A large-scale population based organelle pan-genomes construction and phylogeny analysis reveal the genetic diversity and the evolutionary origins of chloroplast and mitochondrion in Brassica napus L.

backgroundAllotetraploid oilseed rape (Brassica napus L.) is an important worldwide oil-producing crop. The origin of rapeseed is still undetermined due to the lack of wild resources. Despite certain genetic architecture and phylogenetic studies have been done focus on large group of Brassica nuclear genomes, the organelle genomes information under global pattern is largely unknown, which provide unique material for phylogenetic studies of B. napus. Here, based on de novo assemblies of 1,579 B. napus accessions collected globally, we constructed the chloroplast and mitochondrial pan-genomes of B. napus, and investigated the genetic diversity, phylogenetic relationships of B. napus, B. rapa and B. oleracea.

resultsBased on mitotype-specific markers and mitotype-variant ORFs, four main cytoplasmic haplotypes were identified in our groups corresponding the nap, pol, ole, and cam mitotypes, among which the structure of chloroplast genomes was more conserved without any rearrangement than mitochondrial genomes. A total of 2,092 variants were detected in chloroplast genomes, whereas only 326 in mitochondrial genomes, indicating that chloroplast genomes exhibited a higher level of single-base polymorphism than mitochondrial genomes. Based on whole-genome variants diversity analysis, eleven genetic difference regions among different cytoplasmic haplotypes were identified on chloroplast genomes. The phylogenetic tree incorporating accessions of the B. rapa, B. oleracea, natural and synthetic populations of B. napus revealed multiple origins of B. napus cytoplasm. The cam-type and pol-type were both derived from B. rapa, while the ole-type was originated from B. oleracea. Notably, the nap-type cytoplasm was identified in both the B. rapa population and the synthetic B. napus, suggesting that B. rapa might be the maternal ancestor of nap-type B. napus.

conclusionsThe phylogenetic results provide novel insights into the organelle genomic evolution of Brassica species. The natural rapeseeds contained at least four cytoplastic haplotypes, of which the predominant nap-type might be originated from B. rapa. Besides, the organelle pan-genomes and the overall variation data offered useful resources for analysis of cytoplasmic inheritance related agronomical important traits of rapeseed, which can substantially facilitate the cultivation and improvement of rapeseed varieties.