Identification of Root-knot Nematode Resistance Loci in Gossypium hirsutum Using Simple Sequence Repeats
Gossypium hirsutum, upland cotton, is one of the major crops grown in the United States and the world. Upland cotton is cultivated in areas that are ideal breeding grounds for the difficult to manage, southern root-knot nematode (RKN), Meloidogyne incognita. Host plant resistance is the most effective way to control RKN populations. However, resistance used in most breeding programs stems from a few related sources. Novel sources of resistance have been identified but have yet to be introduced into elite breeding lines or genetically studied. The objectives of this study are two-fold. The first is to develop elite germplasm by introgressing RKN resistance from primitive accessions into modern cotton genotypes via backcrossing. The second is to use simple sequence repeats (SSRs) to identify loci associated with RKN resistance in the primitive accessions. The genotypes used will be: 1) inoculated with M. incognita, 2) phenotypically analyzed by measuring the nematode reproduction as eggs per gram of fresh root and host response using a root gall index, 3) genetically evaluated by using SSR markers to detect polymorphisms between the RKN resistant TX accessions and DP90 (susceptible genotype), and 4) analyzed using linkage and mapping software.
Elite germplasm that contains: 1) high yield potential and a high level of RKN- resistance or 2) high fiber quality and RKN-resistance was developed by performing two backcrosses based on phenotypic analyses. A third screen is currently underway to ensure the introgression of the RKN resistance genes. Agronomic tests will need to be done before the germplasm is released. Genetic analyses using SSR-based primer sets of the TX accessions did not yield expected results. Of the 508 primers sets tested, only 31 were polymorphic between the TX accessions and DP90. A bulked segregant analysis approach was used to test the 31 primer sets on the resistant and susceptible bulks of the F2 population but no polymorphisms were seen. However, analyses found that the TX accessions were more genetically similar to Mexico Wild Jack Jones than to Clevewilt 6-3-5. More work needs to be done to understand the mechanism of RKN resistance in the TX accessions.