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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.

Maize meiotic mutant desynaptic (dy) was tested as a candidate recombination modifier gene because its effect is manifested in prophase I. Recombination rates for desynaptic (dy) and its wild type were compared in two ways: (1) segregation analysis using six linked molecular markers on chromosome 1L and (2) cytogenetic analysis using fluorescence in situ hybridization (FISH)-aided meiotic configurations observed in metaphase I. Chromosome 1L map lengths among the six linked markers were 45-63 cM for five F2 dy/dy plants, significantly lower than the wild-type [F.sub.2] map distance of 72 cM. Chromosomes 2 and 6 were marked with rDNA FISH probes, and their map lengths were estimated from FISH-adorned meiotic configurations using the expectation-maximization algorithm. Chiasma frequencies for dy/dy plants were significantly reduced for both arms of chromosome 2, for chromosome arm 6L, and for eight unidentified chromosomes. There was a notable exception for the nucleolus-organizing region-bearing arm chromosome arm 6S, where dy increased chiasma frequency. Maize meiotic mutant desynaptic is a recombination modifier gene based on cytogenetic and segregation analyses.