36 A L . esculentum x L . pennellii backcross recombinant inbred population Vision, T . J . *, Xu, Y . **, Van Eck, N . **, Brown, D . G . *** and Tanksley, S . D . ** **USDA - ARS Center for Agricultural Bioinformatics, Ithaca NY 14853 **Dept . of Plant Breeding, Emerson Hall, Cornell University, Ithaca NY 14853 ***Dept . of Computer Science, University of Waterloo, Waterloo, Ontario, N2L 3G1 Canada We have been developing a recombinant inbred population of tomato derived from a L . esculentum x L . pennellii backcross .   The goal is to have a population that (i) is permanent, (ii) can be grown from seed (rather than tissue culture), (iii) is highly polymorphic for molecular markers and (iv) provides high mapping resolution with a small number of lines .   The population is thus designed to be complementary to other L . esculentum x L . pennellii mapping resources, such as the population of overlapping introgression lines developed by Eshed and Zamir (1995) and an F2 population, recently developed by Tanksley and colleagues (unpublished), which is being immortalized by tissue culture .   The parents of the present population are Lycopersicon esculentum E6203 and a self - compatible accession of L . pennellii (LA716) .   Following a backcross of the F1 into E6203 and two generations of selfing, individuals were genotyped for 119 RFLP markers spanning all 12 chromosomes at roughly even intervals .   All RFLP markers had been mapped on a previous F2 population (Tanksley et . al . 1992) and the majority have also been mapped in the new F2 population mentioned above .   Each individual backcross line has been fixed for segregating loci over the course of three subsequent generations by marker - assisted selection (MAS) .   In addition to accelerating the fixation of segregating loci, MAS has been used to maximize the mapping resolution of the population as a whole, using specialized software designed for that purpose (Vision et al . 2000, D . Brown, unpublished) .   The lines have been re - genotyped for 112 markers in the current, fifth backcross, generation .   Only a small number of heterozygous loci still remain .    Of the 100 original backcross lines, 67 were lost due to reduced levels of germination, seedling survival and fertility in the advanced selfing generations .   In the 2nd selfing generation, survival to flowering and total seed set were measured .   There were inverse correlations between (i)   the proportion of markers homozygous for the LA716 alleles and (ii)   the number of crossovers on all 12 chromosomes with both measured traits .