Cultivated tomato (Solanum lycopersicum L.) accessions have been susceptible to the whitefly-transmitted begomoviruses Tomato yellow leaf curl virus (TYLCV) and Tomato mottle virus (ToMoV) that can cause serious crop damage. S. habrochaites accession LA1777 has been reported to be resistant to TYLCV. To locate putative virus resistance genes, 89 recombinant inbred lines (RILs) previously developed from LA1777 in a tomato background, LA1777 and the susceptible RIL parent E6203, were screened against the begomoviruses TYLCV and ToMoV. An initial study showed 18 RILs had less disease severity to TYLCV or ToMoV. Eight RILs had S. habrochaites alleles at TG27 (restriction fragment length polymorphism marker) on chromosome 1, three RILs had S. habrochaites alleles at TG202 on chromosome 7, and one RIL had S. habrochaites alleles at both marker loci. The RILs with these regions were intercrossed in 10 different cross combinations and F2 seeds were then obtained. The F2 progenies were inoculated separately with both viruses and then evaluated in the field. The F2 plants with less disease severity were selected, but most did not have the markers from the hypothetical resistance regions. The F3 progenies were then inoculated and rated for disease severity to both viruses. None of the F3s demonstrated any increased level of resistance, even if derived from F2s homozygous for the target regions from both chromosomes. All plants from every cross combination were susceptible for both TYLCV and ToMoV, suggesting that there is no begomovirus resistance in the LA1777 RIL population. Some limitations of capturing all genes in an RIL population derived from an outcrossing accession are discussed.
Aliya Momotaz, John W. Scott and David J. Schuster
Aliya Momotaz, Jay W. Scott and David J. Schuster
Solanum habrochaites S. Knapp and D.M. Spooner accession LA1777 have reported resistance to the sweetpotato whitefly (SPWF), Bemisia tabaci (Genn.). An interspecific F2 population of 171 plants between tomato [Solanum lycopersicum L. (formerly Lycopersicon esculentum Mill.)] and LA1777 was bioassayed against adult SPWF in a greenhouse using clip cages. A selective genotyping analysis was used with 11 resistant and 10 susceptible plants to locate resistance genes by testing them with molecular markers spanning most of the tomato genome at about 10-cM intervals. Markers in four regions were found to be associated with resistance, where three of them showed significantly strong associations and one showed a weak association through chi-square and analyses of variance. However, through quantitative trait locus (QTL) analysis using molecular markers, all four regions were identified as major QTLs with logarithm of odds (LOD) values of 4.87 to 5.95. The four QTLs were identified near the markers TG313 on chromosome 10, C2_At2g41680 on chromosome 9, TG523/T0408 on chromosome 11, and TG400/cLEG-37-G17 on chromosome 11. Multiple regression analysis produced similar results as above with fixed effects of single loci as well as interaction among some of the QTLs.