Characterization and Fine Mapping of ds, a Recessive Dense-Spike Mutant Associated with Shortened Spike Axis and...

Spike density is an important component of wheat spike architecture and is determined by the combined effects of spike-axis elongation and spikelet number. In this study, we characterized an ethyl methanesulfonate (EMS)-induced recessive dense-spike mutant, ds, which was obtained from EMS mutagenesis of the wheat cultivar YN21 in 2013 and displays a field-visible compact spike architecture associated with a shortened spike axis, reduced spike internode spacing, and increased spikelet number. Genetic analysis showed that all F₁ plants exhibited normal spikes and that segregation in the F₂ population fitted a 3:1 ratio, supporting control by a single recessive nuclear gene. Through genome-wide marker-based linkage screening, enlarged-population validation, and fine mapping, ds was delimited to an approximately 864.819 kb interval between ID2B2796 and ID2B7615 on chromosome 2B. This interval contains 11 high-confidence annotated genes, including F-box protein, actin, ubiquitin-conjugating enzyme, ribosomal protein L16, RecX, plant cysteine oxidase, PI4KIIγ, and two adjacent GA3OX-family-related genes. Integrated RNA-seq and RT-qPCR analyses showed that the two adjacent GA3OX-family-related genes were expressed in young spikes and exhibited reduced expression in ds-sib relative to WT-sib. These expression data support their retention as plausible, non-exclusive candidates but do not establish causality. Transcriptome analysis further revealed changes in hormone-related pathways, cell-wall organization, carbohydrate metabolism, and transcription-factor regulation. These results provide a reliable genetic basis for further molecular cloning of ds and suggest that altered hormone- and growth-related transcriptional responses may be associated with dense-spike formation in wheat.