Physical and gene organization of mitochondrial DNA in fertile and male sterile sunflower. CMS-associated alterations in structure and transcription of the atpA gene.

To study the molecular basis of cytoplasmic male sterility (CMS) in sunflower (Helianthus annuus), we compared the physical organization and transcriptional properties of mitochondrial DNAs (mtDNAs) from isonuclear fertile and CMS lines. Mapping studies revealed much greater similarity between the two mtDNAs than in previous comparisons of fertile and CMS lines from other plant species. The two sunflower mtDNAs 1) are nearly identical in size (300 kb and 305 kb); 2) contain the same 12 kb recombination repeat and associated tripartite structure; 3) have the same dispersed distribution of mitochondrial genes and chloroplast DNA-homologous sequences; 4) are greater than 99.9% identical in primary sequence; and 5) are colinear over a contiguous region encompassing 94% of the genome. Detectable alterations are limited to a 17 kb region of the genome and reflect as few as two mutations--a 12 kb inversion and a 5 kb insertion/deletion. One endpoint of both rearrangements is located within or near atpA, which is also the only mitochondrial gene whose transcripts differ between the fertile and CMS lines. Furthermore, a nuclear gene that restores fertility to CMS plants specifically influences the pattern of atpA transcripts. Rearrangements at the atpA locus may, therefore, be responsible for CMS in sunflower.