Use of naturally-occurring enzyme variation to detect and map genes controlling quantitative traits in an interspecific backcross of tomato

Summary A backcross between the inbred parents, Lycopersicon esculentum (LA 490, recurrent parent) and Solanum pennellii (LA 716) constitutes ideal material for an investigation of linkage between enzyme marker genes and loci determining quantitative traits (QTL): the chromosomes of the parents are the same in number and are homosequential; the parents differ for well-defined alleles at 12 enzymatic loci, which represent at least 8 of the 12 chromosomes, and large differences exist between the parents for measurable metric traits. The four metric traits investigated in this study were inherited in typical quantitative fashion; the marker segregations fit monogenic ratios for all except four loci, for which there were deviations of three favoured alleles of the recurrent parent, in agreement with frequent observations in interspecific hybrids. Association between QTL and mapped enzyme markers were detected statistically from calculations based on the normal distribution. In this manner a minimum of 21 QTL were mapped. Two pairs of linked enzyme markers permitted application of a three-point mapping procedure by comparing the data with models expected for various positions of QTL. Data for three QTL fitted predictions specified for a locus closer to one marker than the other, and data for one QTL met all criteria save one for a locus between the two markers. Certain QTL coding for stigma exsertion and leaf ratio have effects opposite to those expected from parental values; i.e. , for these traits both parents contribute alleles coding for positive and negative effects, resulting in the observed transgressive variation. When values observed for QTL linked with pairs of independent markers are compared with sums of separate values for each, significant epistatic interactions are detected. Although the extent of epistasis could not be determined precisely, it is probably not large. The paired interactions identified additional QTL whose individual effects were not otherwise detectable. Enzymatic markers offer a number of advantages for the analysis of quantitative genetic variation which include: (a) lack of detectable effects on the morphology and physiology of the soma, (b) codominant expression, permitting exact identification of genotypes, and (c) lack of epistasis, allowing classification of any number of such markers segregating simultaneously. A tester stock with linked enzyme markers on each chromosome therefore could conceivably be utilized to analyze