Behavior of cms-S pseudorestorer genes

--S. Gabay-Laughnan and J. R. Laughnan

Exceptional male-fertile plants which arise in S-type male-sterile (cms-S) maize result from changes at either the nuclear or cytoplasmic level. A class of nuclear revertants in which pollen carrying the newly arisen restorer of fertility (Rf*) fails to function (pseudorestorer) has been previously reported (Maydica 28: 251-263). When ears of plants heterozygous for these Rf* genes are crossed by their respective maintainers, male-sterile and phenotypically male-fertile (50% pollen abortion) plants segregate among the progeny. When cms-S male-sterile testers are crossed by these phenotypically male-fertile heterozygotes there is no seed set on the tester ears. To date, seven strains carrying this class of Rf* gene have been identified in four inbred line-cytoplasm combinations: cms-ML WF9 (2), cms-RD Oh51A (3), cms-S M14 (1), and cms-S B37 (1). All seven strains exhibit 50 percent pollen abortion as expected if they are heterozygous for the nuclear restorer gene (Rf* rf). In two of these seven strains the remaining 50 percent of the pollen grains appear only partially filled. This pollen trait is inherited and it is not surprising that these two strains, when used as pollen parents in testcrosses, produce barren ears. In the remaining five strains, however, the pollen carrying the Rf* gene appears to be normal; nevertheless, in each of these strains the normal-appearing pollen also fails to function.We believe this failure of pollen function results either from transposition and insertion of an Rf element into a gene that controls an indispensable male-gametophyte function, or from a defective Rf gene that codes for a product not fully functional in restoration. We suggest that such a nonfunctional restorer be designated Rf-nf.

In other spontaneous nuclear revertants in which the Rf-carrying pollen is functional, the newly-arisen Rf gene is most often homozygous lethal. These zygotic-lethal Rf genes are interpreted to result either from insertion of Rf into a gene whose function is indispensable for post-zygotic development, or from the deleterious effect of two doses of the Rf gene itself. We suggest that such restorers be designated Rf-lz (lethal zygote).

We have been interested in studying the allelic relationships between the newly arisen restorer genes, those with functional as well as those with nonfunctional pollen. To assist in the analysis the Rf-nf and Rf-lz strains were crossed with the inbred line M825 as pollen parent. This resulted in vigorous F1 plants with excellent tassels. The pollen-bearing F1 plants of the different Rf-nf and Rf-lz strains were intercrossed to test for allelism of their restorer genes. Some of the pollen-bearing F1 plants in the strains carrying Rf-nf genes were crossed onto cms-S male-sterile plants and were also self-pollinated. Surprisingly, these crosses and self-pollinations set seed. Even crosses and self-pollinations involving one of the two Rf-nf strains (the other is still being tested) with partially filled pollen grains, in which this phenotype is evident in the F1 plants, also set seed, although the sets were not always good.

The seven Rf-nf genes in the vigorous background are being further analyzed. These strains can now be tested for homozygous lethality and it appears that at least some are homozygous viable. They are also being converted to the inbred lines in which they arose to determine whether the Rf-nf genes return to the nonfunctional state. The Rf-nf-carrying plants are also being successively self-pollinated to determine whether the pollen viability of the F1 plants is due to hybrid vigor alone; if so, as the strains approach homozygosity the Rf-nf gene will again become nonfunctional. If the homozygous strains retain restorer function it would suggest that modifier genes introduced by M825 are responsible for F1 pollen viability, and that they were selectively retained in the inbreeding procedure. The effects of hybrid vigor resulting from crosses of the Rf-nf strains by inbred lines other than M825 are also being studied.

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