Grain yield components, such as grain number per spikelet (GPSt) and spikelet number per spike, are crucial factors determining wheat yield. Sakuma et al. (2019) demonstrated that the less functional GNI-A1 allele contributed to higher GPSt, resulting in an increase in final grain yield by 10-30%. TN28, a tetraploid pyramidale wheat line, was found to harbor even more GPSt, as many as five grains in a spikelet with an average of 2.61, than ‘Langdon’ (LDN) which carry the less functional GNI-A1 allele with an average of 2.02. This suggests that TN28 harbors novel genes that increases GPSt with effects surpassing GNI-A1.

To identify the genes, we performed QTL analysis using 144 F2 individuals derived from the cross between two tetraploid wheat TN28 and LDN in a previous study in year 2020. We detected a major QTL for grain number and grain weight in a spikelet located on chromosome 7AL, which was named as qGNI-7A. Moreover, a major QTL for GPSt was detected on chromosome 7B named as qGNI-7B and a significant digenic interaction was observed between qGNI-7A and qGNI-7B for GPSt where qGNI-7A can only increase GPSt when they are TN28 alleles at both qGNI-7A and qGNI-7B.

To validate these QTLs of TN28, we performed QTL analysis in year 2022 under two environments with different fertilizer conditions (designated as 22AC and 22AF) using other F2 populations from the same cross. The genetic linkage map was constructed using all three populations.

To evaluate grain number distribution within the spike, we also evaluated grain number along the spike by dividing the spike into several parts (n = 2 to 10; e.g. n= 3 means three parts within the spike: apical, central and basal parts of the spike). qGNI-7A was detected for grain number at the apical 10-20% portion of the spike in 22AC, although the two previously detected QTLs were not detected in either condition.

Also, the interactions between qGNI-7A and qGNI-7B were not reproduced in 22AC and 22AF. All in all, a specific-genotype and specific-environment effects of QTLs on chromosome 7A and 7B may contribute to a higher GPSt in TN28. Better understand their effects and interactions in isogenic backgrounds in multiple environments is needed in the future work.