Articles
MOLECULAR EVOLUTION OF CYTOPLASMIC MALE STERILITY IN PETUNIA
The accepted model of the mt genome in plants is of a master molecule which may be interconverted, via homologous recombination involving pairs of direct or inverted repeats, into subgenomic circular molecules.
It has been suggested that novel open reading frames (ORFs) and therefore novel traits, such as cytoplasmic male sterility (CMS), arose via a mechanism which involves amplification of pre-existing, rare recombinant DNA molecules.
CMS is one of the very few mitochondrial mutations identified and characterized so far in plants.
It is used, in a wide range of plant species, for the commercial production of hybrid seeds.
In Petunia, a mtDNA chimeric sequence termed S–Pcƒ was found to be specific to CMS. The S–Pcƒ locus consists of three ORFs that are co-transcribed.
The first ORF, Pcf, contains parts of the atp9 and coxII genes and an unidentified reading frame, urƒ–s. The second and third ORFs contain NADH dehydrogenase subunit 3 (nad3) and ribosomal protein S12 (rps12) sequences, respectively.
The nad3 and rps12 sequences included in the S–Pcƒ locus are identical to the corresponding sequences on the mt genome of fertile petunia.
In both CMS and fertile petunia, only a single copy of nad3 and rps12 has been detected on the physical map of the main mt genome.
The origin of the urf–s sequence and the molecular events leading to the formation of the chimeric S–Pcf locus are not known.
Recent data from our laboratory indicate the involvement of the homologous recombination mechanism in the evolution of S–Pcƒ and of the CMS trait.
The data suggest that S–Pcƒ was generated by introduction of a subgenomic mtDNA molecule, containing the urƒ–s sequence, into the master molecule via homologous recombination.
In addition, we have recently characterized the first mutation detected so far in S–Pcf and the results indicate the involvement of homologous recombination in the replacement of the 5′ region of S–Pcf with part of the normal cox2 gene.
Molecular analysis of the mtDNA of petunia somatic hybrids that were generated by fusion of CMS and male fertile petunia protoplasts, indicates that fusion of the parental mt and mixing of the mtDNA populations, followed by inter-genomic recombination, have occurred.
The data indicate that sorting out of specific mtDNA sequences has occurred following mtDNA recombination.
In all the cases where male fertility phenotypic expression prevailed, sorting out of mitochondrial DNA sequences containing the CMS-specific S-Pcf locus has been demonstrated.
Taken together, the information obtained in our laboratory, concerning the evolution of CMS in petunia, indicate that the homologous recombination mechanism plays a key role in the evolution of mt-encoded traits.