Abstract
Brassica species (tribe Brassiceae) belonging to U's triangle—B. rapa (AA), B. nigra (BB), B. oleracea (CC), B. juncea (AABB), B. napus (AACC) and B. carinata (BBCC)—originated via two polyploidization rounds: a U event producing the three allopolyploids, and a more ancient b genome-triplication event giving rise to the A-, B-, and C-genome diploid species. Molecular mapping studies, in situ hybridization, and genome sequencing of B. rapa support the genome triplication origin of tribe Brassiceae, and suggest that these three diploid species diversified from a common hexaploid ancestor. Analysis of plastid DNA has revealed two distinct lineages—Rapa/Oleracea and Nigra—that conflict with hexaploidization as a single event defining the tribe Brassiceae. We analysed an R-block region of A. thaliana present in six copies in B. juncea (AABB), three copies each on A- and B-genomes to study gene fractionation pattern and synonymous base substitution rates (Ks values). Divergence time of paralogues within the A and B genomes and homoeologues between the A and B genomes was estimated. Homoeologous R blocks of the A and B genomes exhibited high gene collinearity and a conserved gene fractionation pattern. The three progenitors of diploid Brassicas were estimated to have diverged approximately 12 mya. Divergence of B. rapa and B. nigra, calculated from plastid gene sequences, was estimated to have occurred approximately 12 mya, coinciding with the divergence of the three genomes participating in the b event. Divergence of B. juncea A and B genome homoeologues was estimated to have taken place around 7 mya. Based on divergence time estimates and the presence of distinct plastid lineages in tribe Brassiceae, it is concluded that at least two independent triplication events involving reciprocal crosses at the time of the b event have given rise to Rapa/Oleracea and Nigra lineages.
Highlights
The evolution of land plants, angiosperms, has been characterised by repeated rounds of polyploidization [1] and reticulation [2]
Identification and sequencing of bacterial artificial chromosomes (BACs) mapped to the six R blocks of B. juncea In A. thaliana, the R block is present as a single block between genes At5g01240 and At5g22030 on chromosome 5 [13]
For analysis of R blocks of the A and B genomes, we focused on a specific region between gene IDs At5g14660 and At5g15840 of the R block of linkage group (LG) A10, which contains major QTLs mapped in a cross between Heera and Varuna [28,31]
Summary
The evolution of land plants, angiosperms, has been characterised by repeated rounds of polyploidization [1] and reticulation [2]. Both of these processes appear to have played a major role in the evolution of the Brassicaceae [3,4]. Brassicaceae is a well-defined family with a much conserved floral structure. The most recent taxonomic treatment of the family, which takes into consideration evidence from molecular systematics, identifies 49 tribes, 321 genera and 3,600 species [6]. Relationship between many taxa, remains poorly resolved as a consequence of processes such as convergent evolution and reticulation
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