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The Effect of the Rendement Napole Gene on Pork Quality

M. Ellis, F.K. McKeith, K.D. Miller and D.S. Sutton Department of Animal Sciences University of Illinois Urbana, IL

Introduction

Pig meat quality has become an important issue over the past decade, and interest in improving attributes such as color, water-holding capacity, palatability, and consistency has increased dramatically. A variety of approaches have evolved to improve the quality of pig meat. These approaches include manipulating factors such as genetics, management, nutrition, animal handling, slaughter procedures, and postmortem handling of the carcasses. The focus of this discussion will involve a genetic component of meat quality.

Many factors influence the rate and extent of pH decline in postmortem muscle. Historically, the major contributors to reduced water holding capacity and pale muscle color have been the combination of a rapid pH decline and elevated muscle temperature which combine to produce the Pale, Soft, Exudative (PSE) condition. Monin and Sellier (1985) were the first to suggest another cause for reduced water holding capacity and pale muscle color from studies involving the Hampshire breed. They identified a condition in animals that had normal rates of muscle pH decline postmortem; however, the ultimate pH was relatively low and the meat was of low quality. This so-called "acid meat" condition has been attributed to high muscle glycolytic potential (GP), a measure of the capacity for postmortem glycolysis. High GP in Hampshires is associated with abnormally high levels of muscle glycogen which are converted into lactate after slaughter, resulting in a lower ultimate pH. Naveau (1986) suggested that high GP levels were associated with a single dominant gene known as the Rendement Napole (RN) gene, and subsequent studies have suggested the presence of RN gene in pig populations containing Hampshire ancestry in Sweden (Lundstrom et al., 1996), France (Fernandez and Monin, 1994), and the United Stated (Sutton, 1997; Miller, 1998).

Glycolytic Potential Determination

The major source of muscle carbohydrate is glycogen. Glycogen is a polymer of glucose units linked with á1-4 and á1-6 linkages formed around a "foundation" protein, P-glycogen (Brooks et al., 1996). When energy is required, glycogen is hydrolyzed to glucose by the action of the enzyme glycogen phopshorylase, which cleaves á1-4 bonds. De-branching enzyme is required to cleave the á1-6 bonds. Collectively, the hydrolyzation of glycogen is referred to as glycogenolysis. The glucose products that are produced by glycogenolysis are metabolized via glycolysis. As a result of glucose metabolism, a net yield of ATP occurs supplying the body with an energy source. The end product of this metabolism is lactic acid. In living tissue, lactic acid is transported to the liver and reconverted to pyruvic acid which is used as an energy source. However, in postmortem muscle, lactic acid accumulates and accounts for the pH decline.

Monin and Sellier (1985) suggested the use of glycolytic potential as an index of the muscle's capacity for postmortem glycolysis. Glycolytic potential is an estimate of the levels of all compounds in the muscle that are involved in the transformation of glycogen to lactic acid using the following formula:

Glycolytic Potential = 2([glycogen]+[glucose]+[glucose-6-phosphate])+[lactate]

Assays for the combination of glycogen, glucose, and glucose-6-phosphate combined and for lactate are carried out using standard procedures. Glycolytic potential is an expressed in ìmole lactate equivalent per gram of fresh tissue.

Identification and Classification of RN Genotypes

At present, the RN gene cannot be identified directly, because there is no DNA-based test, and, consequently, an animal's genotype is predicted from its GP relative to the frequency distribution of GP values for the population. The frequency distribution for populations within which the dominant allele of the RN gene (RN^-) is segregating is biomodal (Naveau, 1986; LeRoy et al., 1990; Fernandez et al., 1992). A typical distribution for GP found in a US population is illustrated in Figure 1 (Miller, 1998). Animals in the upper part of the bimodal distribution with the higher glycolytic potential are classified as homozygous dominant (RN^-RN^-) and/or heterozygous (RN^-rn⁺). It is not possible to distinguish between the homozygous dominant and the heterozygote, which is one of the weaknesses of this classification procedure. The animals located in the lower part of the bimodal distribution are classified as homozygous normal (rn⁺rn⁺). The threshold values for glycolytic potential used to distinguish between the upper and lower parts of the frequency distribution vary with population studied and sampling method used. A summary of threshold values reported in the literature is presented in Table l.

