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Update on Breeding for Soybean Aphid Resistance
Tue - July 18, 2017
by Brian Diers, Soybean Breeder, University of Illinois

The soybean aphid (Aphis glycines) is an exotic pest of soybean that was first identified in North American during 2000. Host resistance was recognized as an important management option, and several sources of aphid resistance were identified in accessions from the USDA Soybean Germplasm Collection. The pest can now be controlled by Rag genes (Resistance to Aphis glycines) which have been introgressed into soybean lines adapted to Midwestern growing conditions.

There are currently 10 named soybean aphid resistance genes and alleles:  Rag1, rag1b, rag1c, Rag2, Rag3 , rag3, Rag3b, rag4, Rag5, and Rag6.  Host resistance works by suppressing aphid growth and reproduction on the plant. This type of resistance is called antibiosis. Two other mechanisms of resistance are antixenosis, which describes the plants ability to repel the insect, and  tolerance, the ability of a plant to withstand colonization without impact on yield. A single plant can express one or more types of resistance.

Testing Rag gene combinations
As the number of deployable aphid resistance genes with antibiosis and antixenosis-type resistance continues to expand, so do the options available to breeders. However, the efficacy of gene combinations against known aphid biotypes requires on-going investigation. Aphid biotypes are populations that survive and reproduce on varieties developed for aphid resistance.  There are four known aphid biotypes in North America.

With checkoff funding provided by the North Central Soybean Research Program and the United Soybean Board, we evaluated the differential reaction of soybean isolines carrying different combinations of the genes Rag1, Rag2, and Rag3 to the four known aphid biotypes.

Figure 1. Differential colonization of the eight soybean isolines with combinations of Rag genes by soybean aphid biotype 1 across trials 1 and 2. Means with the same letter are not significantly different (P< 0.05).

We found that all gene combinations, with the exception of Rag3 alone, provided excellent protection against biotype 1 (Figure 1).

Isolines with Rag2 alone or in combination with Rag1 and Rag3 had greater levels of resistance to biotype 2. 

For biotype 3, the Rag1+3 and Rag1+2+3 pyramided lines significantly reduced aphid populations compared with all other gene combinations, while the Rag1+2+3 pyramid provided the greatest protection against biotype 4. Overall, the Rag 1+2+3 pyramided line conferred the greatest protection against all four biotypes.

Rag 2 gene and yield
In 2013, we showed that the resistance gene Rag2 was associated with reduced yield. We have followed that research with a study to determine whether the reduced yield was the result of linkage between Rag2 and a second yield-reducing gene, or because the resistance gene directly reduced yield.

This research was just completed and we found that the yield reduction was the result of a second yield-reducing gene. This is good news, as soybean breeders should be able to break the linkage without much difficulty —allowing for the creation of high yielding, aphid-resistant varieties with the Rag2 gene.

New aphid resistant varieties
Our soybean breeding program continues to develop and release soybean varieties with different combinations of Rag genes conferring aphid resistance. We are backcrossing the aphid resistance genes Rag4 and Rag6 into Rag1, Rag2 and Rag3 varieties. Rag4 and Rag6 are aphid resistance genes that were identified at Michigan State University. Rag4 backcrossing is now complete, and this summer we will select plants with all combination of Rag1-4. The backcrossing will continue until we have sets of lines developed with all combinations of Rag1-4 and Rag6.

The University of Illinois is commercializing four soybean varieties with Rag2, one variety with Rag1, and one variety with Rag1 and Rag2 combined. These varieties are being marketed under the Illini Brand by the Baird Seed Company ( Aphid resistant varieties are also available from other sources. Check with your seed dealer and in seed catalogues about the availability of aphid resistant varieties (Rag) in your area.  
Read the full research report: Differential Reactions of Soybean Isolines With Combinations of Aphid Resistance Genes Rag1, Rag2, and Rag3 to Four Soybean Aphid Biotypes. Journal of Economic Entomology, 2016
Soybean Aphid Field Guide - North Central Soybean Research Program
Scouting for the Soybean Aphid - University of Minnesota
A Visual Guide for Counting Soybean Aphids - North Central Soybean Research Program
Biology of the Soybean Aphid.  Journal of Integrated Pest Management,