Transposition characteristics of the naturally occurring cms-S restorer in inbred line CE1

--John R. Laughnan, Susan Gabay-Laughnan and Janet M. Day

In MNL 63:121 we presented evidence for transposition of the cms-S restorer carried by inbred line CE1, here designated Rf(CE1). This restorer is located in the long arm of chromosome 2 where it ordinarily gives from five to ten percent recombination with the wx gene in wx T2-9d translocation heterozygotes. The testcross: (S) rf rf wx wx X (S) Rf(CE1) N Wx/rf T2-9d wx is used to search for instances of putative transposition of Rf(CE1); these are identified as male parents whose testcross ears show significantly enhanced frequencies of wx kernels. Since the heterozygous male parents in these testcrosses carry S-type cytoplasm, rf (nonrestoring) pollen grains are aborted, so only Rf(CE1) pollen grains function. Hence, on testcrossed ears which exhibit both Wx and wx kernels, the Rf(CE1)-wx recombination rate is given directly by the percentage of wx kernels. Pollen examination with a field microscope confirms that male parents to be testcrossed are heterozygous for both the translocation and the Rf(CE1) restorer.

Occasionally (S) Rf(CE1) N Wx/rf T2-9d wx male parents with significantly higher Rf-wx recombination rates are identified by this procedure. In MNL 63:121 we reported on five such putative cases of Rf(CE1) transposition among 33 tested plants. In another experiment involving Rf(CE1), and the same procedure, there were no putative cases of transposition among 52 tested plants. In the second cycle of this experiment, however, where the 45 Rf(CE1) marked heterozygotes tested were offspring of 16 stable female heterozygotes of the first cycle, there were 11 cases of putative Rf(CE1) transposition. This pattern of low or no activity in the first generation Rf(CE1) heterozygotes, with enhanced Rf-wx recombination rates in later cycles, appears to be the pattern with other Rf marked heterozygotes that have been tested, including spontaneous Rfs I, III, IV, and VI. The Rf(CE1) experiment has been continued through the fourth cycle. New instances of change from control-rate to high-rate Rf-wx recombination are encountered with advancing cycles. Interestingly, high-rate parents may produce both high-rate and control-rate progeny, the latter of which, we believe, are the result of loss, by meiotic recombination, of the most distant (transposed) Rf element.

We have considered the possibility that the cases of testcross ears with high frequencies of wx kernels may not result from Rf(CE1) transposition, but may instead be due to occasional instances of change of S- to "N"-type cytoplasm in smaller or larger sectors of the tassels of Rf(CE1) heterozygous male parents. Lacking S cytoplasm, all pollen from such sectors should be functional, including the nonrecombinant rf wx type, and these would register as increased numbers of wx kernels on testcross ears. The occurrence of such sectors of a size that would explain high frequencies of wx kernels on these ears is not likely, since a pollen check of each male testcross parent is made to confirm that it is heterozygous for both Rf(CE1) and the translocation, before it is involved in crosses. Moreover, there is independent evidence indicating that the S to "N" cytoplasmic change is low in these strains. A direct test of whether this spurious transmission of rf wx through the male parent can account for high wx frequencies on testcross ears is based on the recognition that, if only Rf(CE1) pollen grains function, all offspring are expected to be male-fertile; if rf wx grains are functional because they derived from an S to "N" mutation in the tassel, they are expected to produce male-sterile offspring, since the testcross female parent is (S) rf rf. Table 1 summarizes data on the characteristics of wx progeny from (S) rf rf wx wx X (S) Rf(CE1) N Wx/rf T2-9d wx testcrosses. The tabular information is arranged according to classes of increasing frequencies of wx kernels on testcross ears, a random sample of whose wx kernels were planted to score. While it is evident that male-sterile offspring do occur, their overall frequency, 4.4% of 963 plants scored, is far too low to account for the numbers of wx kernels observed in column 1 classes, and there is no evidence for a correlation of frequency of male-sterile offspring with increased frequency of wx kernels on the ears. The 13 male-sterile plants in class 2 were distributed among 9 of the 23 progenies, and the 18 male-sterile plants in class 4 were distributed among 6 of the 7 progenies, 10 of the 18 plants occurring in a single progeny. In a study similar to this one, involving the restorers of spontaneous origin Rfs I, III, IV, and VI, among 1,924 testcross progeny scored, there were 19 (1.0%) male-sterile progeny, plus 31 (1.6%) plants with exserted anthers that could not be scored. Even if these are considered to be male-sterile, the 2.6% of male-sterile offspring can account for only a small fraction of wx kernels on testcross ears.

The data in Table 1 provide additional information on Rf(CE1) transposition. The 921 male-fertile plants were scored for presence or absence of the translocation, and these data are presented for the six classes based on the frequency of wx kernels on testcross ears. Since all these offspring are fertile and derive from wx kernels, they must represent Rf-wx recombinants. If the Rf-wx exchange occurs between T and wx, the recombinant strand is Rf(CE1) N wx and the resultant offspring will lack the translocation; if it occurs between Rf and T, the recombinant strand is Rf(CE1) T wx, and the resultant offspring carry the translocation. With Rf(CE1) located at its control site, about 5 to 10 map units from wx in the heterozygous translocation complex, the data indicate near equality (60:55) for plants with and those without the translocation. In classes with increasing frequencies of Rf-wx recombinant kernels on the ear, there is a progressive shift in this ratio in favor of plants with the translocation vs. plant without the translocation, as in classes 5 and 6, where Rf-wx recombination frequencies are at their highest, and corresponding percentages of plants with the translocation are 92% and 87%, respectively, with the difference between them not statistically significant by contingency chi-square analysis. This shift in ratio is consistent with the presence of a transposed Rf at a recipient site some distance from the original, or donor, site. As this distance increases the ratio of Rf T wx to Rf N wx progeny is expected to increase, and to reach its maximum when the transposed Rf assorts independently of wx, either because it is far distant from wx in the 2-9 heterozygous translocation complex, or is in another chromosome. The data in Table 1 support the presence of transposed cms-S Rf(CE1) elements; they do not provide information on whether Rf(CE1) is present or absent at the original site in heterozygotes with a transposed Rf. An accompanying article deals with this question.

Table 1. Fertility characteristics of recombinant waxy (wx) progeny from (S) rf rf wx wx X (S) Rf(CE1) N Wx/rf T2-9d wx testcrosses.
wx kernels on testcross ears No. of testcross progenies Total no. of fertile plants No. of plants with translocation: Male-sterile plants
Class % (wx wx)   Present (%) Absent (%) No. (%)
1 0-10 14 115 60 (52) 55 (48) 1 (0.9)
2 10.1-20 23 268 152 (57) 116 (43) 13 (4.6)
3 20.1-30 3 65 49 (75) 16 (25) 0 (0.0)
4 30.1-40 7 174 155` (89) 19 (11) 18 (9.4)
5 40.1-50 9 237 218 (92) 19 (8) 8 (3.3)
6 50.1-60 2 62 54 (87) 8 (13) 2 (3.1)
TOTALS   58 921 688   233   42 (4.4)

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