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Literature review
Introduction
Growth is a complex biological phenomenon controlled by a complex of endo-, para- and autocrine control mechanisms. This has been studied in depth in most farm animals and it has been shown that the insulin growth factor complex (IGF) plays a critical role in growth regulation, together with insulin, thyroid hormones, steroids and the growth hormone (Rejduch et al., 2010). IGF2-gene plays an important role in muscle growth in pigs and has potential to assist pig breeders by using genotypic information in the selection programs.
Meat quality is a complex concept defined by different facets such as the compositional quality (lean to fat ratio) and the palatability quality such as meat colour, tenderness, juiciness, and flavour. The nutritional quality of meat is also considered by the consumer, as being very important. Meat quality traits are measured post mortem. A variety of over-lapping criteria such as chemical, morphological, nutritional, sensory and culinary measures can be used to measure meat quality. The properties of processed and fresh pork is dependent on factors related to the composition of the product including moisture, lipid and protein content. The characteristics of the protein, water and lipid are responsible for the differences in the colour, texture, water holding capacity and tenderness of pork.
This section will provide an overview of the role of IGF2 and its association with meat and fat quality traits and highlight the general factors influencing pork quality.
IGF2, as a major gene
A gene can be considered a major gene when the difference between the mean value of the individual homozygous for the gene and that of individuals not carrying the gene, is equal to or greater than one phenotypic standard deviation of the trait of interest (Sellier and Monin, 1994). Insulin-like growth factors (IGF) are growth-promoting peptides, which are structurally homologous with insulin. Their biological effects are also similar to that of insulin that is synthesized only in the pancreatic islets of Langerhans, while IGF’s are synthesized in tissues throughout the body (Nedbel et al., 2000). IGF’s are a family of hormones which control hyperplasia and differentiation throughout the body. It consists of two hormone ligands: insulin-like growth factor-1 (IGF1) and insulin-like growth factor-2 (IGF2) (Liu et al., 1993). The two IGF cell surface receptors are IGF-Type 1 receptor (IGF1R) and IGF-Type 2 receptor (IGF2R). The IGF1R has an increased affinity compared to IGF2 and binds IGF1 whereas IGF2 is bound at an approximately 10-fold decreased affinity. IGF2R binds IGF2 with an increased affinity compared to IGF1. Insulin binds IGF1R at a decreased affinity than IGF1 and is unable to bind to IGF2R.
The first QTL studies for IGF2 was performed using a candidate gene approach based on an intercross between wild boar and Large White domestic pigs (Andersson-Eklund et al., 1998). A QTL on the short arm of chromosome 2 with moderate effect on muscle mass was detected using a conventional Mendelian inheritance model. The presence of an imprinting effect and an imprinted QTL (paternally expressed) was also detected on the distal tip of chromosome 2 in pigs (SSC2) that has effects on lean meat content (Jeon et al., 1999). The QTL indicated 15.4% of F2 generation’s phenotypic variance of longissimus muscle area, 14% of heart weight and 10.4% of backfat depth. The results indicated that the paternally expressed QTL locates at the same position as IGF2. This result and the fact that both the gene and the QTL were imprinted, made IGF2 gene a possible candidate based on the QTL effect. The allele in the Large White breed found at the IGF2 gene linked to the QTL was associated with larger muscle mass and reduced BFT, but that this QTL had no notable effect on abdominal fat. The IGF2 linked QTL was also found in experimental crosses of Pietrain and Large White pig breeds where results indicated that the QTL at the end of SSC2 was imprinted and paternally expressed (Nezer et al., 1999). Therefore, IGF2 gene was regarded as a potential candidate for the QTL at the distal end of SSC2. The effects on muscle mass and fat deposition were major and of the same magnitude as that reported for the RYR1 gene. These two loci combined explained 50% of the Pietrain-Large White difference for muscularity and leanness. No evidence for interaction between the QTL at IGF2 gene and RYR1 gene locus was found. Sequence analysis (Nezer et al., 1999) found a single nucleotide mutation, G to A transition in IGF2, which increased lean yield by 2.7% (Meadus, 2000).
The QTL at IGF2 and FAT1 on chromosome 4 were the two QTL with the greatest effect on body composition and fatness, present in the wild boar-Large White cross (Andersson et al., 1994; Szyda et al., 2003). The QTL at IGF2 controlled mainly muscle mass whereas FAT1 had a major effect on fat deposition (Jeon et al., 1999). The two QTL loci explained 33% variance for lean meat content in ham, 31% for percentage of lean meat and 26% for the average depth of backfat. The IGF2 microsatellite was also found to be highly polymorphic, with three alleles among wild boars founders and an additional two alleles among eight Large White founders (Jeon et al., 1999). It is important to have polymorphic markers due to more variation amongst the microsatellite markers.
IGF2 explained 25% of the phenotypic variation of leanness in a study using experimental crosses (Sheller et al., 2002). However, it did not influence daily weight gain and pHᵤ of meat. Previous studies have confirmed that the IGF2 gene is an imprinting gene in pigs and constitutes an important QTL for muscle mass and fat deposition. The test reached the genome-wide threshold (P<0.01) for average BFT and loin-eye area. The favorable alleles showed in the Yorkshire breed, when transmitted through the sire, reduced average backfat by 0.1 cm and increased loin-eye area by 1.0 cm², when compared to alleles in the Berkshire breed (Lee et al., 2001; Aslan et al., 2012).