Figure 1. Distribution of live-animal longissimus glycolytic potential values determined from biopsy

Table 1. Literature summary of glycolytic potential (µmol/g) in populations within which the RN gene is segregating

Authors Time of sampling Percent Hampshire rn⁺ mean^a RN^- mean^a Threshold value^a
Minimum^a
Maximum^a

Postmortem:

Enfalt et al., 1997 Post-rigor NA 154 ± 15 251 ± 24 190 107 296

Sutton, 1997 Post-rigor NA 111 189 150 50 240

Lundstrom & Enfalt, 1997 Pre-rigor 50 142 ± 4 225 ± 4 180 110 265

Lundstrom et al., 1996 Pre-rigor 50 142 ± 3 223 ± 2 175 111 263

Biopsy:

Miller, 1998 Live animal NA 154.3 253.4 NA 154 301

Live animal 100 145.7 238.5 185 102 407

Estrade et al., 1993 Live animal NA 142^b 274^b NA 118 297

Live animal NA 187 ± 15 335 ± 30 NA 161 357

Fernandez et al., 1992 Live animal 33 160 240 230 100 340

Live animal 50 200 280 230 160 380

^aìmol/g. ^bMedian. NA = not available

Sampling Methods

Biopsy techniques

The determination of GP for genotype prediction requires a muscle sample. The muscle sample can be taken either from the live animal or postmortem. Sampling live animals requires biopsy equipment which has been developed to ensure adequate sample retrieval with limited injury and discomfort to the animal. Initial techniques for obtaining skeletal muscle biopsies for meat quality evaluation were reported by Schmidt et al. (1971) and Sybesma et al. (1972). The techniques involved the use of "koffler tongs" to remove samples from the live animal; however, local anaesthesia was required. Schoberlein (1976) and Lahucky et al. (1980) developed an improved biopsy technique that reduced injury and eliminated the need for local anaesthesia. Several studies have utilized live-animal biopsies to evaluate correlations between muscle metabolites and pig meat quality (Hennebach et al., 1980; Henneback et al., 1982; Lahucky et al., 1982; Kovac et al., 1985; Lahucky, 1987). Talment et al. (1989) were the first to use biopsy samples to determine GP of muscle. The most widely used approach to obtain muscle samples for glycolytic potential determination is using spring-loaded biopsy equipment. The assumption is that this approach can provide a sample of resting muscle (Fernandez et al., 1992). However, the time required to obtain the sample and freeze it in liquid nitrogen may result in degradation of muscle glycogen. Fernandez et al. (1992) reported that the lactate content of biopsy samples taken from the live animal using spring-loaded biopsy equipment was 5 ± 3 umol g^-1, which indicates that limited glycogenolysis occurred during sampling and storage.

Postmortem sampling

Several authors have used longissimus samples taken postmortem for glycolytic potential determination (Estrade et al., 1993; Lundstrom et al., 1996; Enfalt et al., 1997; Sutton, 1997). This method avoids the problems and issues associated with live-animal sampling and with potential carcass depreciation. However, sampling postmortem will not produce resting muscle metabolite level. Pre-slaughter handling is critical in determining muscle metabolite levels and will influence the accuracy of the assay results. Several environmental factors are likely to affect muscle glycogen levels at slaughter including handling procedures at the farm and during loading, conditions during transport, and handling procedures in the slaughter plant prior to slaughter. It is important when sampling a population that these environmental factors are consistent across all pigs. All of these factors should be closely monitored and controlled if accurate results are to be obtained; otherwise, phenotype misclassifications are likely to occur, particularly in the case of animals with high GP.

Other methods

Lundstrom and Enfalt (1997) proposed a quicker method to predict the RN genotype based on a simple and rapid method to measure glucose levels. Meat juice from slightly thawed longissimus samples was collected and analyzed using a device for checking blood glucose levels in diabetic patients (Glucometer Elite, Bayer Diagnostics). This blood glucose analyzer determines free glucose concentration within the sample very rapidly, with results being available within 60 seconds. Comparison of RN genotype classification based on either GP or this rapid method showed that only 4 of 53 samples were "misclassified" by the glucose test on meat juice compared to classification based on GP values determined on a live-animal biopsy (Lundstrom and Enfalt, 1997). These results suggest that this could be a very rapid, inexpensive method for phenotyping of animals within a population in which the RN gene is segregating.

