In the present work the genetic distance between the Frl and Tm-2 loci was estimated with the purpose of
facilitating selection procedures for resistance breeding to both diseases and of contributing to the mapping of the
Frl locus in the tomato genome. Since the use of individual F2 plants in linkage analyses for disease resistance
can not exclude the mistaken assignment of phenotypes for each pathogen owing to unforeseen interactions
among the pathogens, F3 instead of F2 families were used (Vakalounakis and Williams, 1991).
The inbred line Motelle (Frl+/Frl+, Tm-2+/Tm-2+ was crossed with the inbred line IRB-301-31 (Frl/Frl, Tm-
2/Tm-2). Two hundred twenty two F2 plants were selfed, and from each F3 family different groups of 15-60
seedlings were tested for resistance to either F. oxysporum f.sp. radicis-lycopersici or TMV race 0. Since in F2
population there was departure from Mendelian segregation, contingency chi-square analysis was performed to
test for independent segregation of Frl and Tm-2 loci. Recombination fraction and its standard error was
calculated using the maximum likelihood procedure (Koornneef and Stam, 1992). This analysis was facilitated by
the use of the LINKEM computer program (Vowden et al., 1995).
A very tight linkage, equal to 5.1±1.07 map units, was detected between Frl and Tm-2 loci in tomato (Table
1), Since Tm-2 locus is located on chromosome 9 at position 22 (Mutschler et al, 1987), our finding suggest that
Frl be located on chromosome 9 at position 17 or 27.
The evidence of the very tight linkage between the Frl and Tm-2 loci provides obvious advantages to the
tomato breeders working on disease resistance since the selection procedure for one locus secures with a very
high probability the simultaneous selection of the other locus. The very strong linkage between the Frl and Tm-2
loci may explain the fast appearance of commercial tomato hybrids resistant to F. oxysporum f.sp. radicis-
lycopersici in the European market as soon as the fusarium crown and root rot disease appeared. Because F.
oxysporum f.sp. radicis-lycopersici is an important pathogen of tomato, we have began to test numerous TMV
resistant lines coming from various genebanks for possible resistance against F. oxysporum f.sp. radicis-
lycopersici.
Literature cited:
Koomneef, M. K., and P. Stam. 1992. Genetic analysis. In Methods in Arabidopsis research, pp. 83-99. Eds: C.
Koncz, N.-H. Chua, and J. Schell. Singapore: World Scientific.
Mutschler, M. A., S. D. Tanksley, and C. M. Rick. 1987. Linkage maps of the tomato (Lycopersicon esculentum).
Tomato Genetics Cooperative Report 37:5-34.
Vakalounakis, D. J., and P. H. Williams. 1991. A cotyledon double inoculation technique for evaluating resistance
to anthracnose (Colletotrichum orbiculare) and scab (Cladosporium cucumerinum) in cucumber. Annals of
applied Biology 118:273-282.
Vowden, C. J., M. S. Ridout, and K. R. Tobutt. 1955. LINKEM: A program for genetic linkage analysis. The Journal
of Heredity 86: 249-250.
Table 1. Linkage relationship between the Frl locus for resistance to Fusarium oxysporum f.sp. radicis-lycopersici
and the Tm-2 locus for resistance to TMV arranged in coupling phase and determined from F2 segregation data in
tomato
Segregation locus Frl x locus Tm-2
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