**4. The association study with economic traits in the crossbred of Jinhua and Pietrain**

A crossbred was established by mating the Chinese Jinhua pigs (Central China swine type) with European Pietrain pigs, and obvious segregation of growth, carcass and meat quality characters were observed in this population. Association study of the five HPT key genes' polymorphisms with the economic important traits in the crossbred of Jinhua and Pietrain were carried out by analyzing fourteen polymorphic loci with thirty traits of 463 individuals (Jiang 2011). Both PCR-RFLP and tetra-primer ARMS PCR procedure were utilized for genotyping, and haplotypes were considered as genetic effects.

Molecular Characterization of Hypothalamo-Pituitary-Thyroid Genes in Pig (Sus Scrofa) 553

Chinese breeds, while the Western pig breeds those normally grow faster than the Chinese breeds were almost fixed with the deletion allele. Another LD block consisted of the other *TRH* polymorphisms investigated. No significant associations of this LD block with the

All the four *TRHR* polymorphisms studied existed in one LD block. Significant associations of porcine *TRHR* gene with head weight, carcass length, and electric conductivity, pH value, water holding capacity, intramuscular water and fat content of loin muscle were observed. QTLs of carcass length (Wimmers et al., 2002; Murani et al., 2006) and intramuscular fat content (de Koning et al., 1999) have been reported previously. Individuals with the haplotypes originating from the Pietrain had smaller head weight, longer carcass length and lower conductivity of loin muscle. However, no significant difference among the haplotypes

As the existence of microsatellites in the porcine *CGA* gene, QTL mapping results around the *CGA* gene from different studies were instable. In the crossbred of Jinhua and Pietrain, haplotypes of *CGA* was only associated with intramuscular water content. But if further dissecting by dividing the gene with the microsatellites, the promoter region of this gene

Haplotypes of porcine *TSHB* gene were significantly associated daily weight gain between 90 to 120 days, ham muscle weight, and extremely significantly (P<0.01) associated with carcass weight, carcass length and average backfat thickness. Corresponding QTLs have also been reported in this chromosome region (Grindflek et al., 2001; Knott et al., 2002; Malek et al., 2001; Nagamine et al., 2003; van Wijk et al., 2007; Walling et al., 2000). The homozygous individuals with haplotype originating from Jinhua pig had significantly lower daily weight gain and thicker average BFT than the homozygous individuals with the haplotype originating from Pietrain, which were in accordance with the observations in the two purebreds. Further verification in a Duroc population confirmed the association of *TSHB* gene with the average BFT trait and showed that the allele A of ss181129015 which was popular in Western breeds was associated with lower average BFT. Porcine *TSHB* gene, especially the locus ss181129015 is worthwhile for further study and to be developed as

Individuals with allele A of *TSHR* ss325994932 showed higher daily gain between 120 to 180 days old and lower muscular pH value than the ones with allele G in the crossbred of Jinhua and Pietrain. Several QTLs on ADG around the *TSHR* gene have been reported previously (Edwards et al., 2008; Nagamine et al., 2003). Though individuals with allele A showed better performance in growth and meat quality in the crossbred, investigation on the allele frequencies in different purebreds showed that the allele A appeared in fairly low

showed significant associations with growth and backfat thickness traits.

economical traits were observed.

was observed after bonferroni corrections.

**4.3 Thyrotropin alpha subunit gene (***CGA***)** 

**4.4 Thyrotropin beta subunit gene (***TSHB***)** 

genetic marker for the BFT trait.

**4.5 Thyrotropin receptor gene (***TSHR***)** 

**4.2 Thyrotropin releasing hormone receptor gene (***TRHR***)** 

Fig. 5. Genetic Polymorphisms of porcine HPT axis

#### **4.1 Thyrotropin releasing hormone gene (***TRH***)**

Polymophisms of porcine *TRH* gene were divided into two linkage disequilibrium (LD) blocks. One of the LD block contained locus ss325994933 which was significantly (P <0.05) associated with daily weight gain. Homozygous individuals with the insertion allele showed higher daily weight gain than the ones with the deletion allele. TRH could be considered functional and positional candidate gene of the ADG QTL reported by de Koning et al. (de Koning et al., 2001). However, further investigation on the allele frequencies in different purebreds showed that the insertion allele mainly existed in the Chinese breeds, while the Western pig breeds those normally grow faster than the Chinese breeds were almost fixed with the deletion allele. Another LD block consisted of the other *TRH* polymorphisms investigated. No significant associations of this LD block with the economical traits were observed.
