On the Origin of B-like Chromosomes in Boechera (Brassicaceae)

Boechera is the only genus of the Brassicaceae family in which apomixis occurs. Here our aim was to investigate the genome structure and evolution of sexual and apomictic Boechera diploids (2n = 2x = 14, 15), and to elucidate the origin of heterochromatic B-like chromosomes in apomictic accessions. To this end we used comparative chromosome painting (CCP) analysis with differentially labelled BAC contigs of Arabidopsis thaliana arranged according to the structure of a purported ancestral genome with eight chromosomes (Ancestral Crucifer Karyotype, ACK). First, we constructed comparative cytogenetic maps for sexual and apomictic diploids (2n = 14) and an aneuploid apomictic accession (2n = 15), then we reconstructed the origin of these genomes from the ACK. The Boechera genome shows an extensive collinearity of ancestral genomic blocks with three chromosomes having the ancestral ACK-like structure. The other four Boechera chromosomes originated through reciprocal translocations and inversions, whereby the chromosome number was reduced from x = 8 to x = 7 by centromere inactivation on chromosome B05. Two heterochromatic B-like chromosomes (Het and Del) have been described in apomictic Boechera plants. In eudiploid apomicts (2n = 14), the Het chromosome was identified as one of the B01 homologues comprising blocks A, B and C. In 2n = 15 apomicts, the Het chromosome consists only of blocks A and C and the block has become the Del chromosome. The centromere of Del originated either through centric fission of Het or by a de novo formation. The structure and origin of the Het and Del are important for the further investigation of the apomixis in Boechera and will become useful for a high-quality genome assembly in Boechera.

This work was supported by the European Social Fund (CZ.1.07/2.3.00/20.0189, CZ.1.07/2.3.00/30.0037).