Chromosome location of two Oh51A CMS-S restoring alleles and their potential usefulness in the molecular cloning of rf loci --Gabay-Laughnan, S, Chase, CD Gabay-Laughnan has been searching for exceptional male-fertile plants in a number of CMS-S inbred line backgrounds for three decades. The inbred background exhibiting the highest frequency of nuclear reversion is Oh51A. Twenty-four restorer-of-fertility (Rf) alleles have been identified from 2,635 Oh51A-S plants, a rate just under 1%. Six new Rf cases arose in the 476 Oh51A-S male-sterile plants grown in the summer of 1999. Heritability and transmissibility of these Rf cases will be determined in our 2000 summer nursery.

We are using the wx-marked translocation series to map new Rf alleles to chromosome (Laughnan and Gabay-Laughnan, The Maize Handbook pp. 255-257, 1994). We have previously mapped three of the newly arisen Oh51A Rf alleles to chromosomes 3, 6 and 8 by these means (Gabay-Laughnan, Maydica 42:163-172, 1997) and will be using RFLP marker analysis to more accurately estimate their relative positions on their respective chromosomes. We report here that an additional two Oh51A Rf alleles have been located to chromosome. Rf 81-94-5 is on chromosome 2 according to our crosses with wx T2-9c. Rf 91-1066-3 has also been mapped to chromosome 2, but through use of wx T2-9d. These two newly mapped restorers are of special interest to us since the standard CMS-S restorer, Rf3, maps to the long arm of chromosome 2 (Laughnan and Gabay, Maize Breeding and Genetics pp. 427-447, 1978; Kamps and Chase, Theor. Appl. Genet. 95:525-531, 1997).

We hypothesize that the high rate of nuclear reversion in the Oh51A nuclear background reflects the presence of an active transposable element system. If that is the case, at least some of the new Oh51A Rf alleles may be tagged with a transposable element. Since molecular probes are available for the maize transposable element systems Ac-Ds, I-En (Spm) and Mu, and tester stocks are available to determine the presence of active Ac, En (Spm) and Mu elements, we have begun screening Oh51A for the presence of these three elements by genetic means. The Oh51A inbred line has been crossed with Ac, En (Spm) and MuDR tester stocks. The resulting F1s will be back-crossed by the appropriate tester stocks in Gabay-Laughnan�s winter nursery. When mature ears are harvested, they will be analyzed for the kernel phenotypes indicative of active transposable elements. If Oh51A carries active Ac, En (Spm) or Mu transposons, we will generate segregating populations of CMS-S Rf/rf and CMS-S rf/rf plants for each independent Rf allele recovered in this background. These segregating populations will be screened for transposable element sequences that co-segregate with Rf alleles. Those populations will be used for the molecular cloning of rf loci.
 
 

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