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).