All Brassica species are derived from a common hexaploid ancestor, and this hexaploid ancestor has been further deduced to origin from a diploid species through a whole genome triplication event. The diploid ancestor has 7 chromosomes and resembles the karyotype of tPCK (translocation Proto-Calepineae Karyotype). The confirming evidences for the Brassicas' tPCK ancestor are from below three aspects: (1) The reconstructed genomic segments of the three subgenomes of all Brassica species keep the genomic structure of tPCK; (2) The locations of extant centromeres and the traces of paleocentromeres on the genomes of Brassicas support its ancestral diploid genome as tPCK; (3) The phylogeny tree and evolution analysis based on the whole genome sequences of several sequenced Brassicaceae species find that the Brassicas are evolved from a tPCK genome, such as the tPCK species S. parvula. The determination of the shared diploid ancestor for all Brassica species lays an important foundation for the genetic studies of Brassica crops.
Cheng, F., Lysak, M. A., Mandáková, T., & Wang, X. (2015). The common ancestral genome of the Brassica species. In The Brassica rapa Genome (pp. 97–105). Springer Berlin Heidelberg. https://doi.org/10.1007/978-3-662-47901-8_8