Multiple fertility restorer genes for EP (Z.p.) cytoplasm

--K. J. Newton and E. H. Coe, Jr.

When cytoplasm from perennial teosinte, Zea perennis, was introduced into the A619 maize inbred line, by using A619 as the male parent in a recurrent backcross program, the resulting plants were found to be male sterile (Gracen and Grogan, Agron. J. 66:654, 1974). This type of cytoplasmic male sterility was called cms-EP for Euchlaena perennis (the old taxonomic designation for Zea perennis). Kermicle and Lonnquist (MNL 47:409, 1973) have reported on restorer factors for defective kernels with EP cytoplasm, different from and independent of the restorers of male fertility that are the subject of this note. EP cytoplasm has been found to be male sterile only in A619 or other inbred lines derived from Oh43 (Gracen and Grogan, 1974; Laughnan and Gabay-Laughnan, Ann. Rev. Genet. 17:27, 1983). All other lines tested restore fertility to cms-EP. Restoration is dominant and the mode of restoration is sporophytic; i.e., all the pollen grains of heterozygous, fertility-restored plants appear to be functional.

We have made some observations in an initial attempt to determine the number of nuclear genes involved in restoring cms-EP to fertility. The original cms-EP materials were generously provided by Jerry Kermicle. Apparently, multiple Rf genes for cms-EP exist. The estimates vary depending on which inbred line is analyzed. This is not unexpected because different restorer loci may be involved in the different inbred lines used for comparisons.

For example, cms-EP in A619 (male sterile) was crossed by WF9. All F1 plants were fertile. F2 progeny from self-pollinated F1 hybrids were scored for their fertility status. 19/86 plants were unambiguously male sterile (they were checked several times to ascertain that they did not show "delayed" pollen shedding). Thus, 22.1% of the F2 plants were male sterile and WF9 appears to carry a single dominant Rf gene. B73 also appears to carry a single dominant Rf gene for cms-EP. EP-A619/B73 fertile F1 plants were backcrossed by A619 as the pollen parent. 11/24 backcross progeny plants (45.8%) were scored as male sterile.

In contrast, dominant alleles of multiple restorer genes appear to be carried by the inbred line, W23. Casual observations over two seasons suggested that sterile F2 plants from EP-A619 x W23 hybrids were rare. However, when the fertile F1 plants were backcrossed by A619, male sterile plants were recovered. At least 4/43 backcross progeny were scored as steriles. This suggests that there are a minimum of three loci involved. The simplest hypothesis is that W23 carries dominant alleles for these fertility restoration genes and that the Rf genes act in a duplicate, rather than a complementary, fashion. According to this model, WF9 and B73 would carry homozygous dominant Rf alleles at only one locus. We do not yet know whether the Rf allele is at the same locus in both, nor whether either or both correspond to any of the loci identified in W23. The existence of multiple loci, any one of which can restore fertility, may help account for the rarity of the non-restoring genotype among inbred lines.

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