Genetic control in soybean of resistance to soybean cyst nematode race 4+

Abstract

Race 4+ refers to a population of soybean cyst nematode (SCN), Heterodera glycines, identified for the first time in Brazil, which differs from the classic race 4 for its ability to infect the cv. Hartwig, but not its ancestor PI 437654. Studies were carried out at Embrapa Soybean in Londrina, PR State, Brazil. Resistant genotypes E97-2502-9-3-1 and E97-2502-9-3-5 (types PI 437654) were crossed to the susceptible parent E96-776 (type ‘Hartwig’) to obtain 20 F1 plants, 120 F2 plants and 120 F2:3 families. The greenhouse trials involved those generations plus 20 plants of each parental genotype. Seedlings of each generation and of the race-differential cultivars were transplanted to clay pots and inoculated with 4000 eggs. After 28 days, the root system of each plant was washed and the females collected and counted. The inoculations resulted in high numbers of females in the susceptible genotypes in both crosses, confirming the presence of race 4+. The genetic analysis revealed no reciprocal cross effects, but some transgressive segregation with partial dominance toward resistance, and heritability estimates of medium to high magnitude. Resistant control in cross E96-776 × E97-2502-9-3-1 involved expression of additive, dominant and additive × additive epistatic effects, which suggests the presence of at least two genes. A single-gene hypothesis sufficed to explain the observed segregation in the E96-776 × E97-2502-9-3-5 cross with a simple additive-dominant model showing good fit to the data. The i-locus (expressing black seedcoat color) was associated with resistance.