Exploring Ty Resistance Genes and Genetic Diversity in Improved Tomato Lines Selected from Commercial Hybrids

Background Tomato yellow leaf curl disease (TYLCD), caused by the Tomato Yellow Leaf Curl Virus (TYLCV), threatens global tomato production, making resistance breeding crucial. This study evaluated 17 tomato lines (TLs; F ₆ and F ₇ ) derived from F ₁ hybrids ‘65010’, ‘Nairouz’, ‘SVTD8320’, and ‘Tyrmes’ for 29 traits related to TYLCD resistance, vegetative growth, yield, and fruit quality under natural whitefly-mediated inoculation during the 2022 and 2023 fall seasons. Genetic diversity was assessed using phenotypic (PCV) and genotypic (GCV) coefficients of variation, heritability ( h ² _b ), and genotypic correlation coefficient ( r _g ) for the estimated traits. Results According to PCR-based specific markers, most TLs had multiple homozygous Ty resistance genes (> 2), except for TLs 1 and 3 ( Ty-4 gene only) and TL5 (no Ty genes). TLs showed high TYLCD tolerance with low symptom severity and viral replication, as detected in symptomless plants using TEM scanning. Most traits exhibited significant phenotypic variation across seasons, with 21 out of 29 traits showing high or moderate values of PCV (11.94–65.44%) and GCV (2.45–64.72%), and high h ² _b (75.0-99.6%). This indicates that TLs have high genetic diversity and are suitable for phenotypic selection to improve TYLCD tolerance and productivity. Significant estimates of r _g revealed that selecting for low TYLCD severity and high total yield, average fruit weight, fruit firmness, and fruit TSS content could reduce fruit vitamin C content and number of fruit locules while increasing early and marketable yield, number of plant fruits, fruit equatorial diameter, and fruit β-carotene content. Principal component analysis reduced the traits to five PCs, representing 85.07% of the total variance. Hierarchical cluster analysis classified TLs into four clusters, with Clusters 1 (TLs 1 and 5–12) and 3 (TL4 only) performing better for traits. Conclusion Segregating commercial hybrid generations can be used to develop lines with multiple Ty resistance genes and enhanced traits. Crossing lines between these clusters could improve TYLCD tolerance and yield in future F ₁ hybrids. Some lines harbor TYLCD resistance genes, like TL5, which exhibited high TYLCD tolerance without detecting Ty genes. Further research is needed to identify the specific genes and mechanisms for resistance in these lines.