Isozyme monitoring of genetic variation in Lycopersicon.
Linkages with isozymic loci facilitate the investigation of certain classes of genetic variation. Due to the mapping of 20 isozymic loci on nine of the 12 chromosomes of the cultivated tomato (Lycopersicon esculentum), much progress has been made in these applications, particularly in the analysis of interspecific hybrids. Isozymes can expedite the selective elimination of inferior wild parent germ plasm in backcross transfer of desired genes to the cultivated parent. Allelic isozyme constitution also aids in identification of lines, particularly in evaluating the purity of F1 hybrid cultivars. Advantages that isozymes impart to such investigations are: (1) unequivocal classification of phenotypes, (2) detection of heterozygotes, (3) lack of epistasis between isozyme loci, (4) lack of effect of allelic isozymes per se on morphology or physiology, (5) prolific source of monogenic markers, and (6) phenotyping at early developmental stages. Each of these attributes can be exploited to great advantage, but collectively they constitute a formidable argument for monitoring genetic variation by means of isozymes. Linkages between isozyme loci and qualitative loci can be exploited as in the monitoring of Mi (gene for root-knot nematode resistance derived from L peruvianum) by the very tightly linked Aps-1(1); in similar fashion, Prx-2(1) serves as a useful marker for ms-10 (male sterility). Asp-1 monitoring in the former is more reliable than testing for nematode resistance per se; codominance of Prx-2 alleles of the latter solves problems incurred by the recessiveness of ms-10; in both instances phenotypes can be ascertained at earlier growth stages for isozymes than for economic traits. In the first backcross of the interspecific hybrid L esculentum x Solanum pennellii to the former, the segregation of four quantitative traits was monitored by allelic isozymes at 12 loci, situated on at least eight chromosomes, covering approximately 60% of the known tomato genome. At least five quantitative trait loci (QTL) were found to determine each of the four traits. Each parent contributes alleles with positive as well as negative effects, the greatest balance for stigma exsertion, the trait also exhibiting the greatest extent of transgressive segregation. Three pairs of linked isozymic loci permitted a crude form of three-point mapping of the associated QTL. interactions between QTL linked with pairs of isozymic genes were tested in all possible combinations; 18 of the 274 comparisons showed significant interactions, indicating epistasis.(ABSTRACT TRUNCATED AT 400 WORDS)