Submitted to: Molecular Genetics and Genomics
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/25/2007
Publication Date: 1/4/2008
Citation: Wang, C., Ulloa, M., Roberts, P.A. 2008. A transgressive segregation factor (RKN2) in Gossypium barbadense for nematode resistance clusters with gene rkn1 in G. hirsutum. Mol. Genet. Genome 279:41-52. Interpretive Summary: The southern root-knot nematode is an important pest of cotton and many other crops worldwide. Attack by this nematode increases the severity of problems with Fusarium wilt in cotton. Host plant resistance is one of the most effective, economical, and environmentally safe means of managing root-knot nematodes and Fusarium wilt in cotton. Recently, the gene, rkn1, providing resistance to the root-knot nematode, was identified from crosses of the nematode-resistant Acala variety NemX and the susceptible Acala variety SJ-2. Genetic analyses based on nematode injury and egg production on cotton roots, combined with molecular marker analyses revealed that the gene rkn1 interacted with a gene (named RKN2) in the susceptible Pima variety S-7 to produce plants highly resistant to nematodes. Studies of crosses of resistant NemX cotton with the susceptible Pima S-7 used mapped molecular markers and the incidence of resistance to identify a marker (MUCS088) linked to the gene RKN2. Further analysis suggested RKN2 was located on the same chromosome as rkn1. These results show that a highly susceptible parent can contribute to nematode resistance when crossed with a resistant parent. This process is called transgressive segregation. Highly resistant lines from such crosses can be used as sources of resistance in cotton breeding. Also, the MUCS088 marker can be used to monitor for the presence of RKN2 in breeding populations. The close genetic location of the rkn1 and RKN2 genes provides an important model system for studying transgressive segregation in cotton.
Technical Abstract: Host plant resistance is an important strategy for managing root-knot nematode (Meloidogyne incognita) in cotton (Gossypium L.). Here we report evidence for enhanced resistance in interspecific crosses resulting from transgressive segregation of clustered gene loci. Recently, a major gene, rkn1, on chromosome 11 for resistance to M. incognita in Acala NemX was identified using an intraspecific G. hirsutum cross with susceptible Acala SJ 2. Using interspecific crosses of Acala NemX x susceptible G. barbadense cv. Pima S 7, F1, F2, F2:3, back-cross, and test-cross NemX x F1 (Pima S 7 x SJ 2), parental entries and populations were inoculated in greenhouse tests with M. incognita. Genetic analyses based on nematode-induced root galling and nematode egg production on roots, and molecular marker analysis of the segregating interspecific populations revealed that gene rkn1 interacted with a gene (designated as RKN2) in susceptible Pima S-7 to produce a highly resistant phenotype. RKN2 did not confer resistance in Pima S-7, but when combined with rkn1 (genotype Aa or aa), high levels of resistance were produced in the F1 and segregating F2, F3, and BC1F1 populations. One SSR marker MUCS088 was identified tightly linked to RKN2 within 4.4 cM in a NemX x F1 (Pima S-7 x SJ-2) test-cross population. Using mapped SSR markers and interspecific segregating populations, MUCS088 linked to the transgressive gene from the susceptible parent and was located in the vicinity of rkn1 on chromosome 11. Diverse genome analyses among A and D genome diploid and tetraploid cottons revealed that marker MUCS088 (165 bp and 167 bp) is derived from G. arboreum, A2 diploid genome. These results demonstrated that a highly susceptible parent contributed to nematode resistance via transgressive segregation. Derived highly resistant lines can be used as improved resistance sources in cotton breeding, and MUCS088 can be used to monitor RKN2 introgression in diverse populations. The close genomic location of the transgressive resistance determinants provides an important model system for studying transgressive segregation in plants.