Genetic Variation And Association Analysis In Important Economic Traits In Eucalyptus Urophylla × E.tereticornis

Growth and wood properties are important economic traits for trees and have crucial influences on wood yield and quality.Study on their genetic variation is critical to improve wood yield and quality in tree improvement and breeding.Cross breeding for obtaining heterosis purpose is an important method for the improvement of wood yield and quality,and the key to heterosis obtaining is the choice of appropriate parents based on the relationship between parental genetic information and hybrid performance.Growth and wood formation are regulated by multiple genes,and association study on growth and wood properties will be helpful to find related genomic loci and provide useful markers for molecular breeding.Eucalyptus trees have great contributions to global plantation development and wood production.In the present dissertation,genetic parameters for growth and wood properties were estimated in E.urophylla S.T.Blake×E.tereticornis Smith hybrids derived from an incomplete factorial mating design,and correlations between parental genetic variation and hybrid performance were analyzed.Moreover,genome-wide association studies were performed to find genomic loci related with growth and wood properties using genotyping by sequencing（GBS）derived single-nucleotide polymorphisms（SNPs）.The main conclusions were as follow:（1）Means of hybrids were consistently higher for most traits as compared to maternal open-pollinated families,with significant differences between Changtai and Yong’an trials.Wood properties displayed generally smaller phenotypic variations than growth traits.The narrow-sense heritability estimates for 10-year-old tree height（H10）and diameter at breast height(D₁₀)ranged between 0.13 and 0.22,implying a generally low level of additive genetic control.The additive genetic coefficients（rg）of age 10 with ages after 1.5 years remained stably high for tree height and diameter at breast height,indicating the possible time of age 1.5for early prediction of mature growth.For wood properties,the narrow-sense heritability of wood density(BD₁₀),cellulose content(CC₁₀),hemicellulose content(HC₁₀),lignin content(LC₁₀),and syringyl-to-guaiacyl ratio(S/G₁₀)varied notably from 0.03 to 0.49,suggesting low to moderate levels of additive genetic effect.In addition,dominance effect on growth and wood property traits were generally weak.Values of additive genetic and phenotypic coefficients were significantly positive between growth traits H₁₀ and D₁₀,indicating the possible same genes related to growth traits.Growth traits generally showed positive rg with BD₁₀,CC₁₀,and S/G₁₀ but negative r_g with HC₁₀ and LC₁₀.Such correlations are favorable for pulpwood genetic improvement.The type B coefficients（rB）were significantly departed from one in H₁₀,BD₁₀,CC₁₀,HC₁₀,and S/G₁₀,indicating strong genotype×environment interaction effects on these traits.However,non-significant r _B were found in D₁₀ and LC₁₀,suggesting possibly weak genotype×environment interactions.（2）The genetic distance（GD）between 20 parents（10 E.urophylla females and 10 E.tereticornis males）based on all 184 polymorphic EST-SSR markers ranged from 0.293 to0.783,with a mean of 0.640.Based on the UPGMA dendrogram,20 parents were separated into four clusters.In addition,genetic distance was modified with a subset of selected markers or alleles,including with markers beyond 0.7 and 0.8 in polymorphism information content(GD_PIC>0.7 and GD_PIC>0.8,respectively)as well as with alleles within 0.1–0.9,0.2–0.8,and0.3–0.7 in allele frequency(GD_F0.1–0.9,GD_F0.2–0.8,and GD_F0.3–0.7,respectively),which showed consistently non-significant correlations with hybrid mean value,specific hybridization ability,and most types of heterosis.The correlations were enhanced of significant alleles based genetic distance（GDSA）with hybrid mean value,specific hybridization ability,and heterosis.Genetic distance based on effect-increasing alleles(GD_FA)and effect-decreasing alleles(GD_UA)were significantly positive and negative with hybrid mean value,specific hybridization ability,and heterosis,respectively.The prediction effects were improved obviously.（3）GBS was performed on the hybrid population,from which a mean of 8 014 894 high quality reads per sample were generated,with a mean sequence error of 0.0118%.The proportion of bases with quality score higher than 20 was 97.61%,with a mean of GC content42.16%.A total of 156036 and 156404 high-quality SNPs were obtained for Changtai and Yong’an trial samples,respectively,which locatedquite evenly across the 11 chromosomes of E.grandis genome.For 10-year-old growth traits,a total of 44 SNP loci were associated significantly with H10 at both trials of Changtai and Yong’an,including 18 SNPs within nine different coding regions.There were 68 SNPs associated significantly with diameter at breast height at both trials,including eight SNPs within five different coding regions.Some of the genes were functionally annotated as regulators of plant growth and development.For10-years-old growth traits,a total of 2015 and 464 SNPs were obtained for H₁₀ and D₁₀,respectively,including some within genes regulating plant growth and development.For10-years-old wood properties,a total of 1269,132,290,507,and 32544 SNPs were found to be associated with BD₁₀,CC₁₀,HC₁₀,LC₁₀,and S/G₁₀,respectively,and some of the loci were related with genes regulating cell wall modification,cell organelle development,auxin regulation,cell elongation and division,signal transduction,cellulose synthesis,and cell wall lignification.