Submitted to: Nematropica
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/3/2004
Publication Date: 6/28/2005
Citation: Assuncao, M.S., Atibalentja, N., Noel, G.R. 2005. Soybean cyst nematode, hetorodera glycines, resistance genes in PI89.772 and PI209.332 soybean. Nematropica. 34:165-181. Interpretive Summary: Soybean cyst nematode (SCN) causes $500 million to $1 billion loss in soybean production each year. Planting resistant varieties is used widely by soybean farmers to reduced crop loss caused by the nematode. Several genes have been reported to govern resistance to SCN. These genes come from plant introductions obtained primarily in China. How many genes exist in these sources of resistance and how they interact with each other is not understood completely. The research reported in this paper clarified the number of genes in three important sources of resistance and the interaction of these genes to race 3 of SCN, the most common race of the nematode in the Midwest. The resistance genes were not expressed as simple dominant or recessive genes, but rather as major and minor genes that mask expression of resistance to produce a continuum of resistance from susceptible to highly resistant to SCN. These results are important in helping to explain the wide range of no effective resistance to resistant in the level of resistance of commercial varieties to SCN in the Midwest.
Technical Abstract: The number of resistance genes in PI89.772 and PI209.332 conferring resistance to Heterodera glycines race 3 is not well defined. Crosses of PI89.772 × 'Lee 68', PI88.788 × PI89.772, and 'Lee 68' × PI209.332 were made in the field and greenhouse. Several F1 and F2 families from each cross, 98 F3 families from cross PI89.772 × 'Lee 68', 74 F3 families from cross PI88.788 × PI89.772, and 80 F3 families from cross 'Lee 68' × PI209.332 were tested with an inbred line of H. glycines developed on PI88.788. Approximately 8,000 individual plants growing in pots containing 200 cm3 of sterilized sand were inoculated with 4,000 eggs and J2/pot. Thirty days after inoculation the number of females that developed on each plant was determined. Cluster analysis revealed sets of families with a low mean number of females and low variance, intermediate means and high variance, and high means with a low variance, indicating F3 plants came from, respectively, homozygous resistant, heterozygous or segregating, and homozygous susceptible F2 plants. Thus, resistance classes were considered as quantitative parameters having different levels of resistance as opposed to only two classes, either resistant or susceptible. Chi-square analysis of segregation of phenotypic data indicated two genes confer resistance to race 3 of H. glycines. The three H. glycines resistant parents have at least two genes that express resistance to H. glycines. One gene acts as a major gene (Rhgx) and the other a minor gene (Rhgy) in conferring resistance of the parents PI89.772 (Rhgx1?Rhgx1?Rhgy1?Rhgy1?), PI88.788 (Rhgx2?Rhgx2?Rhgy2?Rhgy2?), and PI209.332 (Rhgx3?Rhgx3?Rhgy3?Rhgy3?) to H. glycines race 3. The same genes may occur in PI209.332 as in PI89.772, but support for this hypothesis must be obtained by studying the cross PI209.332 × PI89.772. The same major (Rhgx) and minor (Rhgy) genes occur in PI89.772 (Rhgx1?Rhgx1?Rhgy1?Rhgy1?) and PI88.788 (Rhgx2?Rhgx2?Rhgy2?Rhgy2?). The phenotypic ratios obtained in this research indicate that epistasis occurs between loci Rhgx and gene Rhgy.