The Occurrence and Frequency of the Rendement Napole Gene

Several authors have used the bimodal distribution of GP as evidence of the presence of the dominant allele (RN^-) within populations, and to date this distribution has only been reported in purebred and crossbred Hampshire populations or in composite lines with Hampshire inclusion (Monin and Sellier, 1985; Naveau, 1986; Fernandez et al., 1992; Estrade et al., 1993; Sutton, 1997; Miller, 1998). However, relatively few studies have investigated muscle GP within other breeds. Talment et al. (1989) and Enfalt et al. (1994) sampled Hampshire and Yorkshire populations and showed that GP in Yorkshires was normally distributed. In comparison with Yorkshires, Hampshires have been shown to have increased GP, lower ultimate pH, reduced water-holding capacity and Napole Yield (a laboratory test of curing and processing yields), greater cooking loss, and higher muscle reflectance scores indicating paler meat (Monin and Sellier, 1986; Enfalt et al., 1994).

A summary of the literature relating to GP levels in populations within which the RN gene is segregating is presented in Table 1. Comparisons between Hampshires and Yorkshires (Enfalt et al., 1994), Hampshires, Large Whites and Pietrains (Monin and Sellier, 1985), and Hampshires, Yorkshires and Swedish Landraces (Essen-Gustavsson and Fjelkner-Modig, 1992) all reported higher glycolytic potential in the Hampshire populations with values being approximately 70-80 umol/g greater than in the other breeds. Miller (1998) reported a difference in glycolytic potential of 92.5 umol/g between American Hampshires (homozygous RN^-RN^- and heterozygous RN^-rn⁺) and American Yorkshires. Yorkshires also had a higher muscle ultimate pH (+0.13) and muscle protein content (+1.5%), and lower drip loss (-1.3%) and cooking loss (-2.8%) than the Hampshires.

Limited gene frequency estimates are available; however, the frequency of the dominant allele is high in Hampshire populations studied. Lundstrom, cited by Fernandez and Monin (1994), reported a frequency for the dominant allele (RN^-) of .5 within a Hampshire population in Sweden. Enfalt et al. (1994) estimated the frequency at .72 from a population of Swedish Hampshires, and Miller (1998) reported a frequency of .63 in a population of purebred Hamphires in the US.

The Effects of the Rendement Napole Gene

Fresh meat quality and pork palatability

Poorer meat quality in Hampshires compared to other breeds has been described by a number of authors (Sayre et al., 1963; Monin and Sellier, 1985). These earlier findings have been confirmed more recently by the National Pork Producers Council "Terminal Line Program Results" (1995) which showed that Hampshires had the lowest ultimate pH when compared to a range of other sire lines and breeds commonly used in the US industry. Several authors have subsequently characterized the poorer meat quality effects in Hampshires and attributed them to the RN gene. A summary of published results on the effects of the RN gene on fresh meat quality traits is presented in Table 2. Longissimus ultimate pH has consistently been shown to be lower for homozygous dominant (RN^-RN^-) and heterozygotes (RN^-rn⁺) with differences in ultimate pH ranging from -.12 (Lundstrom et al., 1996) to -.22 (LeRoy et al., 1996). Only one study has compared homozygous dominant (RN^-RN^-) and recessive (rn⁺rn⁺) animals and found a difference in ultimate pH of -.20 (LeRoy et al., 1996). Lundstrom et al. (1996) analyzed the relationship between GP and ultimate pH of the longissimus and reported a phenotypic correlation coefficient of .86. Miller (1998) reported a phenotypic correlation of .50 between GP and longissimus pH in a population of hybrid commercial animals with Hampshire inclusion.

