TGC REPORT No . 31, 1981 RESEARCH NOTES 18 Tanksley, S . D . Map positions of Prx - 1, Previous studies have revealed that Skdh - 1 and Pgm - 1 . Prx - 1 and Skdh - 1 are about 24 cM apart on chromosome 1 with au being situated between the two isozyme loci . However, the orientation of these loci with reference to the centromere has not been determined . To decide this point, Prx - 1 and Skdh - 1 variants derived from S . pennellii (LA 716) by several backcrosses to L . esculentum were crossed to ms - 32, a marker on chromosome 1 which maps near the centromere, 15 cM from au . The results, given below, indicate that Skdh - 1 is closer to ms - 32 than Prx - 1 . The reduced map distance between Skdh - 1 and Prx - 1 compared with the value obtained from our previous studies (24 cM) is probably due to the fact that the Skdh - 1 and Prx - 1 alleles used here were derived from S . pennellii . Wild chromosomes backcrossed several generations into esculentum are known to experience reduced recombination . These results combined with those previously published indicate the following orientation: Prx - 1 -- 7cM -- au -- 17 cM -- (Skdh - 1, ms - 32) .