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Inheritance of Genes for Resistance to Soybean Cyst Nematode Populations

Dep. of Agronomy, Univ. of Missouri, Columbia 65211

^* Corresponding author.

The arbitrary division of continuous distributions of phenotypes on F₂ plants into ratios of resistant:susceptible categories may not accurately reflect the number and dominance of genes for resistance to pests. Resistance of soybean, Glycine max L. Merr., lines to soybean cyst nematode (CN), Heterodera glycines Ichinohe, populations has been variously defined, representing relatively few cysts per plant phenotypes. This review considers the genetic principles that are relevant to genetic studies involving CN and soybean. The distribution of a phenotype, such as number of cysts per F₂ plant, is determined by the number and dominance of soybean genes for resistance segregating and the frequencies of CN genes for avirulence with which they interact in a gene-for-gene manner. An assumption seems to have been that only one soybean genotype conferred resistance, which implied heterogeneous CN populations. The resistant: susceptible ratios commonly used were deduced to be 3^d:4^N-3^d, with dominant (d) and recessive (r) genes for resistance at N independent loci. An A_ or aa genotype (of d and r genes) at any one locus may condition F₂ plant resistance to homozygous (for avirulence) CN (and parasite clones). Then only one genotype or 3^r gives susceptibility as 4^N–3^r:3^r ratios. Some of the resistant:susceptible ratios were not greatly different, and might be the same with the effects of linkage. Heterogeneous and homozygous CN populations may not be distinctly different, rather the differences are in the distributions of phenotypes they effect with F₂ plants: continuous with heterogeneous but discrete phenotypes, similar to the parental ranges, with homozygous populations. The materials and methods for definitive genetic studies, including allelism and linkage, were discussed.

Received for publication February 8, 1988.