GAMETOPHYTE GENETICS IN ZEA-MAYS L - DOMINANCE OF A RESTORATION-OF-FERTILITY ALLELE (RF3) IN DIPLOID POLLEN

In Zea mays L. plants carrying the S-type of sterility-inducing cytopl asm, male fertility is determined by a gametophytic, nuclear restorati on-of-fertility gene. Haploid pollen carrying the fertility-restoring allele (historically designated Rf3) is starch-filled and functional, whereas pollen carrying the nonrestoring allele (historically designat ed rf3) is shrunken and nonfunctional. Because restoration of fertilit y occurs in haploid tissue, the dominance relationship of restoring an d nonrestoring alleles is unknown. We have tested the dominance relati onship of the restoring and nonrestoring alleles at the rf3 locus in d iploid pollen. The meiotic mutant elongate was used to generate tetrap loid plants carrying both Rf3 and rf3 alleles in the S cytoplasm. Thes e plants shed predominantly starch-filled pollen, consistent with domi nance of the restoring allele. Restriction fragment length polymorphis ms linked to the rf3 locus demonstrated cotransmission of rf3 and Rf3 alleles through heterozygous diploid pollen, providing conclusive gene tic evidence that the restoring allele is the dominant or functional f orm of this restoration-of-fertility gene. We suggest that other S-cyt oplasm restorers result from loss-of-function mutations and propose an alysis of unreduced gametes as a test of this model.