| Heterosis, a widely existing biological phenomenon, has been thought as one of important ways to improve animal and plant productivity. However, because of the complexity of heterosis mechanism and the restriction of studying means, great advance has not been achieved on prediction of heterosis, which limits its further utilization. In recently years, researchers in the world have designed many studies on the basis and prediction of heterosis with molecular biological techniques.In this study, the status of DNA methylation and its effects on hybrid performances were studied, with DNA from Meishan pigs, Large White pigs and F1 individuals of their reciprocal crosses. Then, RAPD analysis on genetic diversity of six pigs was carried out. The main results are as follows:1. Based on the fact that DNA methylation is related with gene expression regulating, methylation-sensitive amplified polymorphism (MSAP) and methylation-sensitive AP-PCR techniques (MS-AP-PCR) were firstly established in the laboratory, in order to explore the relationship between DNA methylation and hyrid performances or heterosis.2. A total of 1508 CCGG sites were amplified by using MSAP technique. In Meishan pigs, 10.3% cytosine was methylated, including 81% hemimethylated and 19% fully methylated. In Largewhite, 10.5% cytosine was methylated, with 79.2% hemimethylated and 20.8% fully methylated. 10.2% cytosine was methylated, with 19.3% fully methylated and 80.7% hemimethylated in F1 of Large white Meishan, 10.2% cytosine was methylated with 17.5% fully methylated and 82.5% hemimethylated in F1 of Meishan Large White. As a whole, cytosine methylation was not significantly different among parental lines and hybrids of reciprocal crosses.3. However, the methylated status in single site was different between parental lines and their hybrids. These differentially methylated sites could be concluded into four major patterns: (1) the same level of methylation in both parental lines and the hybrids, which accounting for averagely 20.4% of all methylated sites; (2) the same level of methylation in either parent and the hybrids, occupying 26.5% of methylated sites; (3) an increased level of methylation in the hybrids compared to the parents, accounting for 33.4% of methylated sites; and (4) a decreased level of methylation in the hybrids, 19.7% of methylated sites belonging to this class. In addition, differentially methylated sites between hybrids of reciprocal crosses were also found in this study, which were called as cross-specific methylation4. The effects of methylaton belonging to pattern (1), (2) and (3) described above on hybrid performances were further analyzed. Consequently, these methylation patterns were significantly correlated with hybrid performances.5. The effects of polymorphic methylated sites on hybrid performances were analyzed. The results showed that direction of the effects on traits were different. Demethylation of some sites contributed possitively to traits expression, while other sites contributed negatively to hybrid performances. So did hypermethylation of sites. Furthmore, methylation change of the same site had different effects on different traits.6. The effects of general methylated sites, neutrol methylated sites and special methylated sites on hybrid performances were also analyzed, As a result, general individual percentage difference and neutrol individual methylation percentage difference had significantly effects on a few traits or no trait, while special individual methylation percentage difference had significant effects on many traits. Moreover, the traits, affected significantly by general individual methylation difference, were also affected significantly by special individual methylation difference, but the contrary was not the case.7. Carcass traits changing could be concluded into three groups, which are: (1) increased, (2) decreased, and (3) fluctuated as special individual methylation percentage difference increased.8. Twenty-two fragments differentially methyla...
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