QtL mapping for ear tip-barrenness in maize

Barren tip on corn ear is an important agronomic trait in maize, which is highly associated with grain yield. Understanding the genetic basis of tip-barrenness may help to reduce the ear tip-barrenness in breeding programs. In this study, ear tip-barrenness was evaluated in two environments in a F2:3 population, and it showed significant genotypic variation for ear tip-barrenness in both environments. Using mixed-model composite interval mapping method, three additive effects quantitative trait loci (QTL) for ear tip-barrenness were mapped on chromosomes 2, 3 and 6, respectively. They explained 16.6% of the phenotypic variation, and no significant QTL × Environment interactions and digenic interactions were detected. The results indicated that additive effect was the main genetic basis for ear tip-barrenness in maize. This is the first report of QTL mapped for ear tip-barrenness in maize. additional key words: quantitative trait loci; Zea mays (L.); mixed-model composite interval mapping; SSR. abbreviations used: MCIM (mixed-model composite interval mapping); QTL (quantitative trait loci). authors’ contributions: Conceived and designed the experiments: JW and JD. Performed the experiments: JD, JM, JC, TA, ZL, ZT, SW and WC. Analyzed the data: JD. Wrote the paper: JD, JW and JC. citation: Ding, J.; Ma, J.; Chen, J.; Ai, T.; Li, Z.; Tian, Z.; Wu, S.; Chen, W.; Wu, J. (2016). Short communication: QTL mapping for ear tip-barrenness in maize. Spanish Journal of Agricultural Research, Volume 14, Issue 3, e07SC01. http://dx.doi.org/10.5424/ sjar/2016143-9325. received: 19 Jan 2016. accepted: 14 Jul 2016. copyright © 2016 inia. This is an open access article distributed under the terms of the Creative Commons Attribution-Non Commercial (by-nc) Spain 3.0 Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Funding: Henan Basic Research Program of China (142102110051). competing interests: The authors have declared that no competing interests exist. correspondence should be addressed to Jianyu Wu: henau1302@126.com.


introduction
Barren tip on corn ear, which shows abortive kernels on the tip of cob tissue, often causes potential yield loss at different ratios (Wang, 2001).Previous studies reported that environmental factors, especially environmental stress at pollination or during grain filling stage, played important roles in the formation of barren tips in maize.For example, stresses like drought, high or low temperature and nutrient deficiency may limit pollination or cause abortive tip kernels after pollination (Zhang et al., 1998;Edreira et al., 2011).Wang et al. (1996) and Zhang et al. (1999) reported a relation between abortive tip kernels and dynamic changes of hormone components.A few studies on the inheritance of resistance to barren tip have been reported in maize, and the mode of inheritance seems to be polygenic (Meng et al., 2007;Li et al., 2008;Cai et al., 2015).However, only limited genetic information is available at the molecular level.In the present study, we report 2 ured.The length of ear tip barrenness was referred to the distance from the bottom of the first round of shrunken kernels to the ear top (Meng et al., 2007).
Standard analysis of variance was performed using the Proc GLM procedure of SAS (SAS Inst., 1999) to determine variation of barren tips among the F 2:3 families in Jiyuan and Zhengzhou locations.The components of variance were estimated using a complete random effects model and broad sense heritability was calculated as defined by Knapp et al. (1985).
Detailed information on the construction of the genetic map was reported previously (Ding et al., 2011).Briefly, all the 225 F 2:3 families were used for genetic map construction, which included 180 polymorphic SSR markers evenly distributed on 10 chromosomes.The genetic map spanned a total length of 1987.7 cM with an average distance of 11.0 cM between markers.For QTL analysis, a full QTL model was selected to detect the effects and locations of multiple QTL by using the software QTLNetwork 2.0 (Yang et al., 2007).The proportion of phenotypic variation explained by a single QTL was calculated by the square of the partial correlation coefficient, and the general contribution for each QTL was calculated from the relative contributions of all the putative QTL involved.

results and discussion
The average performance and the descriptive statistics for barren tip of F 2:3 families as well as the two parents are given in Table 1.Both parents, Chang7-2 and Zheng58, were consistently different in the length of barren tips: Zheng58 had longer barren tip than Chang7-2 in both locations.The means of F 2:3 families for barren tip were close to the parental line Zheng58.The analysis of variance revealed that the genotypic components of variance (σ 2 g ) were significant (p<0.01) for the F 2:3 families.The broad sense heritability was relatively high (H = 0.81), which indicates that much of the phenotypic variance was genetically controlled and suitable for further QTL mapping.
The search for QTL for barren tip followed the method of mixed model composite interval mapping (MCIM).In total, three additive effects of QTL for barren tip were found on chromosomes 2, 3 and 6, respectively (Table 2).Among them one QTL (bin 3.02), the barren tip increasing allele, was contributed by short barren tip parent Chang7-2, and the other two QTL (bin 2.04/05 and 6.06/6.07)had the barren tip increasing allele coming from long barren tip parent Zheng58.Since the means of F 2:3 families usually underestimate dominance by a factor of 1/2, only one QTL (bin 6.06/6.07)was detected with significant dominant effect.In combination, all the QTL explained 16.6% of the phenotypic variation, whereas each QTL individually accounted for between 4.0% and 8.5% of the phenotypic variation.No significant QTL × Environment interactions and digenic interactions were detected.
Ear tip barrenness is an undesirable trait in maize breeding programs, which is usually caused by abortive kernels and unfertilized florets.Previous studies table 2. Estimated additive and dominance effect of QTL detected by the MCIM method for barren tips across two locations (Jiyuan and Zhengzhou).

Bin
Flanking markers additive [a] (cm) Dominance [b] (cm) Var (%) [c]   2.04/05 umc2030-nc131 -0.13 Additive effect; positive values of the additive effect indicate that the Zheng58 alleles increase the barren tip of ear. [b] Dominance effect; positive values of the dominance effect indicate that the heterozygotes have higher phenotypic values than the respective means of two homozygotes. [c] Percentage of genotypic variation explained by each QTL.**significant at p<0.01; *** significant at p<0.001.have pointed at environmental factors that affect tip barrenness (Zhang et al., 1998;Edreira et al., 2011).However, there is limited information about the molecular genetic mechanism in maize.In the present study, we provide evidence of QTL responsible for genetic control of tip barrenness in maize.Both additive and dominant effects QTL were identified for barren tip in maize, which further confirmed that the trait of tip barrenness is a polygenic quantitative character (Meng et al., 2007;Li et al., 2008;Cai et al., 2015).

table 1 .
Means of barren tips length (in centimetres) for parental lines and the F 2:3 population, variance components and broad sense heritability (H) at Jiyuan and Zhengzhou locations.