An unusual mitochondrial DNA plasmid in the genus Brassica

Abstract

Mitochondria of flowering plants contain a diverse collection of small circular and linear plasmid-Iike molecules^1–11 that replicate autonomously from the highly complex and variable chromosomal genome^12,13. Much interest and research has focused on two sequence elements, present in the mitochondrion as linear autonomous replicons in certain cytoplasmic male sterile (cms) lines of corn^1–3 and sorghum^8,9, which in the case of corn are found to be integrated in the mitochondrial chromosome in normal and spontaneously revertant fertile lines^14–16. We describe here an extrachromosomal, plasmid-like DNA species found in the mitochondria of certain spedes of the genus Brassica. Its properties include the following: (1) the plasmid, found thus far only in the diploid spedes Brassica campestris and the allotetraploid Brassica napus, is phylogenetically distributed across a group of 20 Brassica cytoplasms in a manner suggesting its loss or gain on multiple independent occasions. (2) The copy number of the plasmid varies ∼100-fold in those accessions where it is found—from 1/10 to 10 times that of the main mitochondrial genome. (3) The plasmid is 11.3 kilobase pairs (kbp) in size, is linear in conformation, has no detectable internal repeats and is highly conserved in nucleotide sequence. (4) The presence and abundance of the plasmid show a strong association with cytoplasmic male sterility. (5) The plasmid shows no homology to chromosomal mitochondrial DNA from any of the Brassica accessions examined, nor is it homologous to the linear S₁ and S₂ mitochondrial plasmids found in S-cms cytoplasms of corn.