of poultry meat).

While studying the occurrence of giant fibres in pectoralis major and iliotibialis muscles in three different lines (fast growers (FG), slow growers (SG) and intermediate growers (IG)) of turkeys, Remingnon et al. (2000) concluded that, the occurrence of GF is not influenced by genetic background of the birds or type of muscle fibres. However, more fibre cross-section areas were reported in FG line as compared to SG line. Velleman et al. (Reference VELLEMAN, COY, ANDERSON, PATTERSON and NESTOR2002) studied muscle development at 20^th and 25^th day of incubation and found that on 20^th day of incubation, the large bodied groups had longer fibre bundles than others. Guernec et al. (Reference GUERNEC, BERRI, CHEVALIER, WACRENIER-CERE, LE BIHAN-DUVAL and DUCLOS2003) demonstrated that chickens selected for higher breast yield showed larger fibres in breast as well as in sartorius muscle than their controls and the amplitude of this difference increased with age.

Effect of selection for growth rate on muscle damage during turkey breast muscle development was studied by Velleman et al. (Reference VELLEMAN, ANDERSON, COY and NESTOR2003). According to Le Bihan Duval (2004) in chickens, increased breast mass obtained by genetic selection has mainly been achieved by an increased fibre size, while no changes in the metabolic profile were detected.

Effect of growth on protein accretion and degradation

Accretion of muscle is a dynamic process affected by anabolic and catabolic changes determining the size of the muscle. Maruyama et al. (Reference MARUYAMA, KANEMAKI, POTTS and MAY1993) observed that reduced myofibrillar protein degradation in poultry was associated with increased rates of muscle accretion. Lower protein retention efficiency of genetically fat-line chickens over the lean line chickens was due to (Geraert et al. Reference GERAERT, MACLEOD, LARBIER and LECLERCQ1990) increased dietary amino acid degradation and increased uric acid excretion. Gwartney et al. (Reference GWARTNEY, CALKINS and JONES1991) revealed that, feeding cimaterol to chickens rendered heavier muscle weights with higher muscle protein percentages, lower fat content and reduced meat tenderness, characteristics which were reversed on withdrawal of cimaterol. Tesseraud et al. (Reference TESSERAUD, PERESSON and CHAGNEAU1996) found greater tissue protein deposition accompanied by increased protein synthesis and proteolysis in the pectoralis major, anterior latissimus dorsi muscles as well as in liver, during chick growth.

Effect of growth on meat quality traits

It is well known that poultry industry thrives upon improved juvenile body weights for higher meat yields in a shorter time period. But greater growth rates could be related to certain muscle abnormalities and meat quality defects like pale, soft and exudative meat (PSE), deep pectoral myopathy and inherited muscle dystrophy (Wilson et al., Reference WILSON, NIEBERG, BUHR, KELLY and SHULTZ1990). Wilson (Reference WILSON1990) postulated that selection for rapid growth allows muscles to exceed their support systems, resulting into muscle damage. Occurrence of pale soft exudative meat in mature turkey hens has been studied by Barbut (Reference BARBUT1997). Although the real reasons for PSE syndrome could not be explained (Sosnicki et al., Reference SOSNICKI, GREASER, PIETRAZK, POSPIECH and SANTE1998), this has become a growing problem now in the poultry industry, in which meat is characterized by paleness, low WHC and formation of soft gels (Owens et al., Reference OWENS, HIRSCHLER, MCKEE, MARTINEZ-DAWSON and SAMS2000a).

pH

Changes in pH, WHC and rheological properties are reported to significantly affect the storage and processing quality of the meat (Kijowski and Niewiarowicz, Reference KIJOWSKI and NIEWIAROWICZ1978). Offer (Reference OFFER1991) inferred that reduction in pH by 1 unit increased the rate of denaturation by 12 times. The colour of meat is influenced by the pH of the muscle (Yang and Chen, Reference YANG and CHEN1993). Acidic pH of meat resulting into lower WHC with increased cooking and drip losses has also been reported to reduce the tenderness (Northcutt et al., Reference NORTHCUTT, FOEGEDING and EDENS1994) and result in PSE meat (Barbut, Reference BARBUT1997). Allen et al. (Reference ALLEN, RUSSEL and FLETCHER1997) observed that higher pH of meat causing dry, firm and dark (DFD) appearance, resulted in poor storage quality due to faster microbial growth and an unpleasant odour.

