Segregation of isozyme markers and cold tolerance in an interspecific backcross of tomato

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Abstract
An interspecific backcross was obtained between the cultivated tomato, L. esculentum, and a high-altitude, cold-tolerant L. hirsutum, using the former as the recurrent pistillate parent. An individual plant of L. hirsutum which possessed maximum allelic differences for enzyme loci, with respect to those of L. esculentum, was selected as the staminate parent. Allelic differences were found at seventeen enzyme loci, marking eight of the twelve chromosomes of Lysopersicon. Significant distortions in the monogenic segregations were detected at six enzyme loci. Four loci skewed with an excess of esculentum homozygotes and two with an excess of hirsutum heterozygotes. Significant heterogeneity between the segregations of subgroups was found at some loci, when the BC1 population was divided into two subgroups according to their physiological age (plastochron index). This indicates selection at the germination/seedling stage may account for some of the skewness. Differential growth at low temperatures, measured by increments in the plastochron index, was used as the criterion for cold tolerance. Linkages between segregating enzyme loci and genes responsible for cold tolerance were tested via statistical comparisons of the means of plastochron index increments at low temperatures for esculentum homozygotes vs. those of hirsutum heterozygotes at each locus. A minimum of three quantitative trait loci (QT2) responsible for growth at low temperatures were detected, two had positive effects, and the other, negative. One marker locus, Pgi-1, gave a significant and positive effect only at low temperatures.