Diversity and Suitability for Hybrid Production of Different Sources of Cytoplasmic Male Sterility in Maize

The epiphytotic of Helminthosporlum maydis, race T, in 1970 on the Texas male sterile cytoplasm demonstrated the dangers of using a uniform cytoplasm for hybrid corn production. Resistance to H. maydis, race T, was obtained by switching to non-male-sterile cytoplasms. This shift increased cytoplasmic diversity but necessitated the return to detasseling female parents. Sources of male sterile cytoplasms resistant to H. maydis, race T, are known and are being substituted for non-male sterile cytoplasms in the production of commercial corn hybrids. We have proposed that a return to male sterile cytoplasm is practical if precautions are taken to ensure that cytoplasmic diversity is maintained. It was our objective in this study to determine the extent of diversity as measured by differential fertility restoration reactions within most of the known sources o[ cytoplasmic male sterility and to evaluate the suitability of each male sterile cytoplasm for hybrid production in various inbred backgrounds. We, therefore, examined the fertility restoration of 38 sources of cytoplasmic male sterility in 28 inbred backgrounds. The sources of male sterility could be divided into three groups (C,S, and T groups) with similar fertility restoration patterns within groups by 11 of the inbred lines. Cytoplasmic sources B, CH, I), EP, LF, NT, OY, SG, 181, and 234 did not fit the fertility restoration pattern of any of these groups. Although three groups of cytoplasms were distinguished, the fertility restoration reactions with some inbred lines indicated diversity within the groups. Sixteen inbred lines restored certain members of a group of cytoplasms but failed to restore others. Most of the inbreds test,~ formed fully male sterile combinations with several cytoplasmic sources. The feasibility of producing hybrid corn utilizing a physical mixture of male sterile cytoplasms (multiplasm) to insure cytoplasmic diversity is discussed.