Molecular mapping of chromosome segments introgressed from Solanum lycopersicoides into cultivated tomato (Lycopersicon esculentum)

The wild nightshade Solanum lycopersicoides (accessionLA2951) was backcrossed to the cultivated tomato (Lycopersicon esculentum cv ’VF36’), then inbred through single-seed descent for several generations. Over 300 backcross-inbred families thereby derived were genotyped at 139 marker loci, consisting of RFLPs, allozymes, and monogenic morphological markers, to identify introgressed S. lycopersicoides chromosomes and segments thereof. The pattern of genotypes observed in the lines indicated a high degree of overall synteny between the S. lycopersicoides genome and that of tomato. Two putative single-copy RFLP probes revealed secondary loci in this wide cross. Recovery of the L. esculentum genome was more rapid than expected, with an average value in the BC₂ generation of 97.8%, versus the expected value of 87.5%. This was due to widespread segregation distortion that favored L. esculentum alleles as well as a tendency for plants homozygous for in- trogressed segments to be partially or completely male-sterile, thereby preventing the fixation of S. lycopersicoides markers in many lines. Despite these difficulties, nearly every S. lycopersicoides marker (or approximately 98% of the genome, measured in centi Morgans) was represented in at least 1 backcross-inbred line, with only a region on chromosome 4L missing from the population as a whole. Although the extent of transmission and fixation of introgressed segments varied according to chromosome, overall approximately 66% of the S. lycopersicoides genome was represented by homozygous in- trogressions with sufficient fertility to reproduce by self-pollination. An excess of terminal (vs. interstitial) segments was noted, and putative heterozygous substitutions for chromosomes 6, 7, 8, and 10 were found. Recombination within certain introgressed regions was reduced over 100-fold. These backcross-inbred lines are expected to facilitate the genetic analysis of traits identified in S. lycopersicoides and their transfer into horticultural tomatoes. Received: 16 March 1999 / Accepted: 22 June 1999