Agris category code: L10
MOLECULAR GENETICS OF COAT COLOUR IN PIGS
Luca FONTANESI Vincenzo RUSSO 1
abstract
Coat colour in Sus scrofa has been the matter of pioneering genetics studies carried out at the beginning of the last century. Since then, classical genetics studies have assumed that several loci affect this trait in pigs. With the advent of molecular genetics it was possible to identify genes and mutations affecting coat colours and patterns in pigs. Variability in several genes have been shown to affect pigmentation in this species. However, only two of them might play major roles in determining coat colour variation in Mediterranean pig breeds or populations: melanocortin 1 receptor (MC1R, Extension locus) and v-kit Hardy-Zuckerman 4 feline sarcoma viral oncogene homolog (KIT, Dominant White locus). Other genes (ASIP, TYRP1, EDNRB, KITLG and OCA2) might affect coat colour in few breeds/populations or could modify the effect of the two major genes. Polymorphisms in the MC1R and KIT genes can be also used to authenticate mono-breed products obtained from local pig breeds.
Key words: Coat colour / MC1R / KIT/ Polymorphisms / Pig breeds
Invited lecture COBISS: 1.06
1 introduction
One of the first phenotypic traits that has been modified during the domestication process in livestock and that differentiate wild ancestors between the domesticated animals is the coat colour. It is assumed that the occurrence of coat colours different from those of the wild animals is the first sign of domestication in an animal species. Subsequent selection processes have largely determined the current within-species variability for this trait. In pigs, domestication and selection have produced a large variety of coat colours and patterns that are characteristics of different breeds and populations (Porter, 1993; Legault, 1998). Coat colour in Sus scrofa has been the matter of pioneering genetics studies carried out at the beginning of the last century, just after the re-discovery of the Mendel's laws (Spillman, 1906, 1907). Since then, classical genetics studies have assumed that several loci affect this trait in pigs and comparative analyses among species attempted to establish homolo-
gies between these loci across mammals (Searle, 1968). With the advent of molecular genetics it was possible to identify genes and mutations affecting most of the coat colours and patterns in pigs. In this review we will briefly summarize what is currently known in this species, focusing also on practical applications that might be useful to characterize and authenticate pork productions in the Mediterranean area.
2 molecular and morphological
BASIS OF cOAT cOLOUR IN MAMMALS
To understand which genes and how genes are involved in determining coat colour differences, it is important to give an overview of the biochemical and morphological mechanisms determining pigmentation. Pigmentation in all mammals (including the pig) is due to the presence or absence of melanins in the hair and in the skin. Melanins are pigments that derive from the en-
1 Dipartimento di Scienze e Tecnologie Agro-alimentari, Università di Bologna, Bologna, Italy. Correspondence to: luca.fontanesi@unibo.it, vincenzo.russo@unibo.it
zymatic oxidation of the tyrosine amino acid. Two types of melanins are synthesized: eumelanins (black/brown pigments) and pheomelanins (yellow/red pigments). Pigmentation is essentially determined by the distribution of these two pigments producing a black/brown and a yellow/red colour, respectively. Metabolic pathways leading to the synthesis of these two types of melanins are mostly known. The key enzyme of this process is tyrosinase, that catalyses the two first metabolic steps starting from the hydroxylation of tyrosine to dihydroxyphenylalanine (DOPA) and followed by the oxidation of this metabolite into DOPAquinone. Moreover, eumelanins derive from DOPAchrome metabolites, while pheomelanins are produced by 5-S-cysteinylDOPA metabolites (Prota, 1992). Melanins are synthesized and accumulated within melanosomes, which are organelles of the cytoplasm of specialized cells, the melanocytes, lying between derm and epiderm. Melanosomes are transferred into the hair during their growing up, by means of an exocytosis process. During embryo development, melanocytes, starting from the neural crest, migrate in the different parts of the body, conferring pigmentation on the areas where they
are present. In the areas where melanocytes are missing, white spots appear giving the characteristic spotting or white patterns of some breeds. Moreover, in some parts of the body pigmentation can be modified depending on a more or less reduced melanocytes activity.
