Ten newly arisen restorers that were products of independent spontaneous mutations in S male-sterile cytoplasm (cms-S) have been under study. We believe they result from the integration of a cytoplasmic fertility episome at different chromosomal sites. Eight of these ten restorers have now been located to chromosome. Data indicate that restorers I and VIII are located in chromosome 8, restorers IV and VII in chromosome 3, restorers IX and X in chromosome I and, most recently, restorers III and VI in chromosome 2. Some of the inbred lines that ultimately derive from varieties through inbreeding, such as CE1, Tr, CI21E, Ky21 and C103, are homozygous for a naturally-occurring nuclear restorer that was designated Rf3 by D. N. Duvick (Advan. Genet. 13:1-56, 1965). Rf3 is regarded as the standard restorer of cms-S. The Rf3 restorer, as represented in most, if not all, of the inbred lines studied is located in the long arm of chromosome 2 and these restorers appear to be allelic.
Only two of the eight newly arisen restorers located to chromosome are in chromosome 2. Since these new restorers occurred spontaneously, there is reason to believe that such changes should also occur in natural populations. In view of this it seems surprising that so many inbred lines with restoring capabilities for cms-S should carry a restorer located at the same chromosomal site. If, however, most such spontaneous events yield restorers similar to restorers I through X (J. R. Laughnan and S. J. Gabay-Laughnan, 1978, Maize Breeding and Genetics, D. B. Walden, ed.), they would, because of deleterious effects, be at a disadvantage from the standpoint of survival in maize populations. It is possible, therefore, that the Rf3 that is carried in established inbred lines does not carry these associated effects and has therefore survived in maize populations, and has become fixed in the homozygous state in some inbred lines developed from these populations. As a result of these considerations we searched among plantings of varieties of maize obtained from the North Central Regional Plant Introduction Station, Ames, Iowa, for naturally-occurring cms-S restorers. A total of 181 varieties were tested; 89 were nonrestorers and 92 were restorers of cms-S. The restorers identified in this way were tested to determine if they are located in chromosome 2. The procedure involved crossing the restorer strains as male parents with sterile cms-S rf In2a B/rf N b plants. Among the resulting progeny are plants that are male-fertile, have B plant color and carry the inversion 2a (breakpoints = 2S.75-2L.80); their genotype is Rf N b/rf In2a B. These plants are crossed to rf N b homozygous tester plants and the ratio of B to b is scored among the progeny. Since only Rf transmits through the pollen, deviation from a 1:1 ratio for B:b (excess of b) is an indication of linkage. Of the 92 restorer strains 26 had no test, 3 carried B in the tested strain, and 5 had too few progeny to test for linkage. Of a total of 58 that were successfully tested, 11 had ratios not significantly different from 1:1 and 47 had highly significant ratios, thus indicating chromosome 2 as the location of the restorer in 47 of 58 tested strains.
While the inversion testcross procedure used here identifies those restorers that are located in chromosome 2, it does not permit their localization within the chromosome. We have therefore made appropriate crosses of the chromosome-2-linked naturally occurring restorer strains with translocation strains rf T2-9b wx and rf T2-9d wx. Testcrosses of F1 offspring from this cross are expected to provide improved linkage information.
The inverted segment of In2a involves almost 80% of chromosome 2. Even so it is not possible to conclude that the 11 restorers that gave independent B:b ratios in this experiment are not in chromosome 2; if they are, however, they are not at the standard Rf3 position, and further studies are expected to resolve this question for at least some of these restorers.
S. J. Gabay-Laughnan and J. R. Laughnan
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