QTL Mapping And Genetic Dissection Of Tuber Starch Content And Plant Maturity In Potato(Solanum Tuberosum L.)

Potato（Solanum tuberosum L.）is the world’s third largest food crop and plays an important role in ensuring food security and nutrition intake of poor people.Tuber starch content and plant maturity are important agronomic traits of potato.Starch accounts for about 80%of dry matter in potato tubers,which greatly affects the quality of potato fresh food and processing products.Plant maturity is an important variety characteristic because different maturity varieties can meet the market demand in different seasons and regions.Previous studies have shown that these two traits are quantitatively inherited and controlled by multiple genes.However,complex genetic background and tetrasomic inheritance have hindered the genetic analysis and molecular breeding of these traits.To explore the the genetic basis of these two important traits and the genetic relationship bewteen them,we constructed a segregation population（F₁）of 192offsprings from a cross between cultivars Longshu 8（L8,late maturity and high tuber starch content）and Zaodabai（ZBD,early maturity and low tuber starch content）.The genetic linkage maps were obtained for each parent,on which,based on the phenotypic data in six environments,the QTLs for tuber starch content and plant maturity were mapped.The main results are as follows:1.Construction of tetraploid population（F₁）and phenotype evaluationA tetraploid segregation F₁ population LZ was obtained from the cross between the female parent L8 and the male parent ZDB.L8 showed tuber starch content of20.56-23.00%with a mean value of 21.61%and maturity of 111-126 days with a mean value of 119 days across environments.Whereas,ZDB exhibited tuber starch content of11.21-14.09%with a mean value of 12.94%and maturity of 64-86 days with a mean value of 75 days.The two parents and 192 progeny clones were propagated in vitro which were then transplanted into vermiculite in net houses to produce minitubers for phenotyping.The parents and progeny clones were evaluated in years 2012 and 2013 in three typical ecological zones of potato production in Gansu province,including Dingxi,Tianshui and Zhangye.In total,six sets of the data were obtained for plant maturity and tuber starch content,each data set was taken as an environment,i.e.the data was collected at a site in one yeay.Tuber starch content of population LZ behaved continuous variation with an approximate normal distribution of the frequency in all six environments,indicating that tuber starch content is controlled by multiple genes.For plant maturaty,continuous variation was also observed but with a non-normal frequency distribution,suggesting that plant maturity is controlled by a few major and some minor genes.These results show that the segregation population for tuber starch content and plant maturity is suitable for QTL mapping.2.Correlation of tuber starch content and plant maturityThe broad-sense heritability of tuber starch content was 0.82,while the heritability of maturity was 0.93 based on the phenotypic data of population LZ in six environments.The result of high broad-sense heritability indicates that these two traits are mainly controlled by genetic factors.Further analysis indicated that ther were no significant correlation between tuber starch content and plant maturity,though it was significant in Zhangye with correlation coefficients of 0.444 and 0.215 calculated for 2012 and 2013,respectively.The results suggest these two traits may be controlled by different genetic loci.3.Construction of genetic linkage maps of two parentsThe genetic linkage maps for each parent were constructed by using TetraploidMap for Windows based on the method of two-way pseudo-testcross.A total of 817 selected polymorphic markers derived from 85 AFLP combinations（EcoR I/Mse I、Pst I/Mse I and Sac I/Mse I）and 86 SSR primer pairs were applied.The maternal map for L8consisted of 209 markers（58 SSRs and 151 AFLP markers,containing 3 multiallelic,112simplex dominant,10 duplex dominant and 84 double simplex dominant markers）with the average distance of 5.96 cM between markers,overlaid total length of 1245.64 cM.The map comprised of 12 linkage groups,ten of them with whole four homologous chromosomes while two linkage groups,LG3 and LG9,had 2 and 1 homologous chromosomes,respectively.Eleven chromosomes of potato were labeled by 58 SSR markers.Meanwhile,the paternal map for ZDB consisted of 211 markers（51 SSR and160 AFLP markers,containing 4 multiallelic,76 simplex dominant,24 duplex dominant and 107 double simplex dominant markers）with the average distance of 6.21 cM between markers,overlaid total length of 1310.95 cM.The map was composed of 13 linkage groups,9 of them contained whole four homologous chromosomes while the rest had 3.Twelve chromosomes were labeled by 51 SSR markers.Apparently,the quality of the parental linkage maps constructed can basically meet the requirements for genetic analysis and QTL mapping in this study.4.Marker-trait association analysisThrough marker-trait association analysis of variance（single-point ANOVA）and Kruskal-Wallis test,19 markers were associated significantly with tuber starch content in at least 3 environments,8 of which showed positive genetic effects while the others had negative genetic effects on increase of starch content.Sixteen of the above markers were mapped on chromosomes I,III,VI,VIII,X in maternal map and on chromosomes IV,VIII,X,XI,and XII in paternal map.Meanwhile,38 markers were significantly associated with plant maturity in at least 3 environments,20 of them showed positive genetic effects and18 showed negative genetic effects on prolonging growth period.Twenty-five of the above 38 markers were mapped on chromosomes I,V,VIII and XI in maternal map and on chromosomes IV,V,VI,VII,IX,XI,and XII in paternal map.It is worth noticing that there were 7 and 6 markers mapped on chromosome V of parents L8 and ZDB,respectively,implying that chromosome V may contain critical loci for plant maturity.5.QTL analysis of tuber starch content and plant maturityThere were 11 QTL for tuber starch content identified by Interval Mapping using the six sets of phenotypic data collected in six environments.These QTL were mapped on chromosomes I,II,VI,VIII,X and XII of L8 and chromosomes I,II,IV,VIII and XII of ZDB.4 of them mapped on chromosomes I,II and VIII were reproducible in at least 3environments.Reproducible QTL TSC_L01 on chromosome I and TSC_L08 on chromosomes VIII of L8 and TSC_Z08 on chromosome VIII of ZDB had positive effects to increase starch content,which explained 17.52%,10.96%and 13.15%of the phenotypic variation,respectively.Another stable QTL TSC_L02 on chromosome II of L8 explaining 17.19%of the phenotypic variation had negative effects on starch content.Six QTLs of plant maturity were detected on chromosomes II,V and XI of the maternal map and chromosomes IV,V,and VII on the paternal map.An important stable QTL PM_Z05 was located on chromosome V of ZDB.This QTL detected in five environments showed positive genetic effects to prolong plant maturity.It explained24.93%of the phenotypic variation with the one-LOD confidence interval from 45.2 cM to 62.5 cM on chromosome V.In conclusion,potato tetraploid linkage maps were constructed and QTL controlling tuber starch content and plant maturity were detected,demonstrating that the tuber starch content is controlled by multiple genetic loci on chromosomes I,II,and VIII,while the maturity is mainly controlled by one genetic locus on chromosome V.Genetic analysis reveals that tuber starch content and plant maturaty are highly inheritable and independently manipulated by different loci,strongly suggesting a prospect of improving tuber starch content with desirable maturity to meet the demand of potato industry.