Construction Of A Genetic Linkage Map And QTL Analysis Of Lint Percentage In Upland Cotton (Gossypium Hirsutum L.)

Cotton is the leading natural fiber crop and fiber is the main resource of the textile manufacture. The world cotton production was supplied by two tetraploid species, G. hirsutum L. and G. barbadense L., and G. hirsutum L. account for 90% of the total production. However, as the basic raw materials of textile industry, upland cottons generally have lower quality fibers than sea-island cotton, i.e., short or coarse fibers of relatively low strength. Cotton has been playing an important role in the economy of China, one of the largest cotton producing and textile manufacturing countries in the world, with lint yield of 600 million kilograms per year, and the textile manufacturing needs raw cotton 650 million kilograms per year. With the advances in spinning technology and the needs of the people, the better fiber quality will be required. These requirements of fiber quality have attracted a lot of efforts of governments and scientists to improve cotton fiber quality, especially that of upland cotton.Conventional genetic strategies have been employed to enhance the fiber properties of upland cotton for over half a century. However, lint yield, fiber quality and disease resistance are quantitatively inherited, and usually affected by environments; (2) unfavorable correlations exist between lint yield and fiber quality traits, e.g., fiber length, fiber strength and fiber fineness; additionally, the yield and fiber quality must be measured after harvesting. These factors greatly limited the efficiency of conventional breeding efforts in cotton, and it is imperative to develop effective breeding approaches. Recent advances in DNA markers offer plant breeders a rapid and precise alternative approach to conventional selection schemes to improve quantitative traits. Using detailed molecular linkage maps, quantitative trait loci (QTL) affecting economically important traits could be mapped, genetically evaluated and selected through linked markers. At present, the maps developed from interspecific population have been employed to document chromosomal relationships and evolutions in allotetraploid cotton and to locate genes/QTL affecting yield and fiber quality. However, these interspecific maps currently have little use in conventional upland cotton improvement. The linkage maps developed from upland cotton covered only a relatively small part of cotton genome, so the QTLs identified for agronomic and fiber quality traits cannot satisfy the marker assisted selection. Therefor, the comprehensive linkage map covering a wide range of the genome is needed to map QTL precisely and completely. The present study constructed a genetic linkage map with SSR and morphological markers using 270 F_2:7 recombinant inbred lines (RILs) developed from a cross between upland cotton cultivar Yumianl and T586, and the genetic linkage map was used to identify and map the QTL affecting lint percentage. The mainly results were as following:Lint percentage performances of mapping parents and F_2:7 RILsThe lint percentage was tested on the RIL population across four environments (Chongqing-Chongqing-Hainan-Chongqing, 2004-2006). The two mapping parents were markedly different in lint percentage. The lint percentage of high quality cultivar Yumian 1 ranged from 44.00 to 45.89%, with a mean of 43.22%. The lint percentage of multiple dominant gene cultivar T586 ranged from 3.60 to 11.87%, with a mean of 8.08%. The lint percentage of Yumian 1 was 5.4 times more than that of T586.The lint percentage of RILs population mostly ranged between two parents, but transgressive segregation was observed falling beyond both Yumianl and T586, across four environments. Lint percentage segregated continuously in four environments, and the result suggested that the lint percentage was controlled by major and minor multiple genes.The significant variances of genotype and environment tested in four environments indicated lint percentage was affected by both the genotype and environment.Primer pair polymorphism between mapping parentsA total of 349 polymorphic primer pairs were found among 4591 cotton SSR primer pairs between the two mapping parents, and the polymorphic primer pairs accounted for 7.0% of the total primer pairs.Genotyping recombinant inbred lines349 SSR primer pairs revealed a total of 357 polymorphismic loci in recombinant inbred lines, and 8 SSR primer pairs revealed 2 polymorphismic loci per primer pair. Out of these SSR loci, 336 loci segregated as dominant markers (91.6% of the total SSR loci) and 31 as co-dominant markers (8.4% of the total SSR loci).Chi-square examination demonstrated that 112 out of 365 markers (30.7%) including 357 SSR and 8 morphological markers exhibited segregation deviation from the expected 1:1 ratio. The majority of those loci showed a mild deviation from Mendelian segregation (X²<10), and only 31 locus ( 8.5% ) had extremely severe segregation distortion (x²>10).Linkage mapA total of 365 loci, including 357 SSR and 8 morphological markers, were employed to perform linkage analysis, and 346 loci were distributed into 56 linkage groups. Out of these linkage groups, 43 linkage groups were assigned on 24 chomosomes, while 13 linkage groups were not assigned on any chromosome. The linkage map covered 2014 cM with the average distance about 5.8 cM between two markers, accouting for approximately 45.3% of the total recombination length of the cotton genome.QTL analysisBased on interval mapping, 5 QTLs affecting lint percentage were identified, and these QTLs distributed on 5 chromosomes (Chromosomes 6, 7, 12, 14 and 21), explaining 4.8-63.8% of the lint percentage variance. 3 out of 5 QTLs were identified in four environments. The alleles originating from Yumian 1 or T586 increased lint percentage by 2.9 to 7.61 %.