Mitochondrial genotypes with variable parts of Arabidopsis thaliana DNA affect development in Brassica napus lines

Phenotypic, genetic and molecular studies were made of Brassica napus lines with mitochondrial genomes consisting of DNA from both B. napus and Arabidopsis thaliana. The lines were isogenic regarding the nuclear and plastid genomes. Out of 21 lines, 10 were male-sterile, 3 semi-sterile and 8 male-fertile. Screening of the mitochondrial genomes with a dense set of A. thaliana specific markers showed that most lines contained large but variable portions of A. thaliana mitochondrial DNA. Several of the A. thaliana sequences in the mitochondrial genomes lead to the accumulation of novel transcripts. In addition, the restorer line showed different ability to restore male-fertility in the male-sterile lines. These results indicate that CMS is caused by several mitochondrial loci or combinations of loci. Beside petal and stamen morphology, growth rate and adenylate content varied among the lines. Furthermore, we found that the mitochondrial background had a distinct influence on nuclear gene expression. A clear example is the reduced expression of the two B-genes APETALA3 and PISTILATA in the male-sterile lines. From the studies made comparing the mitochondrial loci and the observed phenotypic alterations, our interpretation is that different loci in the mitochondrial genome influence nuclear gene expression via several retrograde signalling pathways.