| The genetic improvement and variety evolution of cotton varieties in China since 1949 were comprehensively studied at three different aspects: phenotypic values, molecular markers and coefficient of parentage. Phenotypic values, which were collected from 4 sets of two-year & multi-site field trials with representative varieties and 2 sets of regional trials, indicated the actual effect of variety improvement and genetic contribution; 42 RAPD molecular markers from 172 entries gave the genetic bases of cotton varieties in China; and coefficients of parentage between 1636 entries showed the utilization situation of variety resources. The main results were:1. In Huanghe Cotton Region (HCR), Changjiang Cotton Region (CCR) and Xinjiang Cotton Region (XCR), the lint yield per unit area had increased at the rate of 7.69 kg/hm2/year, 5.73 kg/hm2/year and 6.16 kg/hm2/year respectively. The yield increase in HCR, CCR and XCR of the recently-bred varieties over the earlier bred were 28.6%, 24.3%and 39.4% respectively; the boll number per plant increasing 8.9%, 19.1% and 27.0% respectively; the lint percentage increasing 7.4%, 7.5% and 12.6% respectively; and the boll weight had a significant variation among varieties. The increase of boll number per plant contributed the most to the lint yield increase, followed by the boll weight and lint percentage.2. In the yield improvement, the genotype effect contributed to the yield increase in HCR, CCR and XCR with the rate of 18.7%, 21.8% and 38.8%, respectively; and the broad genetic effect contributed 44.2%, 45.1 % and 82.0% respectively.3. In terms of the fiber quality improvement of cotton in the nation, the fiber length and strength had increased at the rate of 0.0192mm/year (ICC Standard) and 0.0263cN/tex/year (ICC Standard) respectively; and the Micronaire values among varieties also had significant variation.4. In the population of upland cotton in China, genotype effect contributed to the improvement of fiber length, fiber strength and the Micronaire value with 20.2%, 34.7% and 34.2% respectively.5. The disease resistances of varieties in regional trials to Fususium wilt and Verticillum wilt had been increased gradually in China. The problem regarding resistance to Fususium wilt had been basically resolved; while genetic studies on Verticillum wilt-resistant gene and its QTL positioning work had established a firm foundation for completely settling the problem of resistance of cotton varieties to Verticillum wilt.6. The correlation coefficients between Nei's genetic distances based on RAPDs and Euclidean distances of two groups of phenotypic values were 0.6445(n=1770) and 0.7078(n=7140), which indicated that RAPD could be used to explore genetic diversity among cotton cultivars in China. The results of genetic diversity showed that within cotton cultivars planted in China, in terms of genetic basis, island cotton cultivars were narrower than the upland ones; cultivars released in China since 1949 narrower than the introduced ones from outside of China; hybrids narrower than conventional cultivars; upland cultivars after 1980 narrower thanthose in 1970's; cultivars in CCR narrower than in HCR; and Northwest Cotton Region (NCR) was the narrowest.7. The coefficient of parentage between varieties was analyzed, and results showed that it varied with different periods: specifically, 0.1933 in 1960s, the highest 0.2141 among upland cotton varieties in 1970s, 0.1725 in 1980s, and as low as 0.0848 in 1990s and even decreasing significantly afterwards. The coefficient of parentage also varied with different regions. The average value for upland cotton varieties in CCR exhibited the highest, amounting to 0.4407; with 0.1789 in HCR, the second highest; and 0.1060 in NCR, the lowest.8. Among several important introduced progenitor varieties, Deltapine 15 contributed the most to cotton varieties in China with 30.62%, the Stoneville seconds to it with 15.54%, and followed downwards by King with 8.97%, Uganda with 4.90%, Foster 6 with...
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