The mitochondrial alteration causing S-type cytoplasmic male sterility (cms-S) in maize can be overruled by certain nuclear genes called restorer-of-fertility (Rf) genes. The mode of restoration of these Rf genes is gametophytic in the cms-S system meaning that they act postmeioticallly. Among the many spontaneously occurring Rf genes that we have identified is a class we refer to as pseudorestorer. When "fertile" plants carrying a pseudorestorer gene are crossed as male parents onto rf rf cms-S (male-sterile) testers, or onto male-fertile isogenic maintainer (rf rf) plants with normal cytoplasm, there is no seed set on the ears. Because this class of "restorer" gene produces nonfunctional pollen, we have given it the symbol Rf-nf (MNL 63:122, 1989; MNL 63:122-123, 1989). To date, seven independently occurring spontaneous revertants arising in four inbred line-cytoplasm combinations have been classified as Rf-nf genes.
In the course of studies on the allelic relationships of the Rf-nf genes we found that crossing Rf-nf plants by unrelated, nonrestoring inbred lines yields F1 plants that produce functional pollen. As part of our effort to understand this phenomenon the F1 plants are being successively crossed as male and female parents with each of the two inbreds that constitute the F1. In the course of these crosses we were able to compare the performance of an Rf-nf gene of an F1 (Rf-nf/rf) plant crossed both as male and as female parent. We have observed differential effects on pollen production in the backcross progeny depending on whether the Rf-nf -carrying F1 plant was crossed as the male or female parent. Crosses of the F1 plants as pollen parents often produced progeny segregating sterile plants and, in some cases, all sterile progeny, whereas such crosses should produce all fertile progeny. Crosses of these same F1 plants as female parents gave the expected fertile and sterile plants. Since the backcross progeny in both crosses have the same nuclear constitution the difference cannot be explained by the failure of the Rf-nf gene to express in a particular nuclear background. We have only recently begun studying this phenomenon and have hypothesized that the differential behavior of the Rf-nf genes, depending on whether they were transmitted through the maternal or paternal parent, is due to imprinting.
As a control, we have crossed ears on plants carrying functional Rf genes in inbred nuclear backgrounds, both the standard Rf3 gene and other Rfs that arose spontaneously in our hands, by unrelated inbred maintainer pollen. The fertile Rf-carrying F1 plants were crossed both as male and female parents and exhibited the expected results; crosses of these F1 plants as pollen parents produced only fertile offspring. Since there is no evidence of silencing of functional Rf genes, our Rf-nf genes may represent a unique system in which to study gene silencing in maize.
The unexpectedly sterile plants resulting from crosses of Rf-nf-carrying F1 plants as pollen parents should all carry the Rf-nf allele even though it is not being expressed. We have begun testing such plants to determine if the "imprinting" can be erased by passage of the silent Rf-nf gene through a sporophytic generation and have found that the "imprinting" persists. The resulting sterile plants (now only half of which are expected to carry the Rf-nf allele) have been crossed again by maintainer pollen and were scored in our 1993 summer nursery. There is no indication that a second passage of the silenced Rf-nf genes through a sporophytic generation has erased the imprinting. Since we have carried the crossing of the silenced Rf-nf genes by maintainer pollen as far as we reasonably can, we will now try a different approach. Sterile plants resulting from the backcross of an Rf-nf-carrying F1 plant as pollen parent will be crossed as female parents with pollen from the F1 plants. These sterile plants carry a silenced Rf-nf gene. Will additional copies of the Rf-nf gene also be silenced?
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