Variable presence of the 1.94kb mitochondrial plasmid in maize S cytoplasm and its relationship to cytoplasmic male sterility

Mitochondrial DNA (mtDNA) was isolated from over 100 different maize nucleo-cytoplasmic combinations. DNA preparations were assayed for the presence of the 1.94kb mitochondrial plasmid by agarose gel electrophoresis and hybridization to a recombinant clone of the plasmid. The plasmid was present in all tested inbreds which carried N, male fertile, cytoplasm or the cytoplasmically male sterile (cms) groups,cms-T andcms-C. However, members of thecms-S group differed with respect to the presence of the plasmid. Cytoplasms I, J and S possessed the plasmid, whereas cytoplasms B, CA, D, G, H, IA, ME, ML, PS, RD and VG did not.Cms-S group lines which had spontaneously reverted to fertility (nuclear and cytoplasmic revertants) did not exhibit a concomitant change in 1.94kb plasmid levels, although all such lines showed the previously reported alteration in levels of the linear mtDNAs, S1 and S2. The presence or absence of the plasmid was not correlated with (i) frequency of reversion to fertility, (ii) the degree of male sterility expressed, (iii) the presence or absence of standard nuclear restorer to fertility genes and (iv) nuclear genotype. Latin American races carrying RU cytoplasm possessed the plasmid, as did sweet corn varieties. The relevance of the data tocms and evolution of thecms-S group is discussed.