Maize cytoplasms can now be partially classified using molecular techniques for the isolation and characterization of organelle DNAs. Restriction endonuclease cleavage fragment analysis of mitochondrial (mt) and chloroplast DNAs has demonstrated variation between and within the major cytoplasmic groups N, C, S and T (Pring and Levings, Genetics 89:121-136, 1978; Pring et al., Crop Sci. 20:159-162, 1980) and also among several races of teosinte (Timothy et al., Proc. Natl. Acad. Sci. USA 76:4220-4224, 1979).
In the current survey, mtDNAs from 93 Latin American maize races were examined electrophoretically for the presence or absence of plasmid-like elements and were also subjected to analysis using both BamHI and EcoRI endonucleases. Eighteen races contained plasmid-like mtDNAs. One of these, Conico Norteno, was previously reported (Weissinger et al., MGCNL 55:84-86, 1981; Weissinger et al., Proc. Natl. Acad. Sci. USA 79:1-5, 1982) to contain mtDNAs indistinguishable from the S-1 and S-2 plasmid-like elements of the cms-S cytoplasm. That study also reported the discovery of two other plasmid-like mtDNAs, R-1 and R-2 (previously designated S*-1 and S*-2), which we have now found in seventeen different races, all but one of which are South American.
BamHI digestion of mtDNAs produced ten different fragment patterns, while EcoRI digestion produced eight distinct electrophoretograms. Races were placed into one of eighteen groups, each group being defined by a particular combination of BamHI and EcoRI patterns and the presence or absence of plasmid-like elements. Seven races produced patterns identical to those produced by one or another of the major cms groups (C, T and S) with each group being represented by at least one race. Races from Meso-America and some South American races with Meso-American affinities were separated from other South American races. South American races were divided among three general classes of related groups. There was considerable agreement between these groupings and those derived by other workers on the basis of morphological and cytological affinities. We believe that this work provides the breeder and geneticist with a catalog of cytoplasmic variation of indigenous Latin American maize, and may also provide a model for future classification of maize cytoplasms.
A. K. Weissinger,* D. H. Timothy, C. S. Levings III, and M. M. Goodman
*Current address: Pioneer Hi-Bred International, Inc., P.O. Box 85, Johnston, Iowa 50131
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