In a survey of 25 S-group maize cytoplasms in 36 inbreds, one inbred/cytoplasm combination, cms-L in the inbred strain W182BN, was found to contain two highly abundant double-stranded RNA (dsRNA) molecules. Cms-L in W182BN was given a new designation, "cms-LBN", because cms-L in other inbred backgrounds did not contain these abundant dsRNAs (Sisco et al., MNL 56:82, 1982). Three hypotheses to explain the presence of these dsRNAs in W182BN cms-LBN were:
1) These dsRNAs resulted from a mutation in cms-L during the backcrossing process to inbred W18213N. This would mean that cms-LBN was genetically different from cms-L.
2) These dsRNAs were simply an effect of the W182BN nuclear genome. This would mean that any cms-L cytoplasm backcrossed to W182BN would eventually gain these dsRNAs and that the designation "cms-LBN" should be discarded.
3) These dsRNAs were the genome of a virus that had infected W182BN cms-L plants. This would mean that the dsRNAs might be unrelated either to the cytoplasmic or nuclear genotype of the infected plant.
Evidence against hypothesis (3) was presented by Schuster et al. (pp. 437-444 in R. Goldberg, ed., Plant Molecular Biology, Alan R. Liss, New York, 1983), who showed that single-stranded RNAs homologous to the cms-LBN dsRNAs were present in all S-group cytoplasms and in RU, a type of male-fertile cytoplasm. Recently Finnegan and Brown (Abstract OR-08-06, First Int'l. Cong. of Plant Molec. Biol., Savannah, GA, 1985) have reported data suggesting that the dsRNAs are components of a mitochondrial plasmid found in S-group cytoplasms.
Hypothesis (2) was still likely, however, because Sisco et al. (Plant Sci. Lett. 34:127, 1984) showed that there was a strong effect of nuclear background on the abundance of the dsRNAs. Of 10 inbred backgrounds into which cms-LBN was crossed, eight reduced the amount of the dsRNAs and only two, W182BN itself and a sweet corn inbred strain 2132, maintained the high level of the dsRNAs. To further test hypothesis (2), V. E. Gracen at Cornell crossed five sources of cms-L as female to the inbred strain W182BN. After four crosses to W182BN, our laboratory analyzed the mitochondrial nucleic acids. The abundant dsRNAs characteristic of cms-LBN were not found.
It thus appears that hypothesis (1) is the most likely explanation, and that cms-LBN is different from its progenitor cytoplasm cms-L.
Paul H. Sisco
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