According to Pietrzak et al. (Reference PIETRZAK, GREASER and SOSNICKI1997) irreversible myosin insolubility due to low pH and higher temperature conditions contribute to PSE development in turkey breast muscle. In broiler chicken, Le-Bihan-Duval et al. (1999) reported that selection for live weight and breast muscle yield led to decreased breast muscle drip loss which was partly due to increases in ultimate pH. Rathgeber et al. (Reference RATHGEBER, BOLES and SHAND1999) studied the association of pale turkey meat with lower muscle pH and lower WHC.

In turkey meat, the relationships between pH and meat quality have been studied quite extensively and significant relationships have been reported (Van Laack et al., Reference VAN LAACK, LIU, SMITH and LOVEDAY2000). Berri et al. (Reference BERRI, WACRENIER, MILLET and LE BIHAN-DUVAL2001) reported a significant relationship between higher pH and lower glycolytic potential in breast muscle of broiler chickens selected for breast meat yield compared to the unselected controls. They also observed that rapid pH decline may induce protein denaturation, causing reduction in tenderness, juiciness, and less intense (or pale) muscle discoloration. Rammouz et al. (Reference RAMMOUZ, EL, BABLIE and FERNANDEZ2004) did not find any correlations between pH with Warner-Bratzler shear force value of raw turkey breast meat. Ducloset al. (2007) inferred that poultry meat quality is influenced by the post mortem pH of the meat. They evaluated the issues of low pH poultry meat and tried to establish its linkage with genetic factors affecting growth rate.

Water holding capacity (WHC)

Poultry meat with low pH has been associated with low WHC, which results in cooking and drip losses (Northcutt et al., Reference NORTHCUTT, FOEGEDING and EDENS1994). Addition of polyphosphate to meat has been found to improve WHC by increasing the pH (Yang and Chen, Reference YANG and CHEN1993). Allen et al. (Reference ALLEN, FLETCHER, NORTHCUTT and RUSSELL1998) reported positive correlation between WHC and raw meat pH. LeBihan-Duval et al. (1999) found negative correlations between pH and drip loss. Studies on broilers revealed that the lower pH in pectoralis major was associated with lighter colour (Fletcher, Reference FLETCHER1999) and reduced WHC (Dransfield and Sosnicki, Reference DRANSFIELD and SOSNICKI1999). Wynveen et al. (Reference WYNVEEN, BROWKER, DEMOS and GERARD1999) confirmed that drip loss in low, medium and high pH group were 1.75, 0.81 and 0.75 % of carcass weight. Mallia et al. (Reference MALLIA, BARBUT, VAILLANCOURT, MARTIN and MCEWEN2000) reported that raw, cyanotic, comminuted turkey meat showing dark, firm, and dry appearance having higher pH (6.1 to 6.2), retained more moisture (greater WHC) during cooking (lower cooking loss), and illustrated greater gel strength. Owens et al. (Reference OWENS, MCKEE, MATTHEWS and SAMS2000b) reported that expressible moisture of turkey meat was significantly correlated with pH. Van Laack et al. (Reference VAN LAACK, LIU, SMITH and LOVEDAY2000) suggested that protein denaturation does not contribute to low WHC seen for pale broiler breast meat. Lonergan et al. (Reference LONERGAN, DEEB, FEDLER and LAMONT. S.J.2003) also correlated the meat pH with colour, water holding capacity or texture of the broiler. Further work on identification of candidate genes affecting physical characteristics including meat, flavour and muscle fibre quality, could herald a new era of producing quality chickens (Yang and Jiang, Reference YANG and JIANG2005). Inverse relationship between water holding capacity and cooking yield was confirmed by Pramod (2006).

Extract release volume (ERV)

Fresh meat with a relative low bacterial load releases large amount of extract (High ERV), whereas meat with a higher bacterial number releases less extract (ERV) (Jay, Reference JAY1966. Sinhamahapatra et al. (Reference SINHAMAHAPATRA, BISWAS, DAS and BHATTACHARYA2004) did not find any effect of broiler carcass surface decontamination on the muscle pH, WHC, ERV, appearance, smell, tenderness and overall acceptability.

