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Transgressive Segregation, Heritability, and Number of Genes Controlling Durable Resistance to Stripe Rust in One Chinese and Two Italian Wheat Cultivars

Wheat (Triticum aestivum) cvs. Libellula (LB), San Pastore (SP), and Xian Nong 4 (XN4) possess durable resistance to stripe rust, caused by Puccinia striiformis f. sp. tritici, and cv. Ming Xian 169 (MX169) is highly susceptible to the rust. Inheritance of stripe rust resistance was studied by crossing the four cultivars and evaluating the resistance of parental, F₁, F₂, backcross, and F₃ plants in the fields. Transgressive segregation for resistance was observed in the resistant by resistant crosses of LB × XN4 and XN4 × SP, but not in cross LB × SP. These results indicate that (i) the resistance genes in XN4 are different from those in LB and SP, and (ii) LB and SP share common resistance genes. The number of genes segregating for the resistance was estimated by quantitative methods from the data of F₂, backcross, and F₃ populations. LB and XN4 appear to have two to three resistance genes, and SP appears to have two to four resistance genes when crossed with MX169. The resistance gene number in resistant by resistant cross LB × XN4 was four to five, approximately equal to the sum of the genes in LB and XN4. Similarly, the resistance gene number in cross XN4 × SP was approximately equal to the sum of the genes in XN4 and SP. Broad-sense heritability was high in all crosses except LB × SP. Compared with the three MX169-involved crosses, narrow-sense heritability was higher in LB × MX169 and SP × MX169 crosses than in the XN4 × MX169 cross. The LB × XN4 and XN4 × SP crosses showed moderate narrow-sense heritability.