Genetic Dissection For Gray Leaf Spot Resistance In Maize And Fine Mapping Of A Major QTL

Maize?Zea mays L.?is an important food and forage crop,as well as industrial raw material crop.Gray leaf spot?GLS?is a foliar disease worldwide,and is also one of the major diseases threating maize production in China.Identifying QTL for GLS resistance is useful for understanding resistance mechanism and breeding resistant hybrid variety based on marker-assisted selection.In this study,we developed two doubled haploid?DH?populations and constructed genetic linkage using genotyping-by-sequencing?GBS?.We detected quantitative trait locus?QTL?for GLS resistance based on the disease severity at multiple environments and developmental stages,and confirmed and fine-mapped the major QTL using a BC₃ population and association panel.In addition,we also analysed the correlation between GLS resistance and plant height and tasseling days based on phenotypic correlation coefficient and co-localization of QTL.The major results are as follows:1. Two DH populations?QY and QZ?were successfully constructed using highly resistant Q1 and highly susceptible Ye478 and Zheng58.We investigated the GLS disease severity of the two DH populations for 2 or 3 times?T1,T2 and T3?at three environments,and found these environments could provide morbidity pressuse enough.The GLS scores at the T1 and T2 stages showed a normal distribution,while the GLS scores at the T3 stage showed a segregation distortion toward susceptibility.In addition,the GLS scores of the two populations showed transgressive segregation,indicating GLS resistance is a quantitative trait influencd by multiple genes.The GLS scores between different tested stages in the same environment,and between the environments were positively correlated.Broad-sense heritability ranged from 80.9 to 90.0%at all the three tested stages and the two populations,revealing the GLS resistance in the two DH populations are stably inherited.2. Two genetic linkage maps,including 1,101 and 1,294 bins,were constructed based on65,012 and 43,606 high quality SNP/INDEL markers using GBS in the two DH populations.The total length of the maps was 1479.4 and 1872.1 c M,and the average distance between adjacent bins was 1.36 and 1.44 c M,respectively.3. QTL analysis was conducted in individual DH population using corresponding GLS scores at mutiple stages of the three environments.QTL with overlapping confidence intervals and identical effect directions were integrated into the same consensus QTL.In population QY,25 QTL were detected and distributed on chromosome 1 to 8,explaning6.7 to 26.3%phenotypic variation.These QTL were integrated into 13 consensus QTL,of which three could be detected at multiple environments and stages,three could be detected only at multiple environments and one could be detected only at multiple stages.The consensus QTL q GLS?Y2-1 and q GLS?Y2-2 were detected at the all of three environments,and the former explained 11.2–20.1%of the phenotypic variation and the latter explained 9.9–26.3%of the phenotypic variation.In population QZ,19 QTL were detected and distributed on chromosome 1,2,3,4 and 6,explaning 6.1 to 19.8%phenotypic variation.These QTL were integrated into 8 consensus QTL,of which three could be detected at multiple environments and stages,and one could be detected only at multiple stages.The consensus QTL q GLS?Z2-1 was detected at the all of three environments and stages,and explained 7.8–19.8%of the phenotypic variation.The consensus QTL with overlapping physical intervals between the two DH population and identical effect directions were furtherly integrated.q GLS?Y2-2 and q GLS?Z2-1 were co-localized at the two DH populations,and were integrated into q GLS?YZ2-1,located at 5.2–21.7 Mb genomic region?according to the B73 Ref Gen?v4sequenc?at chromosome 2.This QTL could explain up to 20%of phenotypic variation at multiple environments,suggesting it is a major QTL for GLS resistance.q GLS?YZ3-1overlapped with both q GLS?Z3-1 and q GLS?Z3-2 at the two DH populations,and they were integrated into q GLS?YZ3-1,located at 57.1–190.4 Mb genomic region?according to the B73 Ref Gen?v4 sequenc?at chromosome 3.The resistance allele of these two QTL were derived from the parent Q1.4. To fine map the major QTL q GLS?YZ2-1,a BC₃population was developed by backcrossing the highly resistant DH lines from the QY population with the susceptible parental line Ye478.The GLS disease severity of 828 and 463 plants from the BC₃population were respectively investigated for 4 and 2 times in two years,and the genotypes of these BC₃plants were surveyed using 15 INDEL marker distributed in q GLS?YZ2-1.QTL analysis for the GLS scores in two years,confirmed the q GLS?YZ2-1.Based on the genotypes of highly resistant and highly susceptible BC₃ plants,the q GLS?YZ2-1 was fine mapped to a?2.4-Mb genomic region,ranging from 6,895,568 to9,323,049 bp?according to the B73 Ref Gen?v4 sequenc?on chromosome 2.5. An association panel comprised of 255 genetic materaials selected from Southwest China,the US Corn Belt and Mexico.Regional association analysis was performed and identified 10 markers significantly associated with GLS resistance,with a peak marker located at ID-B1.Pedigree analysis for the association panel revealed that most of the resistant HZ lines carrying Q1 allele at ID-B1 were derived from Tuxpeno or from Tuxpeno‘s derivatives genetic materials,indicating the resistance alelle of the Q1 might be originated from the landrace Tuxpeno.6. Correlation analysis of phenotype showed a weak negative correlation between plant height and GLS resistance,and low negative correlation between tasseling days and GLS resistance.Co-localization analysis of QTL found only one small effect co-localized QTL between GLS resistance and plant height,and this QTL also influenced the tasseling stage.These findings indicated plant height and tasseling days hardly influenced the GLS resistance in this study.This present study dissected the genetic basis of GLS resistance derived from the resistance parent Q1,and fine mapped a major QTL,which provided the basis for improving GLS resistance using marker-assisted selection,furtherly cloning resistance gene and understanding resistance mechanism.