Texture

Warner-Bratzler (WB) shear force press and Texture Profile Analyzer (TPA) are common methods of measuring texture of meat. Texture of chicken breast meat is significantly influenced by the interaction of soluble proteins with collagen as well as temperature. Dawson et al. (Reference DAWSON, SHELDON and MILES1991) reported low level of collagen in chicken breast muscle. There is a negative correlation between tenderness and muscle fibre size (Crouse et al., Reference CROUSE, KOOHAMARAIE and SEIDEMAN1991), which increases with age. Baeza et al. (Reference BAEZA, SALICHON, MARCHE and JUIN1998) found that older ducks yielded tougher breast meat due to greater muscular fibre size (Swatland, Reference SWATLAND1981). The breasts of 8-week-old ducks had a lower mechanical resistance than older age groups (Baeza et al., Reference BAEZA, SALICHON, MARCHA, WACRENIER, DOMINGUEZ and CULIOLI2000) and these differences were non significant between sexes. Le Bihan Duval (2004) stated that ultimate pH could be a relevant criterion for genetic selection on the poultry meat quality because of its strong relationship with colour, WHC and texture of the meat. High body weight broilers have been reported to render tougher breast meat with higher muscle fibre diameter (Pramod, 2006).

Collagen/collagen solubility

Collagen is a connective tissue constituent that is present in all tissues. Morphologically, there are three discrete collagen depots in muscle: the epimysium, perimysium, and endomysium. The intramuscular connective tissue (IMCT) is formed by combination of peri and endomysium, of which perimysium plays major role in determining meat texture. Higher collagen content in processed meat may produce poor emulsifying capacity, poor water holding capacity and gel pocket formation (Gillet, Reference GILLET1987). Bastiaens et al. (1991) reported higher muscle collagen content in slow growing broilers than in fast growers. Duck meat contained more collagen than broiler meat (Smith et al., Reference SMITH, FLETCHER, BUHR and BEYER1993). The contribution of collagen to meat texture is largely determined by its cross-link profile (McCormick, Reference MCCORMICK1999). Baeza et al. (Reference BAEZA, DESSAY, WACRENIER, MARCHA and LISTRAT2002) found that the concentration of connective tissue in pectoralis muscle of control ducks was lower than those selected for higher body weights. Moreover, reduction in collagen content was reported with the increase in fibre size seen with advancing bird age. Breast muscles from high body weight broilers had higher Hydroxyproline and collagen content (Pramod, 2006).

Muscle proteins and calpains

There are three proteolytic systems in muscles, the cathepsins (lysosomal), the calpains (calcium dependent) and the proteasome (ATP ubiquitin dependent). Sams and Birkhold (Reference SAMS and BIRKHOLD1991) reported that calpain I activity was lower, in slower growing female chickens at 1 day and 3 wk of age and in smaller red fibres compared to white fibres. Sams and Birkhold (Reference SAMS and BIRKHOLD1992) revealed that, in chickens, the activity of calpastatin decreased in muscles seven-fold from day 1 to 7 weeks of age. Pringle et al. (Reference PRINGLE, CALKINS, KOOHMARAIE and JONES1993) suggested that myofibrillar protein degradation, and muscle growth, are partly regulated by highly specialized enzymatic activities of the calpain proeolytic system. Pale meat is the result of denaturation of sarcoplasmic proteins which leads to greater scattering of light (Swatland, Reference SWATLAND1993). According to Dransfield (Reference DRANSFIELD1994), rapid decline in pH with higher temperature inactivated the calpain system in the muscle and reduced postmortem tenderness. Studies on the effect of growth rate on the proteolytic capacity of breast muscle (Schreurs et al., Reference SCHREURS, VAN OAR HEIDE, LEENSTRA and DE WIT1995) showed that slow growing birds have a higher ratio of enzyme to inhibitor. Goldberg et al. (Reference GOLDBERG, AKOPIAN, KISSELEV, LEE and ROHRMILD1997) confirmed the role of calpains in degradation of the cytoskeleton. Van Laack et al. (Reference VAN LAACK, LIU, SMITH and LOVEDAY2000) reported that pale and normal samples contained similar quantities of phosphorylase in the myofibrillar fraction, indicating that protein denaturation in pale and normal samples were similar. Broilers with low body weights had more intensely packed protein bands than of heavy broilers (Pramod, 2006).