A new cytoplasmic male sterile genotype in the sugar beet Beta vulgaris L.: a molecular analysis

Cytoplasmic diversity in leaf beet cultivars as revealed by mitochondrial DNA analysis

Mitochondrial (mt) DNA restriction fragment length polymorphisms are convenient markers for identifying cytoplasmic variation among plant cultivars. In an attempt to detect new cytoplasmic genotypes useful for sugarbeet breeding, we have compared the hybridization patterns of mtDNA from three groups of cultivated beets, viz. leaf beet, garden beet, and fodder beet. Utilized as probes were the two sugarbeet mtDNA clones that were capable of distinguishing normal fertile and different sources of male-sterile cytoplasms from one another. The analysis allowed the identification of four chondriome types among 14 leaf beet cultivars examined. Two out of the four chondriome types were found to be different from the previously described fertile or male-sterile chondriome type. Our results thus indicate that leaf beet cultivars and landraces make up the primary cytoplasmic gene pool of the sugarbeet.

Mitochondrial gene variation and phylogenetic relationships in the genus Beta

Restriction fragment length polymorphisms (RFLPs) for three mitochondrial genes, coxI, coxII and atpA, were used to determine mitochondrial (mt) DNA diversity in 21 accessions of the genus Beta representing wild and cultivated species. On the basis of distribution of the RFLP patterns these Beta genotypes were assigned into six distinct chondriome groups. A high degree of heterogeneity was found to exist between the mitochondrial genomes of the sugarbeet cultivar and the wild species of Procumbentes section. The polymorphic fragments from wild Beta species were cloned and subjected to fine mapping. We found that most of the RFLPs are due to sequence rearrangements rather than point mutations. Our data also suggest that the close linkage between coxII and coxI is taxonomically localized to an evolutionary lineage that led to Vulgares and Corollinae species but not to Procumbentes species. This linkage is most likely to have arisen via the mutation(s) that inserted the DNA segment containing coxI downstream of coxII in the common ancestor of Vulgares and Corollinae species. The results are discussed with regard to the taxonomic and phylogenetic relationships of the Beta species.

The sugar beet mitochondrial gene for the ATPase alpha-subunit: sequence, transcription and rearrangements in cytoplasmic male-sterile plants

We have characterized the mitochondrial atpA (the alpha subunit of F1-ATPase) gene from male-fertile cytoplasm (cv TK81-0) of sugar beet. The gene is 1518-bp long and encodes a polypeptide of 506 amino acids. The atpA mRNA sequence is modified by three C-to-U RNA editing events, all of which alter the encoded protein sequences. In order to examine the genome organization of the atpA locus in cytoplasmic male-sterile (CMS) sugar beet, atpA-containing clones were isolated from Owen CMS (TK81-MS) and a different source of CMS [I-12CMS(2)] cytoplasm respectively. The sequences of the atpA coding region from TK81-MS and I-12CMS(2) are identical to each other and to the corresponding TK81-0 sequence. However, the TK81-0 and TK81-MS loci diverge completely 47 bp upstream of the initiation codon, resulting in different 5' transcript termini for the two genes. On the other hand, the point of divergence between the TK81-0 and I-12CMS(2) atpA genes was found to occur after 393 bp 3' to the TAA stop codon. Our results also show the 3'-flanking sequences of I-12CMS(2) atpA to be present elsewhere in the mitochondrial genomes of TK81-0, TK81-MS and I-12CMS(2), suggesting the possible involvement of these repeated DNA elements in the sequence rearrangements.

Different CMS sources found in Beta vulgaris ssp maritima: mitochondrial variability in wild populations revealed by a rapid screening procedure

Mitochondrial DNA (mtDNA) variation in natural Beta maritima populations has been characterized by way of Southern blot hybridizations of total DNA using non-radioactive probes and chemiluminescent detection. It was found that the previously described N ("normal") mitochondrial type could be subdivided into three subtypes. A new mitochondrial genotype (type R) was distinguished in addition to the previously described type S. Both are male-sterile cytoplasms and can produce a. segregation of sexual phenotypes in their progenies depending on the nuclear background. The populations contained at least two to four different mitochondrial genotypes.

