Genetic Diversity Analysis Of Important Inbred Lines And QTLs Identification On Yield-related Traits In Maize

Narrow genetic base resulting from the decrease of genetic diversity is one of important reasons of difficult breakthrough in maize breeding. Therefore, understanding trends of genetic diversity in important inbred lines used in history and genetic basis of yield-related traits is of great importance to utilize maize germplasm resources and broaden genetic bases in maize breeding.In the present study, a set of maize inbred lines developed historically and recently were used to assess the genetic diversity and genetic structure. Meanwhile, parental lines of important hybrids used in maize production in the recent 50 years were used in analyses of genetic and phenotypic diversity to investigate the trends of changes. Furthermore, QTL analyses were conducted for important agronomic characters, especially yield-related traits. The major results are as follows.1. Genetic diversity was assessed for the 241 important inbred lines based on 72 SSR markers. Totally 686 alleles were detected, with an average number of alleles per marker of 9.53, the polymorphic information content of 0.5827 and an average gene diversity of 0.6206. The analysis of genetic population structure revealed that five germplasm origins were included, i.e. domestic landraces with overall proportion of membership of 0.24, American hybrids such as"78599"with 0.119, Lancaster with 0.249, Reid with 0.116 and tropical germplasm with 0.276.2. Analysis of genetic diversity based on 71 SSR markers for the 50 inbred lines which are the parents of 25 most important single-cross hybrids in different decades indicated that the genetic diversity of the parental lines used in the 1970s were the highest and those in the 1980s and the 1990s decreased considerably. However, the genetic diversity in the 2000s increased. On the other side, the analysis of phenotypic variation observed in the field revealed that maturity duration, especially the grain-filling period, increased with the advance of the era. Plant architecture changed towards"source"enlargement and plant density increase. Leave area above ears increased but leaf angle decreased with the advance of the era. The plant height and ear height for the parental lines of the 2000s decreased slightly. The traits for ears and grains changed towards"sink"enlargement. For example, ear width, grain length, grain width and 100-grain weight increased with the advance of the era. Among the quality traits, except the starch content, the contents of fat, protein and lysine decreased. The results suggested that the breeding of maize inbred lines oriented towards the increase of grain-filling period, adaptation to high plant density, large ears and grains, all leading to high yield.3. An F_2:3 population of Ai 2003×Ji 257 was used in quantitative trait loci (QTL) analysis for 28 traits of agronomic importance, mainly including yield-related traits. Totally 139 QTLs were detected for those traits under different environments, some of which were common QTLs detected in different conditions. In addition, eight QTL clusters were detected. Particularly, a major QTL located in the bin of 1.11 had dominance or over-dominance effects on 27 traits, with the percentage of phenotypic variation explained of 40-50%.4. The major QTL located on chromosome 1 was further investigated. Classical genetic analysis revealed that plant height segregated as 3:1 in an F₂ population and 1:1 in a backcross population, suggesting that the QTL for plant height is a recessive gene. Additionally, the dwarfism resulting from the gene was not sensitive to gillerellin. The gene was located on the long arm (1.11) near Dwarf8 (1.09-1.10), one known dwarf gene, with the genetic distance of 20-30 cM. The evidence above indicated that the gene is a new recessive dwarf gene, leading to a significant decrease of plant height.