Chromosome-level Atlantic switchgrass assemblies reveal a supernumerary B-Chromosome with transcriptional and evolutionary impacts

A ~34 Mb supernumerary B-Chromosome was discovered in the diverged Atlantic switchgrass (Panicum virgatum) subpopulation, with Hi-C and long-read depth indicating two to four B copies per nucleus.

• Why it matters: Accessory chromosomes often deviate from Mendelian inheritance, but their origins and impacts in natural plant populations are poorly characterized, limiting understanding of lineage-specific genomic diversity.
• What they did: Chromosome-level assemblies were generated for Atlantic switchgrass, revealing a bipartite B architecture (~8 Mb gene-rich proximal and ~27 Mb repeat-rich distal) and a highly rearranged chimera biased toward the N-subgenome.
• The result: The B-Chromosome is largely restricted to the Atlantic subpopulation with likely a single origin, shows 70% B-positive transmission in open-pollinated progeny from B-positive maternal plants, and is linked to A-Chromosome gene expression differences.