Plastid DNA diversity in natural populations of Beta maritima showing additional variation in sexual phenotype and mitochondrial DNA

Plants of two natural populations of Beta maritima, characterized by high percentages of male-sterile plants, have been investigated for organelle DNA polymorphism. We confirm the two classes of mitochondrial DNA variation previously described: (i) mitochondrial DNA (mtDNA) type N is associated with male fertility, whereas mtDNA type S can cause cytoplasmic male sterility (CMS); (ii) the 10.4-kb linear plasmid is observed in both types of mitochondria and is not correlated with the cytoplasmic male sterility occurring in this plant material. A third polymorphism is now described for chloroplast DNA (ctDNA). This polymorphism occurs within single populations of Beta maritima. Three different ctDNA types have been identified by HindIII restriction analysis. Among the plants studied, ctDNA type 1 is associated with N mitochondria and type 2 with S mitochondria. Chloroplast DNA type 3 has been found both in a fertile N plant and in a sterile S plant. This finding suggests that the chloroplast DNA polymorphism reported is not involved in the expression of male sterility. A comparison with Beta vulgaris indicates that ctDNA type 3 of Beta maritima corresponds to the ctDNA of fertile sugar beet maintainer lines. The three types of Beta maritima ctDNA described in this study differ from the ctDNA of male-sterile sugar beet.

Genomic organization and sequence analysis of the cytochrome oxidase subunit II gene from normal and male-sterile mitochondria in sugar beet

We have cloned and sequenced the cytochrome oxidase subunit II (coxII) gene from both normal and cytoplasmic male-sterile (CMS) sugar beet. The normal coxII (designated NcoxII) locus was found to be located 1491 bp upstream from the gene for cytochrome oxidase subunit I (coxI) on the same DNA strand and to have a 1463 bp intron which split the coding sequence into two exons (382 and 398 bp). The COXII protein contains 260 amino acid residues. We have also found two copies of the coxII gene (ScoxII-1 and ScoxII-2) to be present in the CMS genome. Our results suggest that the NcoxII gene diverges completely from the ScoxII-1 and ScoxII-2 genes 50 bp 5' to the ATG start codon. In addition, the ScoxII-1 and ScoxII-2 sequences could be readily discriminated from each other by the 3' end and the immediately adjacent flanking sequences of the gene: the 3' divergence results in a 101 codon extension of the ScoxII-2 ORF. Northern blot analysis demonstrates that the coxII gene exhibits altered transcript patterns in CMS compared with normal sugar beet. Different genomic arrangements of the coxII gene are considered to be the result of extensive intra- and inter-molecular recombination events involving the repeated DNA elements in the mitochondrial genome.

Cytoplasmic male sterility in beta is associated with structural rearrangements of the mitochondrial DNA and is not due to interspecific organelle transfer

Chloroplast (ct) and mitochondrial (mt) DNAs from four cytoplasmic male sterile (cms) and 22 normal fertile sugar beet lines and accessions of wild beets from the genus Beta have been compared with restriction analyses and Southern hybridizations. We have used restriction analyses of ctDNA as a phylogenetic marker to confirm the taxonomic relationships between the different cytoplasms. According to the ctDNA data, all four cms cytoplasms belong to the same taxonomic section, Beta. Restriction patterns of ct and mtDNA from fertile accessions produced analogous trees of similarity and showed a close correlation between the organellar DNA diversity and the accepted taxonomic classification of the species studied. However, the mt-DNA restriction profiles of the four cms types differed dramatically from each other and from those of all fertile accessions from the genus. No indication of cytoplasmic introgression was found in any of the four investigated cms types. Southern hybridization to mtDNA revealed variant genomic arrangements in the different fertile and cms cytoplasms, indicating that rearrangement of the mitochondrial genome is a common denominator to the different cms systems in Beta. It may, indeed, be a common property to spontaneously occurring cms in all or most species.

Intraspecific diversity of sugar beet (Beta vulgaris) mitochondrial DNA

Mitochondrial (mt) DNA, isolated from different sugar beet populations, was analyzed using BamHI and EcoRI restriction enzymes. It was shown that plants possessing the new mtDNA types are revealed among O-type fertilizers quite frequently. Among cytoplasmic male sterile (cms) plants, which evolved during cultivation of O-type fertilizers, plants with altered mt genome were found.