Genetics of Boar Taint: Implications
for the Future Use of Intact Males
Dept. of Animal and Poultry Science
University of Guelph
Guelph, Ontario, CANADA
Problems with castration
Male pigs used for pork production are castrated very early in life to prevent boar taint or sex taint in the meat. However, castration also removes the source of natural anabolic androgens that stimulate lean growth. As a result, uncastrated (intact) males have improved feed efficiency and greater lean yield of the carcass compared to barrows. The use of intact male pigs has been estimated to improve the profitability of pork production by as much as 30% (deLange and Squires, 1995). Animal welfare concerns about castration of animals is also becoming increasingly important in some countries. The prevention of boar taint without castration is therefore very desirable. This paper outlines work on the genetic control of boar taint.
What causes boar taint?
Boar taint is an off-odor and off-flavor due primarily to high levels of androstenone and/or skatole in pig carcasses. Androstenone is a steroid produced by the testis near sexual maturity. It is concentrated in the salivary gland where it is converted to an active sex pheromone to regulate reproductive function in female pigs. Androstenone also accumulates in the fat, since it is very non polar, and produces taint when the fat is heated. Skatole is produced by bacteria in the hindgut of the pig and then absorbed into the blood stream. Most of the skatole is metabolised in the liver and excreted, but the rest is absorbed into the fat and causes taint. Levels of skatole are higher in uncastrated males, probably due to decreased metabolism and clearance by the liver. Nutrition has very little effect on androstenone production, unless the level of nutrition affects the sexual maturity of the pig. We have found that treatment with somatotropin reduces androstenone levels in fat, and this may be due to increased turnover of fatty tissue with this hormone treatment (Bonneau et al., 1992). Unlike androstenone, levels of skatole can be greatly affected by nutrition and environmental factors. Since skatole is produced by bacteria in the gut, manipulation of the gut microflora can affect skatole production (Jensen and Jensen, 1995). Increasing the gut pH by feeding bicarbonate decreases the synthesis of skatole and increases the synthesis of indole, which does not have as bad an odor as skatole. Feeding sources of fermentable carbohydrate (inulin, sugar beet pulp, and coconut cakes) promotes growth of the microbial biomass and reduces the conversion of tryptophan to skatole. A number of antibiotics can also be used to lower skatole levels by reducing the growth of Lactobacillus bacteria that produce skatole. Management factors can affect skatole levels (Kjeldsen, 1993). Keeping the pigs clean and removing manure by using slatted floors reduces the absorption of skatole from the environment. Restricting feed for 48 hours and withholding feed for 12 hours before slaughter reduces the substrate for skatole production. Increased consumption of water also reduces skatole levels in the pig. Since the clearance of skatole in fat is quite rapid (half-life of 10 hours), any treatments to reduce skatole need only be done in the last week before slaughter.
Genetic factors affecting taint
Genetics dramatically affects the levels of androstenone in the carcass with heritability estimates ranging from 0.25 to 0.87 (Willeke, 1993). Although production of androstenone usually increases near sexual maturity, some pigs do not have a high potential for androstenone production. On the other hand, some pigs are late maturing and would not be mature enough to produce high levels of androstenone when they reach market weight. Levels of androstenone vary among breeds of pigs, with Durocs having dramatically higher levels than Yorkshire, Landrace or Hampshire (Xue et al., 1996; Squires and Lou, 1995) and Large White breeds having higher levels than Landrace breeds (Willeke, 1993). Genetic factors also affect the accumulation of skatole in the carcass. A preliminary study suggested that a recessive gene Skal is linked to high skatole levels, particularly when the production of skatole in the gut is high (Lundström et al., 1994). This is probably due to effects on the metabolism of skatole in the liver, since skatole metabolism is low in some intact male pigs (Friis, 1995; Squires and Lundström, 1997; Babol et al., 1998a, 1998b).
Experimental approaches to developing genetic markers
The development of genetic markers for pigs that are low in boar taint would allow the selection of lines of pigs that are free of taint. Genetic markers can be either anonymous genetic markers that are linked to the trait or involve the genes that affect the trait (candidate genes). Anonymous genetic markers are located close to the genes that control a trait and are thus linked to these genes. Examples of these anonymous DNA markers are microsatellites, which are highly variable tandemly repeated sequences of DNA. In order to find an association between these markers and a particular trait, a population of individuals with a wide variation in the trait is needed. This resource population can be derived from breeding two widely different breeds, such as crossing the European Wild Pig or Meishan pig with a normal commercial pig breed.
Major genes, which have a known function and actually code for proteins that affect the trait, are known as candidate genes. Candidate genes are chosen based on knowledge of the biochemistry and physiology of the trait and can be key regulatory proteins in the biochemical pathways involved in the trait. Candidate genes can also be identified from physical mapping studies. The chromosomal location of a particular trait locus can be determined by linkage analysis between the trait of interest and other traits that have already been mapped. Otherwise, anonymous DNA markers linked to a trait can be physically mapped to a particular chromosomal region. Known genes in the same chromosomal region can then be investigated as candidate genes for the trait. The rapid development of genomic maps for a number of agricultural animals, including cattle, pigs and chickens and extensive human and mouse genomic maps also makes cross-referencing possible. Confirming the validity of a candidate gene will only be possible for major genes with large effects that can be easily measured.