Regions with significant QTL for muscle fibre traits or significant QTL for meat quality were detected on several chromosomes (SSC1, 2, 3, 4, 5, 13, 14, 15 and 16). Loci controlling lean meat content segregated on SSC6. The results presented in the study indicated that loci affecting meat colour and meat quality traits, such as related to water binding capacity, like pH value and conductivity, segregate in many populations including commercial breeds and are located on the p-arm of SSC3. Previous studies from Karlsson & Lundström (1992) and Rosenvold & Andersen (2003) have shown that stress, exercise or fighting results in higher muscle temperature and lactic acid content and faster pH decline. The proportion of slow and fast twitch fibres has been related to insulin resistance, as well as fat catabolism (Simoneau and Kelly, 1997). Any stress during pre-slaughter causes net glycogen depletion and higher ultimate pH (pHᵤ) (Terlouw et al., 2005). A QTL scan in a porcine experimental population based on Duroc and Berlin Miniature pigs confirmed the presence of QTL’s for microstructural muscle properties as well as biophysical parameters of meat quality and traits related to body composition, i.e. pH and lean meat content (Wimmers et al., 2006). A QTL for meat colour was reported on SSC13 (Wimmers et al., 2006; van Wijk et al., 2007). A summary of QTL’s for carcass traits is shown in Table 1.2.
Association between IGF2 gene on meat and carcass traits
Pig breeds can be classified as genetically lean and genetically obese (Wood, 1984) with the two extreme examples being the Duroc (genetically obese) and Pietrain (genetically lean). Selection for a leaner pig results in decreased ratio between fat deposition and lean deposition. Consequently, fat quality is negatively affected. Since the early 1990’s, there has been an interest in the intramuscular fat (IMF) content of pigs since higher IMF levels are associated with improved eating quality of pork (Bejerholm and Barton- Gade, 1986; Jakobsen, 1992, de Vries et al., 1998; Faucitano et al., 2003;Serão et al., 2011). Intramuscular fat content is also determined as seen in some breeds such as the Duroc with higher IMF compared to other breeds (Barton-Gade, 1987; Visser et al., 2003; Burkett, 2009).
The imprinting inheritance mode of the IGF2 gene was reported by several studies (Buys, 2003; van Laere et al. 2003; Van den Maagdenberg, 2007). Van Laere et al. (2003) reported that a G to A transition in IGF2 gene is the causative quantitative trait nucleotide. This single nucleotide mutation adds approximately 3 to 4% more lean meat to pigs. The link of the mutation with the desired phenotype is 100%, regardless the origin of the pedigree (Buys, 2003). It allows for the selection of carcass leanness based directly on the functional nucleotide at the DNA level. The IGF2 gene has an effect on the production of lean meat. Boars tested for IGF2 can be used to either increase or decrease back fat. Boars with IGF2 +/+ genotypes can be used to increase lean yield, while those with IGF2 -/- genotype can be used to decrease lean yield. Actual breeding trials confirmed the use of IGF2 gene where pigs from the selected boars were leaner with a reduced back fat and more uniform compared to those from unselected boars (Sheller et al., 2002; Clark et al., 2014). The carcass leanness measures also showed differences between the IGF2 genotypes and an association study revealed that the ‘A’ allele increased the weight of loin, weight of ham, carcass meat percentage, and decreased average backfat thickness (Liu, 2003). Highly significant effects of the IGF2 mutation on body composition traits were observed, as well as significant effects on growth performance. The influence of the IGF2 gene mutation on meatiness has further been confirmed by a number of studies in various pig populations (Estélle et al., 2005; Van den Maagdenberg et al., 2008a; Gardan et al., 2008; Burgos et al., 2012; Oczkowicz et al., 2012; Clark et al., 2014).
Meat Quality Traits
A complex of biochemical processes are responsible for the conversion of muscle to meat. Carcass temperature and the rate and extent of pH decline are major determining factors of the muscle to meat biochemical processes. In this study, the carcass and meat quality as well as the fat and fatty acid analysis of the IGF2 genotypes was studied. The meat quality traits as shown in the results section are introduced below.
pH
A contributory factor to pork colour is the extent and rate of pH decline in early post-mortem, an important factor in determining the quality of fresh pork. When glycogen metabolism is rapid, lactic acid, is produced and hence the rapid decline in pH (Lonergan et al., 2007). Good quality pork is associated with a gradual decline in pH, therefore with a rapid decline there is an increased chance that the combination of high temperatures and low pH can create conditions that favour the denaturation of proteins. An increasing paleness in meat is inversely proportional to pH therefore a decrease in pH results is associated with an increase in paleness. If the pH decline happens too rapidly after slaughter, resulting in a very low pH at a high temperature, it will result in very pale meat (Barbut et al., 2008). If the pHᵤ is high (where glycogen depletion occurs pre-slaughter resulting in little or no lactic acid production) the meat will be dark and firm with a dry (DFD) surface (Andersson, 2000). DFD meat allows the growth of spoilage organisms which are inhibited at the usual pHᵤ of meat (Newton & Gill, 1981).
