In addition to a classical genetic map that is based on linkage data with isozyme, disease resistance and morphological loci, a 'molecular map' has recently been constructed by Tanksley and co-workers on the basis of loci detected by restriction fragment polymorphisms (RFLP's). Thus far, both mpas are far from being integrated, since only a limited number of markers (mainly isozyme markers) are present on both maps. A fully integrated map is needed in those cases where on wants to use RFLP's as tools in chromosome walking techniques to clone tightly linked genes, that are only known from their variant phenotype. One way to achieve integration, at least for some defined chromosomal segments, is to use isogenic lines (Young et al. 1989). Another approach is to combine the mappin of RFLP- and morphological markers by using populations in which both types of markers segregate. In such a situation, differeneces in recombinatin in the L.esculentum/L. pennellii hybrid (Rick, 1969) will not interfere with the linear order of the loci.
As a first step in constructing a physical/genetic map for chromosome 6 of tomato, we started witha program to improve the accuracy of the map position of the "classical" markers on this chromosome. Thus far, the linkage data used in constructing the map of chromosome 6 have not been combined with procedures that take into account the statistical accuracy of the data. To establish more precise map positions of the various loci on chromosome 6, we have therefore reevaluated the published linkage data, together with new data obtained by ourself for yv/m-2, yv/c, yv/gib-1, m-2/c, m-2/gib-1, c/gib-1 (Koorneef et al. in preparation), using a computer mapping program that has been developed by Dr. P. Stam and is based on procedures described by Jensen and Jorgensen (1975). The results of this exercise are shown if Fig. 1. In broad outline the current map (Fig 1A) is similar to the most recent map published in Tomato Genetics Cooperative report 37 (Fig 1B). Some notable differences, however, deserve to be mentioned. The change in position of def is mainly due to incorporation of the linkage data fro def/Mi, which are based on a relatively large population (Gilbert and Centina, 1965), resulting in an inacurate estimate of the recombination fraction (0.38+/-0.02). The reversion of the order c, sp, B as compared with Tomato Genetics Cooperative 1987 map, is based on the accurate linkage data for this region obtained by Ito and Currence (1964). This order was furthermore confirmed by our finding of a recombinant c/sp in which recombination had also occurred between c and m-2. The loci Adh-2, ms-33 and Pts were not included in this map, because linkage with only one other chromosome 6 marker could be found in the literature. The data for d-2 were omitted because of the inconsistency for the d-2/tl data (Rick et al. 1973). Ves (-vf), which had not been located on the map so far, could be mapped because linkage data with yv and c (Boynton and Rick, 1965) and with coa (Zobel et al., 1969) were available. Gib-1 is a non-germinating, gibberellin responsive dwarf (Groot et al. 1988) for which recently accurate linkage data were obtained (Koornneef et al. in preparation).
Mutants mu and rv-3, which are morphologically very similar, did not show complementation to wild type in the F1 and therefore probably are allelic. Complementation was neither observed between the two potato leaf mutants c and int, an observation also described by Kerr (1960). His finding of wild-type plants in the F2 progeny of this hybrid has not yet been checked by us. To increase the accuracy of the map any further, especially in regions with apparent cluster of loci and to allow mapping of additional markers, new linkage data are being generated.
Figure 1. Linkage map of tomato chromosome 6 (A) based on segregation data
published in Tomato Genetics Cooperative reports 8:15-17, 9:20-21,
10:16-17, 12:43-44, 12:20-21, 14:14-15, 15:24-27, 16:29-30, 18:35-36,
19:31, 20:52-54, 24:22-24, 27:15, 30:20-21 and data obtained by
Koornneef et al. (in prep.) as compared with the map (B) published by
Mutschler et al. (1987). Newly mapped loci are indicated by a arrow.
Literature cited:
Groot, S.P.C. et al. 1988 Planta 174:500-504.
Boynton, J.E. et al. 1965 Tomato Genetics Coorperative Report 15:24-27.
ITO, P. and T.M. Currence 1964 Tomato Genetics Cooperative Report 14:14-15.
Gilbert, J.C. and L. Centina 1965 Tomato Genetics Cooperative Report 15:34.
Jensen, J. and J.H. Jorgensen 1975 Hereditas 80:5-16.
Kerr, E.A. 1960 Tomato Genetics Cooperative REport 10:19.
Mutschler, M.A. et al. 1987 Tomato Genetics Cooperative Report 37:5-34.
Rick, C.M. 1969 Genetics 62:753-768.
Young, N.D. et al. 1988 Genetics 120:579-585.
Zobel, R.W. et al. 1969 Tomato Genetics Cooperative Report 19:30-31.