3 MAIN LOcI AFFEcTING cOAT cOLOUR VARIABILITY IN PIGS
From this short presentation it is possible to predict that a large number of genes may affect coat colour in mammals. For example, in mice more than 300 loci have been shown to affect pigmentation. Variability in several genes have been shown to affect pigmentation also in pigs (Table 1). However, only two of them might play major roles in determining coat colour variation in Mediterranean pig breeds or populations: melanocortin 1 receptor (MC1R, Extension locus) and v-kit Hardy-Zuck-erman 4 feline sarcoma viral oncogene homolog (KIT, Dominant White locus). Other genes (ASIP, TYRP1, ED-NRB, KITLG and OCA2) might affect coat colour in few
Table 1: Genes affecting or associated with different coat colours in pigs
Gene name
Gene symbol
Locus
No. of Molecular SSC1 alleles2 events3
Breeds/ populations4
References
Melanocortin 1 receptor
MC1R
Extension 6 >10
Missense mutations; 2-bp insertion
Many
Kijas et al. (1998; 2001); Fang et al. (2009)
v-kit Hardy-	KIT	Dominant	8	Many	Copy	Many	Johansson Moller et al. (1996);
Zuckerman 4 feline		White			number		Marklund et al. (1998);
sarcoma viral					variation;		Giuffra et al. (1999);
oncogene homolog					splice		Pielberg et al. (2001);
					mutation;		Fontanesi et al. (2010);
					unknown		Rubin et al. (2012)
Agouti signaling	ASIP	Agouti	17	2 (?)	Unknown	Mangalitza	Drögemüller et al. (2006)
protein							
Tyrosinase related	TYRP1	Brown	1	2 (?)	6-bp dele-	Tibetan, Kele,	Ren et al. (2011)
protein 1					tion	Dahe	
Endothelin receptor	EDNRB	Spotting	11	4 (?)	Missense	Jinhua, Glouces-	Okumura et al. (2006; 2010);
beta					mutations;	tershire Old Spot,	Ai et al. (2013);
					unknown	Xiang; Chinese	Wilkinson et al. (2013)
						belted pigs	
KIT ligand	KITLG	-	5	2 (?)	Missense	Berkshire,	Okumura et al. (2008; 2010);
					mutations;	Jiangquhai	Wilkinson et al. (2013)
					unknown		
Oculocutaneous	OCA2 Pink eyed 15 2 (?) Missense Duroc	Fernández et al. (2006)
albinism II	dilution	mutations;
unknown
1 Sus scrofa chromosome. 2 No. of alleles with effect or putative effect on coat colour (including wild type). Question marks indicate that other alleles might be present whose effect should be clarified. 3 Molecular events that create different alleles with effect or putative effect on coat colour. 4 Breeds or populations in which mutated alleles have been described.
breeds/populations or could modify the effect of the two major genes (Table 1).
3.1 MC1R VARIABILITY
The Extension locus was initially characterized at molecular level in mice. This locus encodes for the melanocortin 1 receptor (MC1R), also known as melanocyte stimulating hormone receptor (Robbins et al., 1993), which is a transmembrane proteine of the G-protein-coupled receptors family. Mutations in this gene affect pigmentation in a large number of vertebrates, including the pig (Kijas et al. 1998; 2001).
The MC1R gene is probably the better characterized coat colour gene in Sus scrofa. It is constituted by a single coding exon of about 950 bp that has been sequenced in a large number of pig breeds with different coat colours (Kijas et al. 1998; 2001; Fang et al., 2009). Five allelic groups have been reported so far: wild type alleles (E+); dominant black alleles (ED1 and ED2); the black spotted alleles (Ep); the recessive red e allele. The wild type E+ alleles have been identified in wild boar populations. The only European breed that carries the E+ allele is Mangalitza. Sequence data showed that the Dominant black Extension allele (ED) identified by classical genetics studies is constituted by two different MC1R gene sequences identified as ED1 and ED2 (actually three different sequences that differ from few synonymous mutations are considered for the ED1 allele; Fang et al., 2009). The former sequence may be of Asian origin whereas the latter sequence may be of European origin. These two groups of ED sequences are distinguished from all other alleles by a few missense mutations that might activate the MC1R function, resulting in eumelanin production. The black spotted alleles EP were probably originated from the ED2 allele and produce black spotted phenotypes on a white or red background. These alleles are characterized by a 2-bp insertion in the coding sequence that somatically reverts into a normal and functional gene that produces an ED2 classical phe-notype in coloured spotted regions (Kijas et al., 2001). The recessive red allele (e), derived by two other missense mutations, has been observed in the Duroc. These mutations or just one of them might compromise substantially the function of the MC1R transmembrane protein, leading to the production of pheomelanic pigments and red coat colour in animals homozygous for the e allele. The MC1R gene has been characterized by sequencing or genotyping pigs from a few local Mediterranean breeds, like the Italian Cinta Senese and Nero Siciliano breeds (Russo et al., 2004; Fontanesi et al., 2010), Alentejano and Bisaro from Portugal and Negro Canario from Spain (Ramos et al., 2003; Fang et al., 2009). Many other auto-
chthonous breeds remain to be characterized at this locus even if it could be possible to infer their genotype at the Extension locus based on previous results obtained for other European breeds.
