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Mesopolyploid whole-genome duplication (WGD) was revealed in the ancestry of Australian Brassicaceae species with diploid-like chromosome numbers (n = 4 to 6). Multicolor comparative chromosome painting was used to reconstruct complete cytogenetic maps of the cryptic ancient polyploids. Cytogenetic analysis showed that the karyotype of the Australian Camelineae species descended from the eight ancestral chromosomes (n = 8) through allopolyploid WGD followed by the extensive reduction of chromosome number. Nuclear and maternal gene phylogenies corroborated the hybrid origin of the mesotetraploid ancestor and suggest that the hybridization event occurred ;6 to 9 million years ago. The four, five, and six fusion chromosome pairs of the analyzed close relatives of Arabidopsis thaliana represent complex mosaics of duplicated ancestral genomic blocks reshuffled by numerous chromosome rearrangements. Unequal reciprocal translocations with or without preceeding pericentric inversions and purported end-to-end chromosome fusions accompanied by inactivation and/or loss of centromeres are hypothesized to be the main pathways for the observed chromosome number reduction. Our results underline the significance of multiple rounds of WGD in the angiosperm genome evolution and demonstrate that chromosome number per se is not a reliable indicator of ploidy level.
INTRODUCTION
Hybridization and polyploidization (whole-genome duplication [WGD]) are important evolutionary forces driving genetic diversification and speciation in land plants. In angiosperms, numerous cases of speciation events following this pattern were documented (Grant, 1981; Soltis et al., 2007; Hegarty and Hiscock, 2008). Comparative research using whole-genome and EST sequence data sets uncovered compelling evidence of multiple ancient WGD events in the ancestry of angiosperm lineages (De Bodt et al., 2005; Cui et al., 2006; Soltis et al., 2009; Jaillon et al., 2009). In crucifers (Brassicaceae), the analysis of the Arabidopsis thaliana genome sequence (Arabidopsis Genome Initiative, 2000) suggested the existence of three paleopolyploid WGDs (a, b, and g; Bowers et al., 2003). Whereas the phylogenetic placement of the oldest event (g) is still debated (Soltis et al., 2009), the two more recent WGDs have been shown to postdate the split between Caricaceae and Brassicaceae (Tang et al., 2008). The most recent (a) duplication apparently occurred only within Brassicaceae, and it is equivalent to the wholegenome triplication in the sister family Cleomaceae (Schranz and Mitchell-Olds, 2006; Barker et al., 2009). The exact position of the b event within...