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Cytoplasmic male sterility (CMS) is not only important for exploiting heterosis in crop plants, but also as a model for investigating nuclear-cytoplasmic interaction. CMS may be caused by mutations, rearrangement, or recombination in the mitochondrial genome. With the use of the next-generation sequencing technology, we sequenced the mitochondrial genome of hau CMS line and its maintainer line in B. juneca. The result showed that B. juncea hau CMS mitochondrial genome was assembled into a single, circular-mapping molecule with a size of 247,903 bp with 45.08% GC content. The genome contains 35 proteins, 3 rRNAs, 25 tRNA genes and 29 ORFs of unknown function in addition to the CMS associated gene orf288. The sizes of the maintainer line and another normal type line "J163-4" are both 219,863 bp and the GC content is 45.23%. The maintainer line contains 36 proteins, 3 rRNAs, and 22 tRNA genes and 31 unidentified ORFs. After the comparative analysis the hau CMS line and its maintainer line, specific markers were developed based on their unique ORFs. These can be used as an effective markers to separate the hau CMS line from its maintainer line at the seedling stage. We also confirmed that substoichiometrically different mitotypes coexist in hau CMS lines and its maintainer lines in B. juncea. The number of repeats larger than 100bp in the hau CMS line (16 repeats) are almost twice those of the maintainer line (9 repeats). The 5' region of the chimeric gene orf288 is highly similar to nad5, and the 3' region is also similar to another predicted mitochondrial ORF-orf293 in B. juncea. Phylogenetic analysis of the CMS-associated gene orf288 and four other homologous sequences in Brassicaceae showed that orf288 bore strong similarity to orf263 in Brassica tournefortii, but they still differed from one another. The hau CMS mitochondrial genome was highly rearranged when compared with its iso-nuclear maintainer line mitochondrial genome as the other CMS lines in different crops. The gene orf288 was composed of part nad5, orf293 and an unknown origin in the 5' region as orf263 reported in Brassica tournefortii. It may help us identify the mechanism for natural CMS in B. juncea, and compared with the sequenced mitochondrial genomes in Brassicas, uncover the origin of the hau CMS mitotype and structural and evolutionary differences between different mitotypes. |
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Keywords:Brassica juncea; mitochondrial; cytoplasmic male sterility; orf288; mitotype |
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