Table 2. Literature summary of breed differences in and the effects of the RN gene on meat quality traits

Authors Comparisons pH^a Drip loss, % Muscle reflectance WHC^b FOP^c Napole yield, %

Miller, 1998 RN^-rn⁺ - rn⁺rn⁺ -.15* 2.05* 2.5*^d - - -

(RN^---) - rn⁺rn⁺ -.1*** 1.55** .3^d - - -

(RN^---) Hamp - York -.13*** 1.29** -2.0*^d - - -

Enfalt et al., 1997 RN^-rn⁺ - rn⁺rn⁺ - .6* -.1^e - - -4.7***

Sutton, 1997 RN^-rn⁺ - rn⁺rn⁺ -.18*** 2.53*** 4.9***^d 4.8*** - -3.6**

Lundstrom et al., 1996 RN^-rn⁺ - rn⁺rn⁺ -.12*** 1.1*** 1.4***^e - 5.6*** -6.5***

LeRoy et al., 1996 RN^-RN^- - rn⁺rn⁺ -.22*** - 3.4***^e -3.0* - -7.9***

RN^-rn⁺ - rn⁺rn⁺ -.20*** - 2.5**^e -2.8* - -8.4***

Enfalt et al., 1994 Hamp - York -.13*** 1.9*** .9^e - 2.1** -5.9***

Monin and Sellier, 1985 Hamp - York -.13* - 3.7*^e - 11.0* -

*, **, ***, P<.05, P<.0l, P<.00l, respectively.

^a Ultimate pH of the longissimus.^b Water-holding capacity, %.^c Fiber optic probe. ^d Hunter L* value.^e EEL reflectance.

Increases in drip loss ranging from .6 (Enfalt et al., 1997) to 2.53 (Sutton, 1997) percentage units have been reported for carriers compared to homozygous recessive animals (Table 2). Reduced water-holding capacity may be attributed to two effects: l) the low ultimate pH, and 2) increased glycogen content and the associated reduction in protein content. When muscle ultimate pH was included as a covariate in the statistical analysis, Lundstrom et al. (1996) observed no differences in drip loss and cooking loss between rn⁺rn⁺ and RN^-rn⁺ animals which suggests that the RN gene effects on drip loss and cooking loss are mediated by ultimate pH. The isoelectric point of muscle, at which stage the net electric charge on the muscle approaches zero and water-holding capacity is reduced, is around pH 5.2. Another factor that may contribute to the decreased water-holding capacity of the meat is the reduced protein content of muscle in pigs with the RN^- allele which has been observed by several researchers (Estrade et al., 1993; Lundstrom et al., 1996; Miller, 1998). Estrade et al. (1993) showed a decrease in protein of 5 to 7% within the white fibers of the muscle in RN^- animals compared to rn⁺rn⁺ animals. Water in muscle is bound to both glycogen and protein (2-4g of water/g of glycogen or protein), and more water is likely to be bound to glycogen in RN^- animals due to their elevated levels. Glycogen is more likely to be hydrolyzed postmortem releasing the water, thus decreasing the water-holding capacity of the meat from RN^- animals. However, Lundstrom et al. (1996) evaluated water-holding capacity of RN^-rn⁺ and rn⁺rn⁺ animals on an equal protein basis and still observed differences in water-holding capacity, suggesting other factors are contributing to the lowered water binding characteristics.

Sayre et al. (1963) and Monin and Sellier (1985) observed higher reflectance indicating paler meat for Hampshires compared to Yorkshires. In addition, a number of studies have shown higher muscle reflectance and fiber optic probe measurements for homozygous dominant (RN^-RN^-) and heterozygous (RN^-rn⁺) animals compared to those homozygous recessive individuals (rn⁺rn⁺; Table 2). Miller (1998) reported higher Hunter L* values, indicating paler muscle color, for pigs with glycolytic potentials of greater than 220 ìmol/g compared to animals with values below 180 ìmol/g.

The Napole Yield technique was developed by Naveau (1986) as a laboratory procedure to estimate the yield of Paris style of processing hams. The technique has been used as a measure of water holding capacity and processing yields. In several studies, Napole Yields were lowered for RN^- carrier animals compared to normal genotypes and also for purebred Hampshires compared to Yorkshires (Table 2). These differences appear to be relatively consistent across studies (Table 2), and Napole Yield has been used to classify animals for RN genotype.

