RFLP mapping of Semi-dwarf1
--Vladutu, CI; Phillips, RL

Semi-dwarf1 (Sdw1) is a semi-dominant EMS-induced mutation affecting whole-plant morphology. As described by R.M. Bird and M.G. Neuffer (MNL 59:42), the reduction of height in heterozygous mutants varies from 1/4 to 1/3 of the wild-type plants and is due to internode shortening and not decreased node number. In mutants, the leaves and tassel branches are erect and shorter, and ears are smaller than in wild-type plants. In the complex genetic background of our material we did not notice wilting in Sdw1 heterozygotes. However, among the progeny of a selfed Sdw1 heterozygote, several very short plants with erect leaves and tassel branches (which likely were homozygous Sdw1) showed wilting. Also, the interval between anthesis and silking was generally longer and tillering was suppressed in Sdw1 heterozygotes. Using the waxy reciprocal translocation series, Bird and Neuffer located Sdw1 on the long arm of chromosome 8 about 8 map units from the 8L.35 breakpoint. Mo17 was the source of the pollen subjected to EMS mutagenesis (Bird and Neuffer, MNL 58:71). Because of its deleterious effects, Sdw1 has been maintained as a heterozygote. Seed provided by P. Stinard (Maize Genetics Stock Center) resulted from a cross between Sdw1/+ (+ designates a wild-type allele from either Mo20Y or W23) and a wild-type single cross (L317 x W23). We used bulked segregant analysis to identify useful polymorphic patterns (i.e. distinct bands linked in cis with Sdw1) between heterozygous Sdw1 and wild-type plants segregating from the above cross. DNA was extracted from bulked leaf tissue of six wild-type plants, four mutants, and L317 and W23. The four DNA samples were digested with BamHI, HindIII, EcoRI and EcoRV, Southern-blotted, and probed with 10 genomic and cDNA sequences previously mapped on chr.8: umc124, ucbanp1 (from J. Vogel), pge11 (from S. Hake, S;), umc89, umc12, bnl12.3, csu31, umc93, umc48 and umc30. umc89 (with BamHI and EcoRV) and ucbanp1 (with HindIII) each detected a band present in the mutants and absent in the wild-type controls. umc30 (with BamHI) revealed a more intense band in mutants compared with the wild-type segregants (the respective band was not detected in either L317 or W23).

In 1994 the four Sdw1 heterozygotes were selfed or testcrossed with wild-type segregants. A self and a testcross progeny, involving the same Sdw1 heterozygote, were planted in 1995. Neither the mutant nor the wild-type plant used in the testcross were recombinant for the informative. umc89, ucbanp1 and umc30 fragments. We could not unambiguously score all the phenotypes of the self progeny; however, the testcross progeny segregated in two distinct classes (35 wild-type: 31 mutant). DNA was extracted from the 66 testcross individuals and digested with BamHI and HindIII. No recombination occurred between Sdw1 and umc89 and ucbanp1. Three recombinants occurred between Sdw1 and umc30 (4.5±2.6% recombination). These results indicate considerable shrinkage of recombination values on chromosome 8L in this material compared with other mapping populations. In the UMC 1995 map (Coe et al. MNL69:164) the umc89-umc30 interval is ~25 cM. In a N28 x N28E F2 population (88 individuals) ucbanp1 mapped ~ 10 cM toward the centromere from umc89 (Vladutu, M. S. thesis, 1996). Localized factors such as inversions, heterozygosity for knobs or general background or environmental effects could account for the reduction in recombination values. Alternatively, the recombination values may be underestimated due to the small population size used in this mapping experiment.

In conclusion, Sdw1 is confirmed by RFLP mapping to be on chromosome 8L, close to umc89 and ucbanp1. 

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