Impact of crossbreeding of Thai native chickens on growth
Indigenous chickens are important in developing countries for food security, and the socio-cultural life of the rural community . The Thai native chickens are important genetic resource, as they are adapted to the harsh environmental conditions and have chewy texture and taste that are preferred by consumers in Thailand . Despite these advantages, the growth performance of the native chickens is poor and as a result, crossbreeding with fast growing exotic strains have been encouraged to improve the growth characteristics of the native breeds. In the current study, the KM1 and KM2 crossbred chickens with 50 and 25%, respectively of native Thai chicken background showed significant improvement in growth. The KM1 and KM2 chickens grew about 1.3 and 2.3 folds, respectively when compared with the CH breed.
The improvement in growth of the native Thai breed has come with associated increased in body fatness. Abdominal fat, which is an excessive fat and considered as waste in the slaughtering process. This is a clear positive relationship between growth performance and abdominal fat accumulation. The fat deposition has been correlated with adipocyte enlargement . Improvement in growth and accumulation of fatness associated with crossing breeding of indigenous chickens with exotic breeds have been documented in several other native crossbreeding programs . This has been shown to be due to the pleiotropic effect between body weight and abdominal fat traits . With the faster growth rate of the KM2 chickens because of the higher percentage of exotic genes, it is expected that they will reach slaughter age much faster than the KM1 crossbred and the CH breed.
Contrarily to expectation, the KM1 and KM2 did not show any differences in subcutaneous and skin fat, but they both had values higher than the CH breed. It has been documented that, the weight of the skin is dependent on the amount of subcutaneous fat deposition . However, from the current study, there phenotypic correlation between skin and subcutaneous fats was 0.54. There may be other non-genetic factors contributing towards the relationship between subcutaneous fat and skin fat.
The intramuscular fat (IMF) represents the lipid that is distributed in muscular tissue containing epimysium, perimysium, and endomysium which infiltrates between the muscular fiber bundles. IMF has an influence on meat quality which vary depending on sex, slaughter age and type of muscle . The CH breed has relatively low IMF accumulation in both breast and leg muscle while BR and KM2 have significantly high amount of IMF. The current report suggests that crossbreeding of the Thai native chickens with exotic breeds have the potential to change not only the growth performance but the meat quality as well.
Zhou et al.  demonstrated that the meat from selected for increased fat content have lower in shear force than their control counterparts. Potentially, the meat of the KM1 and KM2 may be more tender than the native CH breed due to their relatively high IMF. However,  did not observe any differences in IMF between native Thai and Barred Plymouth Rock crossbred and the native chickens when breast and thigh muscles were compared. Similarly, crossbreeding Chinese native chickens did not affect breast IMF .
PPARs transcription factors regulating cellular lipid metabolism
In chickens, endogenous lipids are mainly synthesized as lipoprotein in the liver from dietary glucose and then export to extrahepatic tissues by circulation in the blood stream, where the lipoproteins are hydrolyzed by lipoprotein lipase and fatty acids are released for use as energy or accumulation in the cell [13, 16]. At the cellular level, The many types of functional proteins related to lipid metabolism were reviewed in  that included Fatty Acid Binding Proteins (FABPs), insulin-dependent glucose transporter 4 (GLUT4), lipoprotein lipase (LPL) and the fatty acid translocase (CD36), which were stimulated by transcription factor PPAR. PPARs are important cellular regulators which respond to energy status during both fed and fasted states . The PPARs function and mechanism in mammalian species were studied, while many studies focused on the differences in lipid metabolism at the molecular level between avian and other species, and the unique PPARs function in chicken lipid metabolism .
PPARα is one of the transcription factors involved in the regulation of the ketogenesis pathway . Palmitolyation forges the PPARα Mitochondrial 3-hydroxy–3-methylglutaryl-CoA synthase (HMGCS2) interaction which is a nodal point in the ketogenic mechanism, and this complex is transported to the nucleus where it activates PPRE to encode the transcription of HMGCS gene for autoregulation of its own nuclear transcription .
At 6 weeks of age, PPARα mRNA expression in adipose tissue was significantly downregulated in the commercial breed of chicken (BR) compared to CH and KM1. The downregulation may be the effect of cellular energy conservation of Brand KM2 and lead to the remaining fat to enlargement of abdominal fat tissue. From the current study, it appears that mRNA expression of PPARα is not directly dependent on the genetic background due to there was in the stage that unnecessary to use visceral fat as energy sources. In the current study, PPARα transcriptional level in muscle (Figure 3) did not differ among the breeds studied. This may be due to the muscle type as breast muscle require low energy because of lack of movement. We did not observe any significant phenotypic correlation between PPARα mRNA expression level and IMF in both breast and thigh tissues. Thus, the increase in IMF with the muscles of the Thai native crossbreds was not due to changes in PPARα transcription. On the contrary,  reported a positive relationship between PPARα expression and IMF deposition in dwarf chicken which has deletion mutation in 3’UTR of GHR leading to a reduction of body weight and increased IMF accumulation.
However, there appears to be a relationship between PPARγ expression and IMF. The PPARγ mRNA expression of BR and crossbred chicken (KM1 and KM2) were in concordance with the fat deposition traits. PPARγ is one of the most important subtypes, which has activity in oil droplet accumulation within adipose tissue. However there are many related mechanisms such as glucose and fatty acid uptake regulation by LPL, GLUT, and A-FABP [8, 22]. Therefore, these lipid accumulations of BR, KM1 and KM2 could occur putatively via cellular uptake and transport fatty acid for storage as triglyceride. Moreover, the coefficient correlation revealed that PPARγ expression has a moderate positive correlation with abdominal and skin fat. Moreover, Wang et al.  showed that the A-FABP gene is down-regulated when PPARγ is silenced, therefore, PPARγ activity plays a crucial role in cellular lipid accumulation by A-FABP activity especially in lipogenesis and may has potential as target gene for selection against excessive fat deposition in chickens. We have shown that A-FABP is upregulated in concordance with the fatness level of the breed . The phenotypic correlation between PPARγ mRNA expression and IMF for both breast and thigh muscles was positive. This is corroborated by other studies using the Chinese native chickens and female Wuhau chickens .