A summary of the literature relating to the effects of the RN gene on eating quality traits is presented in Table 3. Although fresh meat quality is impaired by the RN gene, some studies have observed advantages in eating quality for animals with the RN^- allele. A number of studies

Table 3. Literature summary of breed differences and the effects of the RN gene on eating quality traits

Authors Comparisons Cooking Loss, % Shear Force, kg Tenderness^a Juiciness^a Flavor^a Acidity^a

Miller, 1998 RN^-rn⁺ - rn⁺rn⁺ 1.57 -.17* -1.4*^b .66^b -.11 -

Miller, 1998 (RN^---) - rn⁺rn⁺ 4.30*** -.14 - - - -

Miller, 1998 (RN^---) Hamp - York 2.8*** -.17 - - - -

Sutton, 1997 RN^-rn⁺ - rn⁺rn⁺ 3.53*** -.27** - - - -

Lundstrom et al., 1996 RN^-rn⁺ - rn⁺rn⁺ 3.0* -.2* -.1^a - - .4**

LeRoy et al., 1996 RN^-rn⁺ - rn⁺rn⁺ .10*** - -1.27**^a -.68^a .87* -

RN^-rn⁺ - rn⁺rn⁺ .09*** - -.45**^a -.25^a 1.42* -

Enfalt et al., 1994 Hamp - York 2.8*** -.3** - - - -

Monin and Sellier, 1985 Hamp - York 2.1* - - - - -

*, **, ***, P<.05, P<.01, P<.001, respectively.

^a0 or tender, juicy, no off-flavor, non acidic - 10 or tough, dry, off flavor, acidic.

^b0 tough, dry, off flavor, acidic - 15 tender, juicy no off flavor.

have observed reductions in Warner-Brazler shear force for RN^- heterozygotes compared to homozygous recessive animals, indicating more tender meat for this genotype (Table 3; Lundstrom et al., 1996; Sutton, 1997; Miller, 1998). In addition, improvements in taste panel tenderness scores (Table 3) for animals carrying at least one copy of the dominant allele compared to homozygous recessive individuals were observed in the studies of Miller (1998) and LeRoy et al. (1996) but not in the study of Lundstrom et al. (1996). These later authors hypothesized that any tenderness advantage may be related to the larger amount of sarcoplasm around the myofibril in RN^- animals due to the higher glycogen content, which may dilute any toughness effect caused by myofibrillar protein. Enfalt et al. (1994) showed shear force advantages for purebred Hampshires compared to Yorkshires. Lundstrom et al. (1996) showed a stronger taste and an increase in acid flavor, for animals which carry RN^- allele. An increase in acid flavor for heterozygous (RN^-rn⁺) compared to homozygous recessive animals was also reported by Sutton (1997), suggesting a potential negative effect on palatability for this gene.

Interestingly, another effect of the gene is to increase cooking loss, which could negatively impact eating quality (Table 3). Several studies have reported higher losses during cooking from RN^- animals compared to homozygous recessive pigs in the range 0.09 to 3.5 percentage units. As discussed above, this may be the result of a combination of reduced water binding characteristics resulting from the low ultimate pH and the breakdown of excess glycogen present in muscle during the cooking process, resulting in moisture release, and an increase in cooking loss for those animals which have high residual muscle glycogen.

Growth and carcass characteristics

There have been a limited number of studies that have examined the effects of the RN gene on growth and carcass characteristics (Table 4). LeRoy et al. (1996) compared the three possible genotypes, namely heterozygotes, and the two homozygotes and found that homozygous dominant (RN^-RN^-) and heterozygous pigs grew faster than homozygous recessive animals.

Table 4. Literature summary of breed differences in and the effects of the RN gene on growth and carcass traits

Authors Comparisons ADG, g Dressing % Carcass length, cm Backfat depth, mm Loin eye area, cm² Carcass Lean, %

Miller, 1998 RN^-rn⁺ - rn⁺rn⁺ 36.0 .2 -1.0 -1.9 1.6 -

Enfalt et al., 1997 RN^-rn⁺ - rn⁺rn⁺ 26.0** -.2 .1 -1.2 - 1.0

LeRoy et al., 1996 RN^-rn⁺ - rn⁺rn⁺ 50.0* - .16 -1.3* 3.3 1.0

RN^-rn⁺ - rn⁺rn⁺ 10.0* - .20 -2.1* 2.9 .9

Enfalt et al. (1997) also reported an advantage in average daily gain of 26 g/day for heterozygotes compared to homozygous recessive animals. These results suggest a small advantage in average daily gain (ranging from 10 to 50 g per day) for animals with theRN^-allele. Miller (1998) reported numerical advantages in growth (+36 g/day) and backfat depth (-1.9 mm) for animals with high glycolytic potential; however, these differences were not statistically significant.

