--J. R. Laughnan, S. Gabay-Laughnan and Janet M. Day
Natural restorers of S-type male-sterile cytoplasm (cms-S) are found in a number of inbred lines of maize. As indicated in another article in this group such restorers, designated Rf3, act at the gametophytic level, meaning that cms-S Rf3 rf heterozygotes produce 50% Rf3-carrying pollen grains that function normally in fertilization, and 50% rf-carrying grains that abort (shriveled pollen). The Rf3 gene carried in inbred line CE1 is located in the long arm of chromosome 2 where it gives from five to ten percent recombination with the wx allele in wx-T2-9d translocation heterozygotes. We have noted that (S) Rf(CE1) N Wx/rf T2-9d wx plants (N refers to a noninterchanged chromosome) crossed as male parents onto (S) rf rf wx wx male-sterile testers (see Table 1 legend) occasionally give strikingly higher Rf-wx recombination rates than usual. The male parents in this testcross are produced by crossing cms-S plants of inbred line CE1 with a highly inbred strain of wx-T2-9d. Since the parents are highly inbred the resulting F1 heterozygotes to be testcrossed are isogenic. Also, since the heterozygous male parents in the testcross have S-type cytoplasm, rf3 (nonrestoring) pollen grains are aborted, so only Rf(CE1) pollen grains are functional. The testcrossed ears exhibit both starchy and waxy kernels and in this particular case the percentage of waxy kernels corresponds directly to the Rf(CE1)-wx recombination rate.
The data in Table 1 are arranged in three groups each derived from a separate cross of a cms-S inbred line plant with a wx-T2-9d pollen parent. Thus, the nine testcrossed plants in family 1064 all came from kernels on the same ear, those in family 1065 from kernels on another ear, and so forth. The Chi-square values for contingency homogeneity tests were not significant at the 0.05 level for the ten plants in family 1065, but the corresponding Chi-square values for the 1064 and 1066 groups were highly significant (0.001 level).
The basis for the highly significant Chi-square values for the contingency homogeneity tests of plants in families 1064 and 1066 is apparent when it is noted (Table 1) that certain male parents when testcrossed gave unusually high frequencies of waxy kernels, as indicated in the # footnote in Table 1. For example, the starchy:waxy testcross progeny from plant 1064-6, when compared with all other
plants in the family, gave a contingency Chi-square value significant at the 0.001 probability level. The same is true for testcross data from 1066-1, -2 and -9. Data for 1066-10 indicate a difference that is significant at the 0.01 level. These higher values for Rf(CE1)-wx ratios suggest that in these plants the Rf gene has transposed to a site more distant from the wx marker than in the controls. If the Rf gene were to transpose nonreplicatively to a site in a chromosome other than chromosome 2, or to a site in chromosome 2 far removed from wx in the translocation heterozygote, the Wx:wx ratio on testcross ears should be 1:1. Since the percentage of Rf-wx recombinants for the exceptional plants ranges from 11.7 to 25.1 it appears that these transpositions are to sites in chromosome 2 that are still linked with wx but at a sufficient distance from it to give elevated Rf-wx recombination values compared with the controls. If this is the case it should be noted that increased recombination rates will be produced whether or not the transposition event is replicative or nonreplicative. Moreover, if a replicative transposition of Rf to another chromosome occurs it can be shown that the Rf-wx recombination value should not exceed 33.3 percent in this testcross system.
Although the data in Table 1, and other data not presented here, suggest
that the Rf3 of inbred line CE1 is transposable it remains to be
shown by tests for linkage with other marker genes that the transposed
element occupies a new site in those plants that show significantly elevated
Rf-wx recombination rates. This analysis is underway.
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