In the process of constructing a maize RFLP map, we have recently positioned the Css (sucrose synthase-2) gene relative to the genes for Sh1 and Wx1. We find that Css maps 32 ± 4 cM from Sh1 and 11 ± 2 cM from Wx1 (Figure 1). This location is in close agreement with that reported by McCarty et al. (MNL 60:60, 1986) using different RFLP markers and in a different F2 population.
Our initial experiments were designed to assess polymorphism around these loci in our F2 population. We nick translated probes prepared from the BamHI insert from the Css clone p21.2 (McCarty et al., MNL 60:58, 1986), the EcoRI insert of the Sh1 clone p17.6 (Sheldon et al., MGG 190:421, 1983), and SalI subfragment #1 from the Wx1 clone pWx5 (Wessler and Varagona, PNAS 82:4177, 1985) and hybridized them against DNAs isolated from the inbred parents B73 and Mo17 digested with various restriction enzymes. As expected, usable polymorphisms were found for each probe. Next, linkage data were obtained by hybridizing each probe to Southern blots containing DNAs from B73, Mo17, B73 x Mo17, and 112 F2 plants. F2 plants were scored as B73-like, Mo17-like, or F1-like for each probe. Map position was determined using the maximum likelihood method. Genetic distances observed were Sh1-Wx1 24 ± 3 cM, Sh1-Css 32 ± 4 cM, and Wx1-Css 11 ± 2 cM.
These mapping experiments demonstrate the utility of RFLPs in genetic studies. The chromosomal location of Css was determined even though no phenotypic mutant had been identified. In addition, we have shown that the F2 population B73 x Mo17)X is recombinationally equivalent to the one used by Helentjaris et al. (MNL 60:118, 1986), at least in this region. This finding should make it easier to correlate the RFLP maps derived in different laboratories.
Wendy Behrendsen, Debi Blair and David Grant
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