Ancestral Chromosomal Blocks Are Triplicated in Brassiceae Species with Varying Chromosome Number and Genome Size

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Abstract
The paleopolyploid character of genomes of the economically important genus Brassica and closely related species (tribe Brassiceae) is still fairly controversial. Here, we report on the comparative painting analysis of block F of the crucifer Ancestral Karyotype (AK; n = 8), consisting of 24 conserved genomic blocks, in 10 species traditionally treated as members of the tribe Brassiceae. Three homeologous copies of block F were identified per haploid chromosome complement in Brassiceae species with 2n = 14, 18, 20, 32, and 36. In high-polyploid (n >or= 30) species Crambe maritima (2n = 60), Crambe cordifolia (2n = 120), and Vella pseudocytisus (2n = 68), six, 12, and six copies of the analyzed block have been revealed, respectively. Homeologous regions resembled the ancestral structure of block F within the AK or were altered by inversions and/or translocations. In two species of the subtribe Zillineae, two of the three homeologous regions were combined via a reciprocal translocation onto one chromosome. Altogether, these findings provide compelling evidence of an ancient hexaploidization event and corresponding whole-genome triplication shared by the tribe Brassiceae. No direct relationship between chromosome number and genome size variation (1.2-2.5 pg/2C) has been found in Brassiceae species with 2n = 14 to 36. Only two homeologous copies of block F suggest a whole-genome duplication but not the triplication event in Orychophragmus violaceus (2n = 24), and confirm a phylogenetic position of this species outside the tribe Brassiceae. Chromosome duplication detected in Orychophragmus as well as chromosome rearrangements shared by Zillineae species demonstrate the usefulness of comparative cytogenetics for elucidation of phylogenetic relationships.