Chromosome location of the three Oh51A pseudorestorer genes and their
usefulness in studying apparent cases of gene silencing
The Laughnan laboratory has been identifying and analyzing spontaneous nuclear restorer genes of cms-S for over two decades. Among the many spontaneous nuclear revertants that have been identified is a class we now refer to as "pseudorestorer" (MNL 63:122, 1989; MNL 63:122-123, 1989). When these phenotypically fertile plants are crossed as pollen parents there is no seed set on the ears; the pollen fails to function. Because this class of "restorer" gene produces nonfunctional pollen we gave it the symbol Rf-nf. To date, eight independently-occurring spontaneous revertants have been identified as Rf-nf genes.
We are using the wx-marked reciprocal translocation series to map the Rf-nf genes to chromosome (Maize Handbook pp.255-257). Three Rf-nf genes arose in the inbred line--cytoplasm combination cms-RD Oh 51A and each has now been located to chromosome. The Rf-nf gene 81-67-9 is in chromosome 3 according to our crosses with wx T3-9c and wx T3-9(8447). Rf-nf 79-21-27 has been mapped to chromosome 6 by use of wx T6-9(4505) and wx T6-9(4778). Rf-nf 79-23-27 has been placed on chromosome 8 by use of wx T8-9d and wx T8-9(043-6). We have previously mapped newly arisen Rf genes to chromosomes 3 and 8 but this is the first case of an Rf gene in chromosome 6.
In the course of studies on the allelic relationships of the Rf-nf genes, we found that crossing an Rf-nf/rf plant by an unrelated inbred line yields F1 plants that produce functional pollen grains. Crosses of these F1 plants as pollen parents often produce progeny segregating male-sterile plants and, in some cases, all male-sterile progeny (MNL 68:105-106, 1994). Since restoration of cms-S is gametophytic, all progeny of a cross cms-S rf/rf x cms-S Rf/rf are expected to be fertile. Crosses of these same restored F1 plants as female parents give the expected fertile and sterile plants. Therefore, the appearance of sterile plants in the crosses of the F1 plants as male parents cannot be explained by the failure of the Rf-nf gene to express in a particular nuclear background. We have been studying the basis for this apparent "gene silencing". Now that we have located the three Oh51A Rf-nf genes to chromosome, we are in a position to take a unique approach to the analysis of this phenomenon.
By crossing each Oh51A Rf-nf gene with its respective nonrestoring (rf) wx-marked reciprocal translocations we effectively link the Wx gene to the Rf-nf gene. The heterozygote can be symbolized Rf-nf N Wx/rf T wx, where N stands for nontranslocation (or normal) and T for translocation. Since the wx-marked translocations are carried in nuclear backgrounds unrelated to Oh51A (e.g. W23, M14 or W23/M14), these F1 plants should exhibit functional pollen. By crossing pollen from these plants onto a cms-S rf T wx/rf T wx tester strain we can follow the Rf-nf gene by its linkage to Wx. This will allow us to determine if the apparent gene silencing is due to unexpected transmission of rf (kernels will be wx) or to silencing of Rf-nf (kernels will be Wx).
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