Anonymous markers or candidate gene markers can be used in marker assisted selection (MAS) breeding programs. In this procedure, breeding animals are selected based on the presence of genetic markers for the trait of interest, in addition to performance records. The use of anonymous markers for MAS requires that the marker is always associated with the trait,over several generations and within a variety of populations. Candidate gene markers should not have these limitations and can be used immediately in commercial populations. The limitations of using candidate genes are that many traits are affected by a large number of genes, each with small effects. For these traits, it may not be possible to identify candidate genes with major effects.
Genetic markers for boar taint
A significant effect of different haplotypes of Swine Lymphocyte Antigen (SLA) and the microsatellite marker S0102 on androstenone level in fat has been proposed (Bidanel et al., 1997). This suggests that a quantitative trait loci (QTL) for androstenone is located on chromosome 7 between these two markers. Other work, based on a selection experiment for low androstenone, proposed a major two-allele gene affecting androstenone levels in fat (Fouilloux et al., 1997). Their model suggests that the ‘low androstenone’ allele is completely dominant over the ‘high androstenone’ allele. There was no evidence for a linkage of this gene with the SLA system. Thus it appears there are at least two major genetic effects on androstenone accumulation in fat. No anonymous markers for skatole in fat have yet been reported.
Our work in developing markers for low boar taint pigs has focused on the identification of candidate genes. We have characterized the biochemical pathways involved in androstenone biosynthesis and purified the components of the enzyme system which converts pregnenolone into the first 16-androstene steroid and studied its component parts, cytochrome P450c17 and cytochrome b5 (Meadus et al., 1993). Cytochrome P450c17 also converts pregnenolone into precursors of the androgens and estrogens, so complete inhibition of this enzyme system would also inhibit androgen synthesis. We have recently identified two variants of cytochrome b5 in testis by Western analysis, a low molecular weight form and a high molecular weight form (Davis and Squires, 1999). Levels of the low molecular weight form (but not the high molecular weight form) of cytochrome b5 were correlated with both the rate of 16-androstene steroid synthesis and fat androstenone concentrations. Levels of mRNA for total cytochrome b5 were also correlated with levels of androstenone in fat. These results indicate that increased levels of the low molecular weight isoform of cytochrome b5 are linked to a higher level of androstenone production in pig testis. This suggests that levels of this protein in the testis could be used as a marker to reduce boar taint. Our work is continuing to develop genetic markers for low androstenone based on cytochrome b5.
Recent research has shown that some pigs are unable to metabolise and excrete skatole efficiently, so they will have high taint when the production of skatole by the gut bacteria is high. We have extensively studied the metabolism of skatole in the liver of pigs and identified the major metabolites that are formed by Phase I metabolism (Diaz et al., 1999). We have shown that the enzymes CYP2E1 (Squires and Lundström, 1997), CYP2A6 (Diaz and Squires, in preparation) and aldehyde oxidase (Diaz and Squires, submitted for publication) are involved in this process. We have also found that the Phase II metabolism of skatole metabolites by sulfotransferase is related to the clearance of skatole in the liver (Babol et al., 1998a, 1998b). Pigs with high levels of these enzymes have low levels of skatole in the fat, since skatole is rapidly metabolised and cleared from the body. Pigs with low levels of these enzymes can have high levels of skatole in the fat if the amount of skatole absorbed from the gut or the environment is high. We are continuing this work to develop genetic markers for pigs that will efficiently metabolise and clear skatole, so they will not be tainted from skatole.
Other Issues affecting use of intact males
Interfering with testicular function can also reduce boar taint. Testicular function and boar taint can be decreased in mature male pigs by treatment with GnRH agonists (Xue et al., 1994). Schneider et al. (1998) treated pigs with GnRH agonist from day 135 through to day 164 and Reid et al. (1996) administered a 4 month formulation beginning at day 66 to reduce gonadal steroid secretions to castrate levels. However, while Ziecik et al (1989) found that administration of a GnRH antagonist decreased testis weight in neonatal pigs, there was no treatment effect by six months of age. This suggests that GnRH agonists can be used to decrease boar taint only when the treatment is given close to the onset of puberty. Testicular function can also be reduced by immunocastration, which involves immunizing pigs against GnRH (Bonneau et al., 1994; Hennessey et al., 1997; Oonk et al., 1998). It is likely that a vaccine for immunocastration will be commercially available in the future.
The use of intact males for pork production also requires that quality control procedures be in place to insure that tainted meat does not reach the consumer. Reliable and rapid methods to detect boar taint are needed. The available quantitative assays for androstenone in fat include chromatography, immunoassay and colorimetry. All these methods include extraction, purification and concentration steps and are too slow for on-line use in a packing plant. Recently, various types of electronic sensors have been investigated for detection of boar taint (Bourrounet et al., 1995; Annor-Frempong et al., 1998). It may be possible to incorporate appropriate sensors into an electronic nose for on-line detection of taint.
Significance to the Industry
We have identified the major genes that are responsible for high levels of boar taint from both skatole and androstenone and we now need to use this information to develop genetic markers so that we can eliminate boar taint by genetic selection. These simple DNA based tests to determine the genetic potential of a pig for taint would be run with a blood sample in a similar manner to the DNA test for Porcine Stress Syndrome. Since the genetic variability for boar taint already exists in the pig population, we will be able to select for taint free pigs once our DNA markers have been developed. This will dramatically improve the profitability and address animal welfare concerns in the pork industry.
The Animal Research program of OMAFRA, the Research Grants program of NSERC and Ontario Pork have provided financial assistance for this research.
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