Peanut root-knot nematode [Meloidgyne arenaria (Neal) Chitwood race 1] is a soilborne disease in peanut fields. The worm attacks the root system of peanut and markedly reduces its yield (see figure at right).
Application of nematicides is costly and raises environmental concerns. Only a moderate level of nematode resistance has been found in various peanut cultivars; however, near-immunity to nematode infection was discovered in several wild diploid Arachis spp.
Genes controlling resistance to nematodes were introgressed into peanut through interspecific hybridization and resulted in the release of the first nematode resistant cultivar COAN (Simpson and Starr 2001). Subsequent breeding efforts have been carried out in multiple breeding programs to pyramid other desirable agronomic traits with nematode resistance. Screening for nematode resistance by greenhouse inoculation is labor intensive and time consuming. In addition, the plant roots inoculated with nematodes need to be stained for egg and gall quantification which is a destructive assay. In our lab, and in collaboration with USDA scientists Corley Holbrook and Patty Timper, we developed a reliable DNA molecular marker to screen segregating populations for nematode resistance trait (Chu et al. 2007). This marker has a 6% recombination frequency with the resistance gene.
A second trait for which marker-assisted selection is being performed is high oleic to linoleic acid (O/L) ratio. It has been shown that oleic acid can reduce the risk of heart disease by decreasing LDL synthesis. A high O/L ratio also increases the shelf life of peanut products. Peanut seeds contain ~50% oil. Oleic and linoleic fatty acids comprise ~80% of peanut oil. Normally the O/L ratio in peanut cultivars is 3:1. F435, a high oleic acid spontaneous mutant with an O/L ratio of 35:1, was identified in 1987 (Norden et al. 1987, Peanut Sci 14:7). Two delta-12-desaturases (ahFAD2A and ahFAD2B), the enzyme responsible for converting oleic to linoleic acid, also were discovered in peanut (Jung et al. 2000).
Dysfunction of both genes is required to produce a high O/L peanut. We showed that a mutant form of ahFAD2A is prevalent among runner-type cultivars and therefore selection for a mutant ahFAD2B typically is sufficient to achieve high O/L (Chu et al. 2007, 2009).
Markers for both nematode resistance and high O/L have been used to combine the traits in high oleic ‘Tifguard’. Cost-effective, highly predictive molecular assays were designed and validated by correlation with phenotypic analyses for these two traits. Using molecular markers has more than halved the time for conversion of normal oleic acid Tifguard, a nematode resistant cultivar, to a high-oleic acid Tifguard (Chu et al. 2011).