A molecular linkage map of tomato based on a cross between Lycopersicon esculentum andL. pimpinellifolium and its comparison with other molecular maps of tomato

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Abstract
The wild species related to the cultivated tomato, Lycopersicon esculentum Mill., are a rich source of useful genes for germplasm improvement and varietal development. Characterization and utilization of exotic germplasm can be accelerated by the use of molecular linkage maps. Recently, we have identified an accession (LA722) within L. pimpinellifolium Jusl., a closely-related, red-fruited wild species of tomato, which exhibits a number of desirable agricultural characteristics including salt tolerance, disease resistance, and high fruit quality. The limited DNA marker polymorphism between L. esculentum and L. pimpinellifolium, however, restricts the use of the high-density molecular map of tomato, which was previously constructed based on a cross between L. esculentum and L. pennellii Corr., for marker-assisted identification and utilization of useful genes in LA722. To overcome this problem, we have constructed a linkage map based on restriction fragment length polymorphisms (RFLPs) which were identified between LA722 and a fresh-market tomato breeding line (NC84173). The mapping population (consisting of 119 BC1 individuals) was genotyped for 151 RFLP markers, including 17 germination related and 2 potassium transporter cDNAs. The DNA markers spanned approximately 1192 cM of the tomato genome with an average distance of 7.9 cM between markers. The length of the map and the linear order of the markers were in good agreement with those of the previously published molecular maps of tomato, however, there were considerable differences in the distribution of recombinations along the chromosomes. Comparison of all seven published molecular maps of tomato, which were constructed based on different inter- and intraspecific crosses, indicated that some chromosomal regions were more stable than others in terms of the frequency of recombinations. Similarities and differences among tomato molecular maps are discussed in relation to phylogenetic relationships between parents of the mapping populations. In comparison, a L. esculentum ×L. pimpinellifolium map should be more useful than other interspecific maps for marker-assisted exploitation of genetic variation that exists within L. pimpinellifolium or L. esculentum; the latter is because of the extensive introgression of genes from L. pimpinellifolium into L. esculentum, which occurred over time by natural means or through plant breeding.Key words: Lycopersicon esculentum, L. pimpinellifolium, genetic marker, molecular map, recombination, restriction fragment length polymorphism (RFLP), tomato.
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