3.2. KIT VARIABILITY
The KIT gene encodes the mast/stem cell growth factor receptor. This is a large protein with an extracellular domain consisting of 5 Ig-like domains, a transmembrane region, and a tyrosine kinase domain (Ray et al. 2008). KIT plays key roles in melanogenesis, erythro-poiesis, spermatogenesis and T cell differentiation (Bes-mer et al. 1993; Yoshida et al. 2001). Its functional role in melanogenesis derives from its involvement in driving the melanocyte migration from the neural crest along the dorsolateral pathway to colonize the final destination in the skin (Besmer et al. 1993).
Extended variability at the KIT gene is responsible for the allelic series of the Dominant White (I) locus (Table 2; Johansson Moller et al., 1996; Marklund et al., 1998); Giuffra et al., 1999; Pielberg et al., 2001; Fontanesi et al., 2010; Rubin et al., 2012). The recessive wild type allele "i" is constituted by a normal single copy KIT gene that is associated with a solid or wild type coat colour. The Dominant White coat colour of several important commercial breeds, like Large White and Landrace, is determined by the duplication of the KIT gene (copy number variation) and by the presence of a splice mutation in intron 17 in one of the duplicated copies, that causes the skipping of exon 17 (allele I1). The duplicated region is of about 450-kb. To complicate the allelic series at this locus, the number of KIT gene copies could be more than two but in all cases at least one copy should carry the splice mutation on intron 17. These Dominant white alleles are indicated with I2, I3, etc., with different numbers of detected copies. In the I2 and I3 alleles, the splice mutation is in one or two copies of the three-copies gene forms, respectively (Pielberg et al. 2002). There are other evidences that support the presence of alleles with a larger number of copies and combinations with the splice mutation (Johansson et al. 2005; our unpublished data). The presence of another Dominant white allele, IL, has been hypothesised. This allele should carry a single copy of a mutated KIT gene (with splice mutation) that should be lethal if homozygous (Pielberg et al. 2002; Johansson et al. 2005). Two normal KIT copies are present in the Ip allele that causes the presence of pigmented regions (patches) in white pigs. The IBe allele, a single copy KIT gene with a regulatory mutation, determines the belted phenotype of the Hampshire and Cinta Sense pigs (Giuffra et al., 1999; Fontanesi et al., 2010). Other two dupli-
Table 2: KIT alleles and their effects
Allele symbol	Allele name	No. of KIT copies	No. of splice mutations	Intron 18 deletion	Effect	Breeds/ Populations
i	Wild type	1	0	No	Wild type (solid coat colour)	Many- mainly local breeds
I1	Dominant white	2	1	Yes	White	Large White, Landrace, Belgian Landrace
I2	Dominant white	3	1	Yes	White	Large White, Landrace, Belgian Landrace
I3	Dominant white	3	2	Yes	White	Large White, Landrace, Belgian Landrace
IP	Patch	2	0	Yes	Black spots on white background	Pietrain (Large White, Landrace, Belgian Lan-drace)
IL	Lethal	1	1	?	?	?
IBe	Belt	1	0	No	white belt at level of shoulders and forelegs	Hampshire, Cinta Senese
IRn/Id	Roan/Dilute	1	0	Yes	Grey - white and coloured hairs intermingled	Local populations (Large White, Landrace, Belgian Landrace)
cated regions (4.3-kb duplication located about 100-kb upstream of the KIT gene; and 23-kb duplication located about 100-kb downstream the gene, which in turn contains another duplication of 4.3-kb) have been identified in Hampshire and might be involved in determining the belted phenotype in this breed (Rubin et al., 2012). Another allele with a single copy of the KIT gene without the splice mutation and, possibly, determining a spotted phenotype, has been hypothesized to segregate in white pig populations (Johansson et al. 2005). This allele has been named IBe*, suggesting that might be similar to the IBe allele or it is the same allele but in a different genetic background. Earlier classical genetics studies on coat color segregation between and within populations suggested the presence of an additional allele (Id also indicated as IRn), giving a gray-roan phenotype and dominant over the i allele (Hetzer 1948; Lauvergne and Canope 1979). Markers in the KIT gene (including an indel in intron 18: intron18-g.29_32delAGTT) were used to demonstrate that the Id allele is determined by a single KIT copy gene with another putative regulatory mutation in a grey-roan Sicilian pig population (Fontanesi et al., 2010).
