Two satellite repeats reveal B chromosome structural diversity in Atractylodes lancea

Atractylodes lancea in the family Asteraceae is an important plant in traditional herbal medicine. Although its genome has been reported, this species harbors B chromosomes whose DNA composition and biological significance remain largely unexplored. Here, we dissect the origin, structure, and diversification of A. lancea B chromosomes, using an integrated genomic and cytogenetic approach. Illumina sequencing of bulk samples and RepeatExplorer2 analysis identified two satellite repeats as major components of B chromosomes: CL2, a centromeric and pericentromeric repeat shared with A chromosomes, and CL47/59, a B chromosome-specific repeat localized to the interstitial region of B chromosomes. Fluorescence in situ hybridization uncovered striking variation among B chromosomes in repeat distribution, as well as in length and the arm ratio. CL47/59 abundance was positively correlated with B chromosome length, implicating its amplification during B chromosome enlargement. Integrating these genomic and cytological observations, we propose a model for B chromosome diversification in A. lancea, involving a pericentric inversion and the differential accumulation of CL2 and CL47/59 repeats.