Nature Communications, Published online: 31 August 2026; doi:10.1038/s41467-026-76979-2 Early genomic and karyotypic evolution that underpinned grasses (Poaceae) diversification remains incompletely resolved. Here, the authors report genome assembly of Streptochaeta spicata and reveal the ancestral karyotype, subgenome dominance following genome duplication, and the origin of spikelet.