A new search for restorer cytoplasm: the restorer cytoplasm for the gene ms10 most probably does not exist in maize

Since the suggestion concerning the hypothesis of the existence of restorer cytoplasms for some of the known and currently available male sterile (ms) genes in maize, a relatively limited amount of research effort has been made in order to test the hypothesis. Considering the importance of such a phenomenon, we designed a large, two-part experiment to test the idea more seriously. In the first part, 50 randomly chosen, medium-late open-pollinated (OP) varieties of maize from the Zemun Polje (ZP) collection were tested for the presence of the restorer cytoplasm in some currently known ms-genes in maize (ms1, ms2, ms3, ms4, ms5, ms6, ms7, ms8, ms9, ms11, ms12, ms13, ms17, ms22, ms23, and ms24). In the second part, the whole ZP collection of maize germplasm (more than 4,000 entries) was tested for the presence of the restorer cytoplasm for the gene ms10. After the first basic screening of OP varieties, more than 70 nonsegregating "candidates" were identified; however, after additional screening of the collection and the direct testing with respective homozygous ms-testers, all of them showed segregation, indicating that the restorer cytoplasm does not exist, especially the gene ms10. While performing this experiment, we discovered almost a hundred sources of male sterile cytoplasm, which were distinguished by their overwhelming frequency of male sterile plants in segregating test progenies.