Whole-Genome Triplication and Species Radiation in the Southern African Tribe Heliophileae (Brassicaceae)

The unigeneric tribe Heliophileae includes c. 90 Heliophila species, all endemic to southern Africa. The tribe is morphologically the most diversified Brassicaceae lineage in every aspect of habit, foliage, flower, and fruit morphology. Despite this diversity, virtually nothing is known about its origin and genome evolution. Here we present the first in-depth information on chromosome numbers, rDNA in situ localization, genome structure, and phylogenetic relationship within the Heliophileae. Chromosome numbers determined in 27 Heliophila species range from 2n=16 to 2n=c. 88, but 2n=20 and 22 prevail in 77% of species. Chromosome-number variation largely follows three major lineages (A, B, and C) resolved in the ITS phylogeny. B-clade species possess mostly 2n=20, whereas 2n=22 is the dominating number in C clade. The number and position of 5S and 45S rDNA loci vary between species and cannot be employed as phylogenetically informative characters. Seven species with different chromosome number and from the three ITS clades were analyzed by comparative chromosome painting. In all species analyzed, 90% of painting probes unveiled three homeologous chromosome regions in Heliophila haploid chromosome complements. These results suggest that all Heliophila species, and probably the entire tribe Heliophileae, experienced a whole-genome triplication (WGT) event. We hypothesize that the mesohexaploid ancestor arose through hybridization between genomes resembling the Ancestral Crucifer Karyotype with n=8. The WGT has been followed by species-specific chromosome rearrangements (diploidization) resulting in descending dysploidy towards extant quasi-diploid genomes. More recent neopolyploidization events are reflected by higher chromosome numbers (2n=32-88). The WGT might have contributed to diversification and species radiation in the Heliophileae.