Mapping QTL Affecting The Vertical Distribution Of Soybean [Glycine Max (L.) Merr.] Pod Number

High yield is the main objective of soybean breeding,and pod number is an important factor in soybean yield,and is also a quantitative trait regulated by multiple genes.Because of the differences of soybean varieties and sowing environments,pods were unevenly distributed in the upper,middle and lower segments of the main stem of soybean.This study focused on exploring the relationship between pod number in different parts of plant and further exploring the QTL regulating its expression.Through the analysis of spatial distribution of pod number in soybean plants,it can further reveal the spatial difference of vertical distribution of pod number in soybean,which can be used for reference in formulating and implementing breeding objectives of pod number in soybean;verify and supplement the QTL regulating pod number by FW-RIL population;and obtain MQTL which is conducive to carrying out breeding practice at the same time,hope to fine mapping of QTL and screening and pre-selection of candidate genes.The measurement provides a reference.Field experiments were conducted in Harbin,Heilongjiang Province,in 2016.One hundred and sixty families of soybean FW-RIL population(Kenfeng 14 × Kenfeng 15)×(Heinong 48 ×Kenfeng 19)was used as genetic population.The experimental materials were divided into two sowing dates(May 7,May 17)for sowing.Random block design was adopted and the repetition number was 2.Field investigation was conducted after the first ripening period and before harvest.The number of one pod,two pods,three pods and four pods of the main stem from the growth point of the stem tip to each node of the cotyledon node were investigated.The above ground part of the plant was divided into three parts according to the number of nodes(i.e.the ratio of nodes in the upper,middle and lower parts was x : x ± 1 : x).The number of one seed,two seeds,three seeds and four seeds per pod in the upper,middle and lower parts of the plant were counted respectively.Using a genetic linkage map containing 275 polymorphic SSR markers,12 pod number traits were mapped at two sowing dates.Using the inclusive complete interval mapping(ICIM)method to identify QTL.A total of 59 QTL regulating pod number of soybean plants published by previous scholars from 17 literatures were collected.Meta-QTL analysis was performed based on the QTL identified in this study.The following results:(1)The results of F test showed that there were significant a difference among genotypes(families)of all traits in FW-RIL population under two sowing dates(P < 0.01).Therefore,this population is suitable for variance analysis and QTL mapping.From the point of view of the spatialdistribution of pod numbers on plants,pod numbers are concentrated in the middle of the plant,with more pods in the middle and less pods in the upper and lower parts.The number of pods was three pods > two pods > four pods > one pod.The results of pairwise data t test showed that sowing date had a significant effect on PNMB and PNMC of 12 traits(P < 0.05),and PNLA and PNLB were significantly affected by sowing date(P < 0.01).The results of correlation analysis of 12 traits showed that 24 groups of pod number traits,such as PNUA and PNUB,PNUB and PNUC,PNUA and PNNA,had significant correlation(P < 0.05)and above levels at two sowing dates.ANOVA analysis showed that the pod number of soybean plants was easily affected by genotype,different parts of the plant,interaction between genotype and sowing date,and interaction between genotype and different parts of the plant(P < 0.01).(2)Seventy six QTL controlling the number of soybean pods were located at the LOD threshold of 2.5.The number of QTL located in the upper,middle and lower parts of the main stem was 33,28 and 25,respectively.The number of QTL located in the first,second,third and fourth pods of the main stem were 18,20,27 and 27,respectively.The phenotypic variability explained(PVE)by a single QTL ranged from 0.96% to 16.85%.Among the 72 QTL,40 had inclusion,overlap or cross regions with those previously identified by scholars,and the other 32 were used to supplement pod number QTL.(3)A total of 37 MQTL were identified with two or more constituent QTL.Among them,Mq PN-C2-2 has a high phenotypic contribution rate(11.70%)while the number of related QTL reaches the maximum,which is 20.Therefore,Mq PN-C2-2 can be considered as a better MQTL selection for fine mapping and candidate gene screening.