Along with possible growth advantages, there is evidence of decreased backfat depth for RN^-homozygotes and heterozygotes compared to rn⁺ homozygotes; however, the differences are relatively small (Table 4). LeRoy et al. (1996) reported decreases in backfat depth of 1.3 and 2.1 mm for heterozygotes (RN^-rn⁺) and homozygous dominant (RN^-RN^-) animals, respectively, compared to homozygous recessive pigs (rn⁺rn⁺). Enfalt et al. (1997) also reported a trend for decreased backfat (-1.2 mm) and increased carcass lean percentage (+1.0%) for RN^- homozgyotes and heterozygotes compared to rn⁺ homozygotes. Miller (1998) reported a small advantage in gain:feed ratio for pigs with high compared to low glycolytic potential.

Summary

The RN gene can be detected within a population based on the glycolytic potential of the pigs measured either on biopsy samples taken from the live animal or on postmortem muscle samples. If the RN gene is segregating within the population, a bimodal frequency distribution for glycolytic potential is observed. The breakpoint between the two distributions is used as the threshold value to determine the predicted genotype of the individual pigs. A number of studies have investigated the effects of the gene on fresh meat and eating quality. Decreased ultimate pH and water-holding capacity and paler muscle color have been reported by several authors; some studies have indicated tenderness advantages for animals carrying the dominant allele. Data on the effects of the gene on on-farm performance and carcass characteristics are limited; however, a small advantage for RN^- animals in terms of average daily gain, decreased backfat depths, and increased carcass lean content has been observed.

Literature Cited

Brooks, A.B., Fahey, T.D., and White, T.P. 1996. Exercise Physiology: Human Bioenergetics and Its Applications. Mayfield Publishing Co., Mountain View, CA.

Enfalt, A.C., Lundstrom, K., Lundkvist, L., Karisson, A., and Hansson, I. 1994. Technological meat quality and the frequency of the RN-gene in purebred Swedish Hampshire and Yorkshire pigs. 40th IcoMST, The Hague, Netherlands.

Enfalt, A.C., Lundstrom, K., Hansson, I., Johansen, S., and Nystrom, P. 1997. Comparison of non-carrier and heterozygous and heterozygous carriers of the RN^- allele for carcass composition, muscle distribution and technological meat quality in Hampshire-sired pigs. Livestock Prod. Sci. 47:221.

Essen-Gustavsson, B., Karlstrom, K., and Lundstrom, K. 1992. Muscle fiber characteristics and metabolic response at slaughter in pigs of different halothane genotypes and their relation to meat quality. Meat Sci. 3l:l.

Estrade, M., Vignon, X., and Monin, G. 1993. Effect of the RN^-gene on ultrastructure and protein fractions in pig muscle. Meat Sci. 35:313.

Fernandez, X., Tornberg, E., Naveau, J., Talmant, A., and Monin, G. 1992. Bimodal distribution of the muscle glycolytic potential in French and Swedish populations of Hampshire crossbred pigs. J. Sci. Food Agric. 59:307.

Fernandez, X., and Monin, G. 1994. A major gene affecting pork quality: the RN gene. Meat Focus, 332.

Hennebach, H., Albrecht, V., and von Lengerken, G. 1980. Investigation for prediction of meat quality by use of shot biopsy in live pigs. Archiv. Tierzucht, Berlin. 23:183.

Hennebach, H., and von Lengerken, G. 1982. Investigation for prediction of meat quality by use of shot biopsy in live pig. Archiv Tierzucht, Berlin. 25:131.

Kovac, L., Sidor, V., Mlynek, J., Vavrisinova, K., and Lahucky, R. 1985. Evaluation of pork meat quality by use of biopsy method in relation to other premortal diagnostic methods and possibility of its practical use. 4th Int. Symposium. Rational Prod. and meat quality under the Large Scale Prod.Conditions, Nitra, Czechoslovakia, September 24-26th.

