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Abstract: 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 [...]

We determined the relative positions of the tandem-repeat molecular cytogenetic marker B77, translocation breakpoints, and telosome arms in Gossypium hirsutum cytogenetic stocks by fluorescence in situ hybridization (FISH) analysis of meiotic quadrivalents in 16 single and 2 double translocation heterozygotes and five monotelodisomics. Results delimited the B77 FISH locus to the right arm of the D-subgenome chromosome 14 (14R) and the short arm (14sh), respectively. By equating 14R with 14sh and 14L (left) with 14Lo (long), the findings established a unified nomenclature for the arms of chromosome 14. Previously reported chromosome 14 arm locations were confirmed for four of the five translocations involving chromosome 14, namely NT1L-14L (2780), NT2R-14R (2B-l), NT14L-23R (2777), and NT14R-24R (2781), whereas the location of breakpoint T6L-14L was not confirmed and was reassigned to arm 14R. When used as a probe on Southern blots, the B77 signal was associated with a terminus of the D-subgenome RFLP linkage group (LG) D04 by linkage analysis of an interspecific [F.sub.2] population, now known to be chromosome 20. However, additional codominant DNA marker information in the affected region excluded the B77 polymorphism detected by Southern blot hybridization from chromosome 20 and, indeed, from the remainder of the genome.

A library of 'introgression lines' containing Solanum lycopersicoides chromosome segments in the genetic background of cultivated tomato (Lycopersicon esculentum) was used to study factors affecting homeologous recombination. Recombination rates were estimated in progeny of 43 heterozygous introgressions and whole-chromosome substitution lines, together representing 11 of the 12 tomato chromosomes. Recombination within homeologous segments was reduced to as little as 0-10% of expected frequencies. Relative recombination rates were positively correlated with the length of introgressed segments on the tomato map. The highest recombination (up to 40-50% of normal) was observed in long introgressions or substitution lines. Double-introgression lines containing two homeologous segments on opposite chromosome arms were synthesized to increase their combined length. Recombination was higher in the double than in the single segment lines, despite a preference for crossovers in the region of homology between segments. A greater increase in homeologous recombination was obtained by crossing the S. lycopersicoides introgression lines to L. pennellii--a phylogenetically intermediate species--or to L. esculentum lines containing single L. pennellii segments on the same chromosome. Recombination rates were highest in regions of overlap between S. lycopersicoides and L. pennellii segments. The potential application of these results to breeding with introgression lines is discussed.

The wild nightshades Solanum lycopersicoides and Solanum sitiens are closely affiliated with the tomatoes (Lycopersicon spp.). Intergeneric hybridization with cultivated tomato (Lycopersicon esculentum) is impeded by strong reproductive barriers including hybrid sterility and suppressed recombination. Conservation of genome structure between these nightshades and tomato was studied by construction of a genetic map from F[sub 2] S. sitiens × S. lycopersicoides and comparison with existing maps of tomato. Owing to self-incompatibility of the F[sub 1] , two hybrid plants were crossed to obtain a population of 82 F[sub 2] individuals. Using 166 previously mapped RFLP markers and 5 restriction enzymes, 101 loci polymorphic in the S. sitiens × S. lycopersicoides population were identified. Analysis of linkage between the markers resulted in a map with 12 linkage groups covering 1192 cM and one unlinked marker. Recombination rates were similar to those observed in tomato; however, significant segregation distortion was observed for markers on 7 out of the 12 chromosomes. All chromosomes were colinear with the tomato map, except for chromosome 10, where a paracentric inversion on the long arm was detected. In this region, S. sitiens and S. lycopersicoides share the same chromosomal configuration previously reported for potato (S. tuberosum) and pepper (Capsicum), suggesting that of tomato is derived. The 10L inversion explains the lack of recombination detected among homeologous chromosomes of intergeneric hybrids in this region. On this basis, we recognize two principle genomes, designated L for the Lycopersicon spp., and S for S. lycopersicoides and S. sitiens, the first examples of structural differentiation between tomato and its cross-compatible wild relatives.Key words: tomato, Solanum lycopersicoides, Solanum sitiens, RFLPs, comparative linkage maps.Les solanacées sauvages Solanum lycopersicoides et Solanum sitiens sont très apparentées à la tomate (Lycopersicon spp.). Cependant, l'hybridation intergénérique avec la tomate cultivée (L. esculentum) est inhibée par d'importantes barrières reproductives dont la stérilité des hybrides et une suppression de la recombinaison. La conservation de la structure des génomes chez ces trois espèces a été examinée en produisant une carte génétique à partir d'une population F[sub 2] issue d'un croisement S. sitiens × S. lycopersicoides et en comparant celle-ci avec les cartes existantes chez la tomate. En raison de l'auto-incompatibilité de la F[sub 1] , deux hybrides ont été croisés pour obtenir une population de 82 plantes F[sub 2] . En utilisant 166 marqueurs RFLP dont la position était déjà connue et cinq enzymes de restriction, 101 locus polymorphes au sein de la population S. sitiens × S. lycopersicoides ont été identifiés. Une analyse de linkage a permis de produire une carte à 12 groupes de liaison totalisant 1192 cM, un seul marqueur ne montrant de liaison à aucun autre. Les taux de recombinaison étaient semblables à ceux observés chez la tomate, cependant des distorsions significatives de la ségrégation ont été observées pour des marqueurs sur 7 des 12 chromosomes. Tous les chromosomes étaient colinéaires par rapport à la carte de la tomate, à l'exception du chromosome 10 sur lequel une inversion paracentrique a été notée sur le bras long. Dans cette région, le S. sitiens et le S. lycopersicoides ont la même configuration chromosomique que chez la pomme de terre (S. tuberosum) et le poivron (Capsicum), une observation qui suggère que c'est la configuration de la tomate qui a divergé. L'inversion sur 10L explique l'absence de recombinaison détectée entre les chromosomes homéologues dans cette région chez les hybrides. Sur la base de ces observations, on distingue deux types génomiques principaux : le génome L chez les espèces du genre Lycopersicon et le génome S chez le S. lycopersicoides et le S. sitiens. Il s'agit du premier exemple d'une différenciation génomique entre la tomate et des espèces sauvages apparentées avec lesquelles elle peut être croisée.Mots clés : tomate, Solanum lycopersicoides, Solanum sitiens, RFLP, cartographie génétique comparée.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]