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Meiotic chromosome pairing was studied in introgression lines of cultivated tomato, Lycopersicon esculentum (= Solanum lycopersicum), containing 1 or 2 chromosome segments from the wild species Solanum lycopersicoides. Genomic in situ hybridization (GISH) was used to compare the relative lengths at diakinesis of the different introgressed segments and to measure the chiasmate arm frequency for the chromosome pair involved in the introgression(s). Longer segments generally produced stronger GISH signals than shorter segments. GISH signal intensity also depended on whether or not an introgressed segment encompassed the centromeric region. For example, a 29 cM segment that included the centromeric region produced a stronger GISH signal than a 42 cM segment that did not. In each line the chromosome arm containing the homeologous segment showed a reduction in chiasmate arm frequency that was most pronounced in lines with long segments. This reduction was accompanied by an increased chiasmate arm frequency on the other arm. Double introgression lines, heterozygous in repulsion phase for 2 introgressions on opposite chromosome arms, showed a lower frequency of chiasmata than single introgression lines. Pairing failure, indicated by the presence of univalents, was highest in the double introgression and whole chromosome substitution lines. These results are discussed with respect to observations of suppressed recombination in these stocks and potential practical implications for reducing linkage drag in breeding programs.

The goal of the present experiments was to transfer the chromosomes of Solanum sitiens (syn. Solanum rickii) into cultivated tomato ( Lycopersicon esculentum). By crossing an allotetraploid L. esculentum x Solanum sitiens hybrid to sesquidiploid L. esculentum x S. lycopersicoides, a trigenomic hybrid (2n+14=38) was obtained. Analysis of the latter by GISH (genomic in situ hybridization) indicated it contained a full set of 12 S. sitiens chromosomes, plus two extras from S. lycopersicoides. This and other complex hybrids were pollinated with Lycopersicon pennellii-derived bridging lines to overcome unilateral incompatibility. A total of 40 progeny were recovered by embryo rescue, including diploids and aneuploids (up to 2n+8). In order to determine the origin of chromosomes and the location of introgressed segments, progeny were genotyped with RFLP markers. S. sitiens-specific markers on all chromosomes, except 6 and 11, were detected in the progeny. Several S. sitiens chromosomes were transmitted intact, either through chromosome addition (i.e., trisomics) or substitution (i.e., disomics). Recombination between S. sitiens and L. esculentum was detected on most chromosomes, in both diploid and aneuploid progeny. A monosomic alien addition line for S. sitiens chromosome 8 was identified, and the extra chromosome was stably transmitted to approximately 13% of the backcross progeny. This study demonstrates the feasibility of gene transfer from S. sitiens to L. esculentum through chromosome addition, substitution, and recombination in the progeny of complex aneuploid hybrids.