Fibre typing
Colour variations between muscles are due to differences in pigment content and muscle metabolism and therefore is a major determinant of meat colour. Red muscles (oxidative muscles) depend on an oxidative metabolism which requires large amount of myoglobin for oxygen supply and storage. Glycolytic muscles uses glycogen as an energy source, have a pale colour and are hence referred to as ‘white muscles’ (Jeong et al., 2012). Colour differences between muscles with different metabolism, are possible to observe with the naked eye. Differences in muscle fibre type composition are greater between muscles than differences between animals from different genotypes or breed (Lefaucher & Gerrard, 1998). Maltin et al. (1997), examined muscle fibre characteristics of Longissimus dorsi from pigs of eight breeding stock companies and reported that variation in fibre size and type existed among the population of pigs. Those differences did however not contribute to differences in the sensory quality measurements, such as juiciness or pork flavour. In a recent study, Clark et al. (2014) indicated that muscle fibre cross-sectional area and intermediate fibre area (Type 11A) appeared to be reduced in IGF2 Aᵖᵃᵗ pigs as compared with Gᵖᵃᵗ pigs.
Meat colour
Meat colour is of utmost importance as it forms the basis for consumers for product selection (Dransfield, 2008) and is dependent upon myoglobin content and the amount of oxygen available for reacting (Hur et al., 2004). The colour of pork is determined by many factors including genotype, breed, gender, diet, muscle type and extrinsic factors such as pre-slaughter handling and slaughter procedure (Rosenvold & Andersen, 2003), which influences pH decline, furthermore, storage conditions and storage time (Faustmann & Cassens., 1990). Brewer et al. (2001) reported the stabilization of colour parameters were unaffected by bloom time. Variations in meat colour have been observed among pig breeds (Oliver et al., 1994; Blanchard et al., 1999). Recent literature where IGF2 was studied (Van den Maagdenberg et al., 2008a; Burgos et al., 2012; Clark et al., 2014) has shown that the colour of the Longissimus dorsi was affected by the A allele with higher colour values, and may result in paler meat.
Water holding capacity (WHC)
Drip loss is also an important factor for meat quality as it affects consumer perception and nutritive value of finished products (Muchenje & Ndou. 2010). In a study by Joo et al. (2000) where pork quality categories were investigated, highly marbled meat with less drip loss was observed in pork Longissimus dorsi. In the same study, the increased IMF appeared to affect the WHC of pork loin during cold storage. Protein characteristics are responsible for the water holding capacity in pork. Meat proteins have no net charge at pH 5.1. As the pH of meat reaches pH 5.1 (isoelectric point), the WHC drops drastically because of protein denaturation. An increased pH decline at high carcass temperature cause an increased drip loss in fresh pork chops and less WHC (Andersson, 2000). Product weight loss of between 1-3% drip loss represent an economic loss to both processors and retailers and can be as high as 10% in PSE products (Melody et al., 2004). Therefore, understanding the process of drip loss and preventing drip loss is important in the meat industry. Lean meat contains approximately 70% water and most of the water is held within the structure of the muscle and muscle cells. Loss of water may involve different mechanisms and may occur at different times during storage (Huff-Lonergan & Lonergan, 2005).
ACKNOWLEDGEMENTS
ABSTRACT
LIST OF TABLES.
LIST OF FIGURES
LIST OF ABBREVIATIONS
CHAPTER ONE: Introduction and literature review
1.1 Introduction
1.2 Aims and objectives
1.3 Literature review
1.4 Conclusions
CHAPTER TWO: The Frequency of two Major Genes, RYR1 and IGF2, in the South African pig population
2.1 Introduction
2.2 Material and Methods
2.3 Results
2.4 Discussion
2.5 Conclusions
CHAPTER THREE: The effect of the IGF2 gene on meat and fat quality traits in South African Large White and Landrace pig populations
3.1 Introduction
3.2 Material and methods
3.3 Results
3.4 Discussion
3.5 Conclusions
CRITICAL REVIEW AND RECOMMENDATIONS
REFERENCES
ADDENDUM
Addendum A Breed effects on percentage total fatty acids and fatty acid ratios: Belly fat
Addendum B Breed effects on percentage total fatty acids and fatty acid ratios: Backfat
Addendum C Breed effects percentage total fatty acids: Longissimus dorsi muscle
Addendum D Genotype effects percentage total fatty acids and fatty acid ratios: Belly fat
Addendum E Genotype effects percentage total fatty acids and fatty acid ratios: Backfat
Addendum F Genotype effects on percentage total fatty acids and fatty acid ratios: Longissimus dorsi muscle
Addendum G Breed*Genotype effects on percentage total fatty acids and fatty acid ratios: Belly fat
Addendum H Breed*Genotype effects on percentage total fatty acids and fatty acid ratios: Backfat
Addendum I Breed*Genotype effects on percentage total fatty acids and fatty acid ratios: Longissimus dorsi muscle