3.3 OTHER GENES
Polymorphisms in other genes have been associated with coat colour in different pig populations. However, their effects seems restricted to few breeds. In addition, in most cases the causative mutations have not been identi-
fied or demonstrated yet. In particular, Drögemüller et al. (2006) have demonstrated that the a* allele at the Agouti locus is associated with the markers in the agouti signalling protein (ASIP) gene in the Mangalitza breed. However, the causative mutation, supposed to be a regulatory mutation, has not been identified yet. A 6-bp deletion in the tyrosinase related protein 1 (TYRP1) gene is responsible for the Brown coat colour locus in Chinese pig populations (Ren et al., 2011).Variability in the endothelin receptor beta (EDNRB) gene has been shown to be associated with spotted or belted phenotypes in a European pig breed (Gloucestershire Old Spot) and in several Chinese breeds (Bamaxiang, Dongshan, Ganxi, Jinhua, Rongchang, Shaziling and Tongcheng) (Okumura et al., 2006, 2010; Ai et al., 2013; Wilkinson et al., 2013). Polymorphisms in the KIT ligand (KITLG) gene might be involved in determining the typical coat colour pattern of the Berkshire breed similar to what is also present in the Jiangquhai breed (Okumura et al., 2008, 2010; Wilkinson et al., 2013). Fernandez et al. (2006) investigated the oculocutaneous albinism II (OCA2) gene in Iberian Duroc pigs and identified polymorphisms that could explain variability in colour intensity in animals of this breed. This gene is the product of the Pink-eyed dilution coat colour locus. A quite large number of other coat colour genes selected according to their role in affecting pigmentation in mice, have been studied in pigs (Okumura et al., 2010). However, variability in these genes might not be involved in determining variability of coat colour phenotypes in different pig breeds. Additional studies
might be needed to evaluate their role as modifier or the major coat colour loci.
4	USE OF POLYMORPHISMS IN cOAT
colour genes for pork authentication
Due to the recent increase of the market of local and typical products, a large number of local breeds have been rediscovered by farmers who have established consortia with the aim to protect, valorise and characterise the products obtained from local populations. As the meat obtained from these pigs are usually sold at a higher price than that of commercial pigs, there is the need to guarantee pig farmers as well the consumers about the authenticity of the meat through the production chain by means of traceability and authenticity systems (D'Alessandro et al., 2007). To verify the information reported on the product labels, DNA systems based on the analysis of coat colour genes have been applied. The systems use polymorphisms in the MC1R and KIT genes as several mutations are fixed in breeds/populations with different coat colours. However, coat colour cannot be observed on meat products and these markers can indirectly identify the colour of the animals, and in turn identify or exclude the breed of origin of the pork products. Usually, local pig breeds are coloured and should be homozygous for the i allele at the Dominat white locus. The duplication breakpoint test (Giuffra et al., 2002) can identify if meat comes from pigs that carry alleles with duplicated KIT genes. This test can be easily applied to exclude that the meat has been produced with white pigs instead of coloured (local) pigs. The use of polymorphisms in the MC1R gene can establish if meat comes from Duroc or Wild Boars to complement results obtained with analyses of the KIT gene.
5	conclusions
Variability in several genes contributes to explain difference of coat colours and patterns between and within pig breeds and populations. Two major genes (MC1R and KIT) with many alleles are the most important determinant of this phenotypic trait in pigs. Characterization of variability in coat colour genes may contribute to evaluate biodiversity in local pig populations. Analysis of polymorphisms in these two genes can be also used for breed authentication of pork products. Several other genes have been shown to affect coat colour in a few breeds. However, other studies should be carried to understand their role in affecting coat colour in this species.
6 acknowledgments
This work has been carried out in the framework of the INNOVAGEN Project.
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