Lahucky, R., Rajtar, V., Kovac, L., and Sidor, V. 1980. Investigation of biochemical values in muscle tissue by biopsy assays in pigs. Scientific Symposium on Early Performance Information in Pigs, Leipzig, GDR, November 5, p. 79.

Lahucky, R. 1987. Recent findings using the muscle shot biopsy to evaluate meat quality in pigs. Pig News and Information 8:291.

LeRoy, P., Naveau, J., Elsen, J.M., and Sellier, P. 1990. Evidence for a new major gene influencing meat quality in pigs. Genet. Res. Camb. 55:33.

LeRoy, P., Monin, G., Elsen, J.M., Cartiez, J.C., Talmant, A., Lebret, B., LeFaucheur, L., Mourot, J, Juin, H., and Sellier, P. 1996. Effect of the RN genotype on growth and carcass traits in pigs. 47th EAAP, Lillehammer, Norway.

Lundstrom, K., Anderson, A., and Hansson, A. 1996. Effect of the RN gene on technological and sensory meat quality in crossbred pigs with Hampshire as terminal sire. Meat Sci. 42(2): 145.

Lundstrom, K., and Enfalt, A.C. 1997. Rapid prediction of RN phenotypic in pigs by means of meat juice. Meat Sci. 45:127.

Miller, K.D. 1998. The detection and characterization of pigs with differing glycolytic potential levels within United States Swine populations. Ph.D. Thesis, University of Illinois, Urbana-Champaign, Illinois.

Monin, G., and Sellier, P. 1985. Pork of low technological quality with a normal rate of muscle pH fall in the immediate postmortem period: The case of the Hampshire breed. Meat Sci. 13:49.

Naveau, J. 1986. Contribution à l'étude du determinisme genetique de la qualité de la viande porcine. Heritabilité du rendement technologique Napole. J. Rech. Porcine en France 18:265.

NPPC. 1995. Genetic Evaluation: Terminal Line Program Results. National Pork Production Council, Des Moines, IA.

Sayre, R.N., Briskey, E.J., and Hoekstra, W.G. 1963. Comparison of muscle characteristics and postmortem glycolysis in three breeds of swine. J. Anim. Sci. 22:1012.

Schmidt, G.R., Zuidam, L., and Sybesma, W. 1971. Biopsy technique and analysis for predicting pork quality. J. Anim. Sci. 34:25.

Shoberlein, L. 1976. The shot biopsy - a new method of sampling the muscle tissue. J. Anim. Sci. 32:457.

Sutton, D.S. 1997. The meat quality and processing characteristics of RN carrier and non-carrier pigs. Ph.D. Thesis, University of Illinois, Urbana, IL.

Sybesma, W., Minkema, D., van der Wal, P.G., and Walstra, P. 1972. Muscle biopsy analysis and meat quality. 23rd Ann. Mtg. European Assoc. Anim. Prod., Verona, Italy, October 5-9.

Talment, A., Fernandez, X., Sellier, P., and Monin, G. 1989. Glycolytic potential in longissimus dorsi muscle of Large White pigs as measured after in vivo sampling. Proc. 35th ICoMST, Copenhagen, Denmark, pp 1129.

Table 1. Literature summary of glycolytic potential (µmol/g) in populations within which the RN gene is segregating
Authors	Time of sampling	Percent Hampshire	rn⁺ mean^a	RN^- mean^a	Threshold value^a	Minimum^a	Maximum^a
Postmortem:
Enfalt et al., 1997	Post-rigor	NA	154 ± 15	251 ± 24	190	107	296
Sutton, 1997	Post-rigor	NA	111	189	150	50	240
Lundstrom & Enfalt, 1997	Pre-rigor	50	142 ± 4	225 ± 4	180	110	265
Lundstrom et al., 1996	Pre-rigor	50	142 ± 3	223 ± 2	175	111	263
Biopsy:
Miller, 1998	Live animal	NA	154.3	253.4	NA	154	301
	Live animal	100	145.7	238.5	185	102	407
Estrade et al., 1993	Live animal	NA	142^b	274^b	NA	118	297
	Live animal	NA	187 ± 15	335 ± 30	NA	161	357
Fernandez et al., 1992	Live animal	33	160	240	230	100	340
	Live animal	50	200	280	230	160	380
^aìmol/g. ^bMedian. NA = not available

