Extensive chromosomal arrangements between Brassicaspecies
Genome-wide synteny analysis was conducted using syntenic orthologous genes both within and between species for Brassica rapa . Firstly, the genome of Chinese flowering cabbage was compared to two published genome assemblies of different strains of this species, B. rapaZ1(Belser et al., 2018) andB. rapa var. pekinensis (Zhang et al., 2018). The SyMAP map reveals that these three Brassica rapa assemblies retain well conserved overall genome architecture except a translocation event between chromosome 1 and chromosome 3 that differentiates our assembly to the other two assemblies (Fig. 5A). Next, we performed the comparison between B. rapa var. parachinensis and two highly continuous assemblies of the B. oleracea genome (Belser et al., 2018; Liu et al., 2014). Besides the different chromosome numbers (i.e.B. rapa var. parachinensis ; AA genome, n=10 and B. oleracea ; CC genome, n=9 ), we observed extensive chromosomal rearrangements between these two species (Figure 5B). Only 2 chromosomes (Chr1 and Chr2) showed minimal changes since their divergence from a common ancestor. The extensive chromosomal rearrangements that occurred during the course of Brassica genome evolution is different from the observation in Oryza , one of the well-studied genus models in monocot, in which the karyotype of most diploid species is well-conserved, even over 15 million years evolutionary history(Stein et al., 2018).