Plasmid-like mitochondrial DNAs from Latin American maize races

There are two unique plasmid-like mitochondrial DNA (mtDNA) molecules associated with the S-type of cytoplasmic male-sterile (cms) maize (PNAS 74:2904-2908, 1977). These molecules have terminal sequences repeated once in reverse polarity (MGCNL 52:96-98, 1978), a characteristic often involved with insertional events in lower organisms. The spontaneous reversion of cms-S maize to fertility, correlated with the disappearance of these two molecules and changes in the mtDNA, has been associated with a transpositional event (Science 209:1021-1023, 1980).

Mitochondrial DNAs from 61 accessions of Latin American maize races were examined by agarose gel electrophoresis, BamHI restriction endonuclease fragment analyses, and electron microscopy of molecules under denature/renature and heteroduplex conditions.

The Mexican race, Conico Norteno, contained two molecules which we cannot distinguish from the S-1 and S-2 molecules of cms-S. Test crosses to establish the identity of this cytoplasm as cms-S are not yet complete, but about one-half the plants of Conico Norteno in those plantings were completely or partially male-sterile. Fertile and nonfertile pollen on the partially sterile plants is indicative of gametophytic control, a characteristic of the cms-S system. Conico Norteno was the only race, of the 17 Meso-American races examined, to exhibit the cms-S like traits. This observation has important implications. It establishes that a male-sterile system can exist and be maintained in indigenous populations of maize.

From the 44 South American accessions, 12 races (Coroico, Racimo de Uva, Kcello, Marron, Mochero, Guaribero, Moroti, Mishca, Araquito, Pollo, Enano, and Chirimito) exhibited the same distinctive digestion fragment pattern, and each contained two plasmid-like DNAs similar to those of cms-S cytoplasm. We have temporarily designated this cytoplasm S*.

One of these molecules, S-2*, appears identical to the S-2 molecule in cms-S, although there may be micromolecular heterogeneity. Each has a molecular length of about 5450 nucleotides, the same BamHI cleavage sites, and terminal inverted repeats of about 155 nucleotides.

The other molecule, S-1*, has a length of about 7460 nucleotides whereas S-1 of cms-S has a length of about 6450 nucleotides. BamHI digestion of S-1* produces fragments of about 4560 and 2420 nucleotides, while S-1 of cms-S has no BamHI cleavage site. Heteroduplex analyses of S-1 and S-1* showed that these molecules have terminal homologous sequences of about 165 at one end and a homologous region of about 4719 nucleotides at the other end. Within the ends of S-1* is a terminal inverted repeat of about 152 nucleotides (Fig. 1).

The similarities of the S* plasmid-like molecules to, and partial homologies with, those of cms-S could imply their involvement in a male-sterile mechanism. However, we have found no male-sterility in the 12 races having the S* cytoplasm. Neither have we observed other phenotypic traits associated with this cytoplasm.

These molecules contain terminal inverted repeats, a characteristic common to transposable elements. They may have been derived by sequence excision from the large circular molecules of the mt genome. It is possible that these 12 races acquired the plasmid-like DNAs independently of each other. However, the disjunct distribution, geographically and racially, of the S* cytoplasm cannot preclude the possibility that these 12 races share a common ancestral cytoplasm. Finally, we have demonstrated that the presence of plasmid-like DNAs in maize is not a unique phenomenon, restricted to a single cytoplasm, the cms-S.

Figure 1.

A. K. Weissinger, D. H. Timothy, C. S. Levings, III, W. W. L. Hu and M. M. Goodman

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