Evaluation of soybean aphid resistance, yield qtl confirmation, and genetic gain in soybean

Soybean Aphid Resistance in Exotic Soybean Lines The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is a common insect pest of soybean [Glycine max (L.) Merr] in China and is also found frequently in other Asian countries. It was identified in the US in 2000 and has since spread throughout the soybean growing regions of the US and Canada. Currently, there are four known soybean aphid resistance genes (Rag1 through Rag4) and a fifth gene, Rag5, which was identified at the Rag2 locus. Soybean aphid biotypes have also been identified that can overcome Rag1 and Rag2. Soybean aphid resistance was examined in the greenhouse in a collection of F2 families developed from 19 soybean aphid resistant exotic plant introduction (PI) lines with the objective of increasing the understanding of the genetics of soybean aphid resistance. In addition, 104 soybean aphid resistant PIs were haplotyped in the regions of Rag1 and Rag2 to gain information about the commonalities and differences in the genetic regions surrounding these genes. Resistance appeared to be controlled by a single dominant gene in 14 populations, two dominant genes in one population, a single recessive gene in one population, and three populations had no clear Mendelian segregation ratio. The region around Rag2 was frequently significantly associated with resistance, indicating that this locus may be important in soybean aphid resistance. Haplotyping results indicated that only three PIs had a haplotype similar to the Rag1 source in the Rag1 region, and that 69 PIs had a haplotype similar to the Rag2 source in the Rag2 region. The haplotype data was inconclusive at identifying patterns of soybean aphid resistance. Genetic Progress in Soybean Yield Between 1924 and 2010, US average soybean yields have increased at the rate of 21.3 kg ha-1 annually. This yield gain has occurred due to improvements in soybean genetics, the production environment, or increased atmospheric CO2 levels. The objectives of this research were to examine the rate of yield gain in northern soybean maturity group (MG) II, III, and IV cultivars and to compare the effects of rotation on yield gain for soybean following eleven years of continuous corn or following a corn-soybean rotation. The experiment was performed in 2010, using 45 MG II soybean cultivars, 40 MG III cultivars, and 45 MG IV cultivars that were released between the 1923 and 2008. Yield data was adjusted using maturity as a covariate. Overall, the rate of raw yield gain was 22.8 kg ha-1 yr-1 and the rate of maturity-adjusted yield gain 19.8 kg ha-1 yr-1, suggesting that a large portion of the historic yield gains can be attributed to genetic improvements. There was a significant rotation effect, with cultivars grown following continuous corn yielding on average 377 kg ha-1 more than cultivars grown in a corn-soybean rotation. No significant differences in rate of yield gain between prior planting treatments were observed at any of the six test locations. Generally, disease symptoms and pathogen and pest levels were equal to or higher in the corn-soybean rotation plots compared with the continuous corn plots. The lack of difference in yield gain from the two rotation treatments indicates that modern cultivars may not be more adapted to the stresses of the modern-day corn-soybean rotation production environment than older cultivars. Yield QTL Confirmation in Soybean Increasing yield is one of the primary objectives that breeders are working to improve in soybean. After initial mapping, yield quantitative trait loci (QTL) should be validated, confirming that the QTL are true yield increasing alleles that act consistently across multiple genetic backgrounds and environments. In order to confirm a QTL, the Soybean Genetics Committee has set rules: the QTL must be confirmed at an experiment-wise error rate of 0.01 in a population developed from a separate set of meiotic events from the original population. Yield QTL confirmation was performed on four yield QTL that were identified by Guzman et al. (1) in a mapping population developed using PI 68658 as the donor parent and Lawrence as the recurrent parent, and each yield allele provided an additive effect of 40 to 80 kg ha-1. The yield QTL were examined in two confirmation populations: three QTL were studied in a population composed of three sets of near isogenic lines (NILs), and the effects of all four QTL were studied in a segregating recombinant inbred line (RIL) backcross population. The three NIL populations were unable to significantly identify marker-yield associations, while the yield QTL linked to marker Satt300 (chromosome 5) was found to be significantly associated with yield (p < 0.001). The PI 68658 allele at this marker had an additive effect of 68.4 kg ha-1, accounted for 10.7% of the phenotypic variation for yield, and was not associated with height, lodging, or maturity across the six environments the experiment was grown. 1. Guzman, P.S., B.W. Diers, D.J. Neece, S.K.S. Martin, A.R. Leroy, C.R. Grau, T.J. Hughes, and R.L. Nelson. 2007. QTL associated with yield in three backcross-derived populations of soybean. Crop Sci. 47:111-122