Genetic Mapping of the TtGL-2A Long-Grain Locus in Tetraploid Wheat

Tetraploid wheat (Triticum turgidum), the progenitor of common wheat, provides rich genetic resources for wheat genetic improvement. TDI-1 is a long-grain cultivated emmer wheat (T. turgidum ssp. dicoccum) accession collected in our laboratory. It was crossed with TDU-1, a short-grain durum wheat (T. turgidum ssp. durum) accession. The F₁ hybrids exhibited heterobeltiosis (i.e., performance superior to the better parent) for grain length, thousand-kernel weight, and other kernel traits. In the F₂ population, grain length showed a strong positive correlation with thousand-kernel weight (r = 0.77), and the ratio of plants with the short-grain parental phenotype, heterobeltiosis phenotype, and long-grain parental phenotype was approximately 1:2:1 (χ² ≈ 0.697, p > 0.7), suggesting that a major locus plays a primary role in controlling grain length in this population. Combined with phenotyping of F_2:3 families, homozygous long-grain and short-grain bulks constructed from F₂ individuals were genotyped using bulked segregant analysis based on 120K-SNP array. The results showed that polymorphic SNPs were mainly concentrated on the short arm of chromosome 2A, leading to the inference that this region harbors a locus regulating grain length, tentatively designated TtGL-2A. Twelve SSR markers were developed on the short arm of chromosome 2A, mapping TtGL-2A to an 8.1 cM genetic interval between markers 2AS-280t26 and 2AS-280t29, corresponding to the physical interval 121.2-158.8 Mb, with a LOD score of 12.3. Using the diagnostic marker 2AS-95t3 to genotype 253 wheat accessions, the proportion of long-grain allelic variation TtGL-2A_TDI in landraces, cultivars, and introduced lines is 40.27%, 75.86%, and 82.35%, respectively. Transcriptome sequencing of grains at 15 days post-anthesis from the parental lines TDI-1 and TDU-1 identified 11 differentially expressed genes within the TtGL-2A interval. These results lay a foundation for map-based cloning of TtGL-2A and provide an efficient marker for molecular breeding of wheat yield.