Table 2. Literature summary of breed differences in and the effects of the RN gene on meat quality traits
Authors	Comparisons	pH^a	Drip loss, %	Muscle reflectance	WHC^b	FOP^c	Napole yield, %
Miller, 1998	RN^-rn⁺ - rn⁺rn⁺	-.15*	2.05*	2.5*^d	-	-	-
	(RN^---) - rn⁺rn⁺	-.1***	1.55**	.3^d	-	-	-
	(RN^---) Hamp - York	-.13***	1.29**	-2.0*^d	-	-	-
Enfalt et al., 1997	RN^-rn⁺ - rn⁺rn⁺	-	.6*	-.1^e	-	-	-4.7***
Sutton, 1997	RN^-rn⁺ - rn⁺rn⁺	-.18***	2.53***	4.9***^d	4.8***	-	-3.6**
Lundstrom et al., 1996	RN^-rn⁺ - rn⁺rn⁺	-.12***	1.1***	1.4***^e	-	5.6***	-6.5***
LeRoy et al., 1996	RN^-RN^- - rn⁺rn⁺	-.22***	-	3.4***^e	-3.0*	-	-7.9***
	RN^-rn⁺ - rn⁺rn⁺	-.20***	-	2.5**^e	-2.8*	-	-8.4***
Enfalt et al., 1994	Hamp - York	-.13***	1.9***	.9^e	-	2.1**	-5.9***
Monin and Sellier, 1985	Hamp - York	-.13*	-	3.7*^e	-	11.0*	-
, , **, P<.05, P<.0l, P<.00l, respectively.
^a Ultimate pH of the longissimus.^b Water-holding capacity, %.^c Fiber optic probe. ^d Hunter L* value.^e EEL reflectance.

Table 3. Literature summary of breed differences and the effects of the RN gene on eating quality traits
Authors	Comparisons	Cooking Loss, %	Shear Force, kg	Tenderness^a	Juiciness^a	Flavor^a	Acidity^a
Miller, 1998	RN^-rn⁺ - rn⁺rn⁺	1.57	-.17*	-1.4*^b	.66^b	-.11	-
Miller, 1998	(RN^---) - rn⁺rn⁺	4.30***	-.14	-	-	-	-
Miller, 1998	(RN^---) Hamp - York	2.8***	-.17	-	-	-	-
Sutton, 1997	RN^-rn⁺ - rn⁺rn⁺	3.53***	-.27**	-	-	-	-
Lundstrom et al., 1996	RN^-rn⁺ - rn⁺rn⁺	3.0*	-.2*	-.1^a	-	-	.4**
LeRoy et al., 1996	RN^-rn⁺ - rn⁺rn⁺	.10***	-	-1.27**^a	-.68^a	.87*	-
	RN^-rn⁺ - rn⁺rn⁺	.09***	-	-.45**^a	-.25^a	1.42*	-
Enfalt et al., 1994	Hamp - York	2.8***	-.3**	-	-	-	-
Monin and Sellier, 1985	Hamp - York	2.1*	-	-	-	-	-
, , **, P<.05, P<.01, P<.001, respectively.
^a0 or tender, juicy, no off-flavor, non acidic - 10 or tough, dry, off flavor, acidic.
^b0 tough, dry, off flavor, acidic - 15 tender, juicy no off flavor.

Table 4. Literature summary of breed differences in and the effects of the RN gene on growth and carcass traits
Authors	Comparisons	ADG, g	Dressing %	Carcass length, cm	Backfat depth, mm	Loin eye area, cm²	Carcass Lean, %
Miller, 1998	RN^-rn⁺ - rn⁺rn⁺	36.0	.2	-1.0	-1.9	1.6	-
Enfalt et al., 1997	RN^-rn⁺ - rn⁺rn⁺	26.0**	-.2	.1	-1.2	-	1.0
LeRoy et al., 1996	RN^-rn⁺ - rn⁺rn⁺	50.0*	-	.16	-1.3*	3.3	1.0
	RN^-rn⁺ - rn⁺rn⁺	10.0*	-	.20	-2.1*	2.9	.9