A genetic map of the Nicotiana alata S locus that includes three pollen-expressed genes

The S locus of solanaceous plants includes separate genes that control the self-incompatibility phenotype of the pistil and of the pollen. The gene co ntrolling the self-incompatibility phenotype of the pistil encodes an extra cellular ribonuclease, the S-RNase. The gene(s) controlling the self-incomp atibility phenotype of pollen (the pollen-S gene) has yet to be identified. As part of a long-term strategy to clone the pollen-S gene by chromosome w alking, a detailed map of the region near the S locus of Nicotiana alata wa s generated using a total of 251 F-2 plants. The map spans an interval of a pproximately 2.6 cM and contains five markers as well as the S-RNase gene. Two markers were detected with heterologous probes that also detect sequenc es linked to the S locus of Solanum tuberosum and the homologous region of the Lycopersicon genome. Three markers were identified by differential disp lay using N. alata pollen RNA as a template. One of these markers is a poll en-expressed sequence, 48A, which detects a polymorphic marker no more than 0.5 cM from the S locus. RNA blot analysis indicates that the 48A gene is expressed primarily during pollen development after the completion of meios is and is therefore a candidate for the pollen-S gene. The utility of these markers and the possible involvement of 48A in the molecular mechanism of self-incompatibility are discussed.