Association Mapping and Genetic Diversity of a Soft Winter Wheat Germplasm Collection.

Exploring the natural diversity and identifying phenotypic trait variations caused by sequence polymorphisms are crucial steps in unlocking the genetic potential of plants towards crop improvement. In the recent years, substantial attention has been given to genome-wide association studies (GWAS) as popular alternatives to the traditional bi-parental QTL analysis in identifying significant marker-trait associations (MTAs). In the present study, GWAS using data from the Illumina 9K iSelect SNP assay is being carried out on a collection of 240 soft red winter wheat cultivars and breeding lines evaluated for yield, plant height, and heading date across multiple environments for 2014-2015 seasons. The effects of allelic variation for vernalization (Vrn), photoperiod (Ppd), and height (Rht) genes, and their combinations to yield, heading date, and height of the SRWW panel are also being examined using allele-specific SNP markers. A compressed mixed linear model (CMLM) approach, implemented in GAPIT (Genome Association Prediction Integrated Tool) package in R, identified at least 75 significant markers associated with the evaluated traits for the 2014 season. Significant marker loci contributed approximately 4-14% of the variation for measured traits. Allelic diversity for the major growth habit genes was also observed for the panel, a characteristic of the diverse nature of the population under study. Preliminary marker analyses indicated significant effects of Ppd-B1, Ppd-D1, and Rht-D1 on height and heading date. Semi-dwarf lines with photoperiod insensitivity at Ppd-B1 and sensitivity at Ppd-D1 were observed to be higher yielding in Fayetteville and Stuttgart, AR locations for 2014. Genotyping-by-sequencing (GBS) is currently being used to increase the depth of marker coverage for GWAS. Additionally, the identified significant MTAs will be validated using a population of spring wheat varieties at CIMMYT in Mexico for the 2017 season. Results suggest the utility of association mapping approaches for marker-assisted breeding towards genetic improvement of wheat.