| Heterosis, a widely existing biological phenomenon, has been successfully exploited on breeding of many plants and animals. The efforts for exploring the mechanism of heterosis have lasted for nearly one century. However, many aspects remain unknown. Rice, one of the most important crops and showed greatest success on heterosis application, has deeply studied for the genetic basis of heterosis in recent years. Based on the rice materials with advanced studies on genetic basis of heterosis, this work was carried out for further dissecting the molecular basis of heterosis on gene differential expression level. The main results are as follows:1. By using differential display technique, the differential gene expression was compared between parents and hybrid in elite hybrid rice, Shanyou 63. Three types of differential expression patterns were detected: 1) expressed only in one parent and the hybrid (DMP1 or DMP2), 2) expressed only in two parents (ABF1), and 3) expressed only in one parents or only in the hybrid (UNP1, UNP2 or UNF1).2. In a diallel cross including 8 elite rice parental lines and 28 hybrids, totally 135 differentially displayed cDNA.bands were detected in flag leaf based on 6 primer combinations. The number of each differential banding patterns, which is variable in different crosses, was subjected to correlation analysis with hybrid performance, heterosis and heterozygosity of 6 important traits (heading date, plant height, biomass, seeds per panicle, 1000-grain weight and yield). No significant correlation was found between dominant-like patterns (DMP, and DMP2) and all the trait data. Significant positive correlations were detected between parent-specific expressions (UNP, and UNP2) and heterosis or heterozygosity. For hybrid-specific expressions (UNF,), however, significant negative correlations were found with the relationships to heterosis or heterozygosity for almost all the traits.3. Based on the presence or absence of specific cDNA band in different hybrids, T-test was performed for all the trait data. More than 60 cDNA bands were detected with significance on various trait data. For most significant bands, the heterosis or heterozygosity values of band-absent group were higher than that of band-present group.4. Using large-scale differential display analysis with 65 primer combinations in Shanyou 63 cross, more than 1000 differentially displayed cDNA bands were detected in flag leaves. Among them, 326 bands were recovered and cloned. Totally 384 cDNA clones were subjected for further confirmation.5. Expression analysis by high-density cDNA dot hybridization among the parents and hybrid of Shanyou 63 cross suggested that more than 40 per cent genes were differentially expressed. Compared to parents both increased expression and decreased expressions were found. A considerable number of genes were differentially expressed between flag leaves and seedlings.6. In three crosses of hybrid rice showing significant difference for heterosis, the expression analysis in seedlings also suggested significant difference of expression and some genes were differentially expressed with the same trend as the difference of heterosis.7. Homology search of 34 cDNA sequences showing significant differential expression suggested more than half of the sequences have no match in the database. For the homologous sequences, some are functional genes that are critical for plant development and some are regulatory genes. Out of the 34 cDNAs. 12 have been located on rice chromosomes.8. It is widely accepted that DNA methylation has involved in gene regulations. In this study, a new efficient method (Methylation-sensitive amplification polymorphism. MSAP) was developed and utilized to detect cytosine methylation in rice and this technique was further confirmed by southern analysis.9. Based on the MSAP technique, at least 16.5 per cent of the CCGG sites were cytosine methylated. Tissue-specific methylation was detected at many sites. A lot of sites with hypermethylation...
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