Resistance and virulence in the soybean-Aphis glycines interaction

Aphis glycines Matsumura, the soybean aphid, first arrived in North America in 2000 and has since become the most important insect pest of domestic soybean, causing significant yield loss and increasing production costs annually in many parts of the USA soybean belt. Research to identify sources of resistance to the pest began shortly after it was found and several sources were quickly identified in the USDA soybean germplasm collection. Characterization of resistance expression and mapping of resistance genes in resistant germplasm accessions resulted in the identification of six named soybean aphid resistance genes: Rag1, rag1c, Rag2, Rag3, rag4, and Rag5 (proposed). Simple sequence repeat markers flanking the resistance genes were identified, facilitating efforts to use marker-assisted selection to develop resistant commercial cultivars. Saturation or fine-mapping with single nucleotide polymorphism markers narrowed the genomic regions containing Rag1 and Rag2 genes. Two potential NBS-LRR candidate genes for Rag1 and one NBS-LRR gene for Rag2 were found within the regions. Years before the release of the first resistant soybean cultivar with Rag1 in 2009, a soybean aphid biotype, named biotype 2, was found that could overcome the resistance gene. Later in 2010, biotype 3 was characterized for its ability to colonize plants with Rag2 and other resistance genes. At present, three biotypes have been reported that can be distinguished by their virulence on Rag1 and Rag2 resistance genes. Frequency and geographic distribution of soybean aphid biotypes are unknown. Research is in progress to determine the inheritance of virulence and develop DNA markers tagging virulence genes to facilitate monitoring of biotypes. With these research findings and the availability of host lines with different resistance genes and biotypes, the soybean aphid-soybean pest-host system has become an important model system for advanced research into the interaction of an aphid with its plant host, and also the tritrophic interaction that includes aphid endosymbionts.