Chromosome triplication found across the tribe Brassiceae

Abstract

We have used an ∼8.7-Mb BAC contig of Arabidopsis thaliana Chromosome 4 to trace homeologous chromosome regions in 21 species of the family Brassicaceae . Homeologs of this segment could be identified in all tested species. Painting of pachytene chromosomes of Calepina, Conringia , and Sisymbrium species (2 n = 14, 16), traditionally placed in tribe Brassiceae , showed one homeologous copy of the Arabidopsis contig, while the remaining taxa of the tribe (2 n = 14–30) revealed three, and three Brassica species (2 n = 34, 36, and 38) and Erucastrum gallicum (2 n = 30) had six copies corresponding to the 8.7-Mb segment. The multiple homeologous copies corresponded structurally to the Arabidopsis segment or were rearranged by inversions and translocations within the diploidized genomes. These chromosome rearrangements accompanied by chromosome fusions/fissions led to the present-day chromosome number variation within the Brassiceae . Phylogenetic relationships based on the chloroplast 5′- trn L (UAA)– trn F(GAA) region and estimated divergence times based on sequence data of the chalcone synthase gene are congruent with comparative painting data and place Calepina, Conringia , and Sisymbrium outside the clade of Brassiceae species with triplicated genomes. Most likely, species containing three or six copy pairs descended from a common hexaploid ancestor with basic genomes similar to that of Arabidopsis . The presumed hexaploidization event occurred after the Arabidopsis – Brassiceae split, between 7.9 and 14.6 Mya.