Inheritance of aphid resistance in PI 567541B and PI 567598B, identification of aphid resistance QTL in PI 567598B, and a new aphid biotype in Michigan

The soybean aphid (Aphis glycines Matsumura) has become a very important pest of soybean [Glycine max (L.) Merr.] in North America since it was first reported in 2000. In 2005, four new plant introductions (PI) with aphid resistance: PI 567543C, PI 567597C, PI 567541B and PI 567598B were identified. Since then, other sources of aphid resistance have been identified, but only in two sources has genetic and molecular characterization been conducted. The objectives of this research were to: (1) determine the inheritance of antibiosis resistance in PI 567541B and PI 567598B, (2) determine if a different soybean aphid biotype exist in Michigan and (3) identify and map quantitative trait loci (QTL) underlying aphid resistance in PI 567598B. Field studies were conducted to determine the inheritance of antibiosis resistance in PI 567541B and PI 567598B. The two resistant PIs were crossed with one or two susceptible soybean lines and the F1 and F2 plants and F2:3 families were evaluated for aphid resistance. All F 1 plants were found to be susceptible to soybean aphids. The plants in seven F2 populations segregated in a 15 susceptible to 1 resistant ratio, which is the expected ratio for a trait controlled by two recessive genes. The segregation data shows that two recessive genes are involved in the resistance in PI 567541B and PI 567598B. This information will be useful for breeders to design efficient breeding schemes for developing soybean cultivars with resistance to aphids. To achieve our second objective, 188 F2 individuals from a cross between Titan and PI 567598B were genotyped with 109 polymorphic simple sequence repeats (SSR) markers. Both single marker analysis (SMA) and composite interval mapping (CIM) methods were used to determine locations of QTLs. SMA revealed 24 markers associated with aphid resistance. QTL mapping by CIM identified a putative QTL on LG J. The SSR markers flanking these resistance genes can be used in marker-assisted selection for aphid resistance in soybean breeding programs. With the testing of several soybean aphid resistant genotypes, it was expected that resistant biotypes would evolve. In a field study in 2006, Dowling, a resistant check was found to be susceptible to the soybean aphid. A greenhouse study was conducted to compare the effect of the aphids which overcame the resistance in Dowling and aphids collected in the field in 2006. Dowling was found to be susceptible to both aphid colonies. In a follow up greenhouse study Dowling was found to be resistant to aphid colonies which had been raised in a growth chamber and greenhouse since 2002. These two studies indicate that, there is a difference in the feeding behavior on Dowling by aphids collected in 2002 and 2006 and suggests that a new soybean biotype may have evolved in Michigan.