One hundred and ninety three parental lines obtained from 26 countries for an international rice molecular breeding program were evaluated using 101 well-distributed simple sequence repeat (SSR) markers. An overall genetic diversity of 0.68 and an average of 6.3 alleles per locus were revealed, indicating a high level of genetic variation in these lines. Cluster analysis of the 193 accessions showed three major groups and nine subgroups. Group I corresponded to the classical indica subspecies, whereas groups II and III belong to the japonica subspecies. Indica and japonica differentiation accounted for only 6.5% of the total variation in the entire sample and 93.5% was due to within-subspecies diversity. Differentiation among eco-geographic regions accounted for 24% of the diversity within the subspecies. Larger amounts of the eco-geographical differentiation were resolved within japonica than within indica. The largest indica-japonica differentiation based on the single locus level was detected by markers on chromosomes 9 and 12, while the smallest differentiation was detected by markers on chromosomes 4 and 8. Furthermore, genetic differences at the single-locus and two-locus levels, as well as components due to allelic and gametic differentiation, were revealed between indica and japonica and among the main geographic regions. The multilocus analysis in genetic diversity showed a higher proportion of variation caused by predominant non-random associations of different loci within and among the classified subspecies and geographic subdivisions. The results suggest that selection for eco-geographical adaptation on multilocus associations was largely responsible for the maintenance of the extensive variation in the primary gene pool of rice.

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In this article, we used PCR analysis with arbitrary primers (AP-PCR) to screen for overlapping bacterial artificial chromosome (BAC) clones and assembly of contigs. A rice BAC library with three genome equivalents was used to prepare pooled BAC DNA. Twenty-two arbitrary primers were used to survey the pooled BAC DNAs and individual BAC DNAs. Each primer identified 1-10 loci, and the average was 4.4 loci. There were 1-5 overlapping clones in each locus, and the average was 2.5 clones. A total of 245 BAC clones were identified as overlapping by AP-PCR and the identities were confirmed by DNA-DNA hybridization. The 245 BAC clones were then assembled into 80 contigs and 17 single-clone loci. The results indicated that PCR analysis with arbitrary primers is a powerful tool in screening for overlapping BAC clones with high accuracy and efficiency. The use of AP-PCR analysis should speed up the construction of physical maps of the plant and animal genomes, as well as the rice genome.Intensive efforts are being made to construct high-resolution physical maps of human, animal, and plant genomes through the generation of ordered overlapping DNA fragments. Because of their capacity for housing large fragments of exogenous DNA, yeast artificial chromosomes (YAC) (1) and bacterial artificial chromosomes (BAC) (2) are used to generate contigs of large genomes. Several approaches are used to identify overlapping clones. DNA-DNA hybridization with mapped DNA markers, combined with chromosomal walking, is widely used to identify overlapping YAC clones. A YAC contig map of the human genome has been constructed by using this technique (3). In plants, physical maps of overlapping YAC clones of chromosomes 2 (4) and 4 (5) of Arabidopsis have been constructed by colony hybridization of YAC libraries. With the use of 1,383 DNA markers, the rice physical map of YAC covers about 50% of the rice genome (6). To obtain more complete coverage of the rice genome, a higher-density marker map is being constructed (7). Obviously, the major limitation of this approach to generate a physical map is the requirement of a high-density DNA marker map of the target genomes. Furthermore, repetitive DNA sequences present in eukaryote genomes and chimeric YAC clones create enormous difficulties in chromosome walking (6,8).Another approach to assemble overlapping clones is based on DNA fingerprinting of random clones (9, 10). The restriction fragments of each DNA clone show typical banding patterns when separated in high-resolution gels and͞or probed with DNA probes. Overlapping clones are identified by shared restriction fragments. By use of DNA fingerprinting of cosmid clones, a high-quality physical map of the nematode Caenorhabditis elegans genome was constructed (11). Though the DNA fingerprinting approach is effective in mapping a small genome, its application to a large genome such as the human genome can be difficult (3).Sequence tag site (STS) content mapping is used for the identification of YAC clones (12). Early application of the met...

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Erosion is one of the important reasons for the thickness decrease and perforation of the pipe walls. Understanding the gas–solid two-phase flow pipe erosion mechanism is the basis for monitoring pipe erosion. According to the structural characteristics and working conditions of the gas–solid two-phase flow pipeline in a gas transmission station, a gas–solid two-phase flow pipe erosion finite element model was established and validated by combining it with field test data. Then, the gas–solid two-phase flow pipeline erosion characteristics under different pressures, solid contents, throttle valve openings, and pipe diameters were studied. On this basis, a maximum erosion rate prediction equation was put forward after verification by using actual wall thickness detection data. Results show the following: (1) The absolute error of the maximum erosion rate between the model results and the test datum is ≤10.75%. (2) The outer cambered surface of the bend after the throttle valve is the most seriously eroded areas. (3) The maximum erosion rate increases with pressure, solid content and throttle valve opening increasing, but, along with the change of the pipe diameter, the maximum erosion rate increases at first and then decreases with pipe diameter increasing for throttle valve openings of 20% and 30%, and it decreases with pipe diameter increasing for a throttle valve opening of 50%. (4) A maximum erosion rate prediction equation, involving pressure, solid content, opening of the throttle valve, and pipe diameter, is proposed and is verified that the absolute percentage error between the prediction equation calculation results and the field test datum is ≤11.11%. It would seem that this maximum erosion rate prediction equation effectively improves the accuracy of predicting the gas–solid two-phase flow pipe erosion rate in a gas transmission station.

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