International Maize and Wheat Improvement Center

About: International Maize and Wheat Improvement Center is a nonprofit organization based out in Texcoco, Mexico. It is known for research contribution in the topics: Population & Agriculture. The organization has 1976 authors who have published 4799 publications receiving 218390 citations.

Genetic analysis of adult plant, quantitative resistance to stripe rust in wheat cultivar ‘Stephens’ in multi-environment trials

Abstract: The wheat (Triticum aestivum L.) cultivar ‘Stephens’ has been grown commercially in the USA Pacific Northwest for 30 years. The durable resistance of ‘Stephens’ to stripe rust (Puccinia striiformis f. sp. tritici) was believed to be due to a combination of seedling and adult plant resistance genes. Multilocation field trials, diversity array technology (DArT), and simple sequence repeat (SSR) markers were used to identify quantitative trait loci (QTL) for resistance. Recombinant inbred lines were assessed for stripe rust response in eight locations/years, five in 2008 and three in 2009. The data from Mt. Vernon, WA, differed from all other environments, and composite interval mapping (CIM) identified three QTL, QYrst.orr-1AL, QYrst.orr-4BS, and QYrpl.orr-6AL, which accounted for 12, 11, and 6% of the phenotypic variance, respectively. CIM across the remaining six environments identified four main QTL. Two QTL, QYrst.orr-2BS.2 and QYrst.orr-7AS, were detected in five of six environments and explained 11 and 15% of the phenotypic variance, respectively. Two other QTL, QYrst.orr-2AS and QYrpl.orr-4BL, were detected across four and three of six environments, and explained 19 and 9% of the phenotypic variance, respectively. The susceptible parent ‘Platte’ contributed QYrpl.orr-4BL and QYrpl.orr-6AL, with the remaining QTL originating from ‘Stephens’. For each environment, additional minor QTL were detected, each accounting for 6–10% of the phenotypic variance. Different QTL with moderate effects were identified in both ‘Stephens’ and ‘Platte’. Significant QTL × environment interactions were evident, suggesting that specificity to plant stage, pathogen genotype, and/or temperature was important.

Comparative SNP and Haplotype Analysis Reveals a Higher Genetic Diversity and Rapider LD Decay in Tropical than Temperate Germplasm in Maize

Abstract: Understanding of genetic diversity and linkage disequilibrium (LD) decay in diverse maize germplasm is fundamentally important for maize improvement. A total of 287 tropical and 160 temperate inbred lines were genotyped with 1943 single nucleotide polymorphism (SNP) markers of high quality and compared for genetic diversity and LD decay using the SNPs and their haplotypes developed from genic and intergenic regions. Intronic SNPs revealed a substantial higher variation than exonic SNPs. The big window size haplotypes (3-SNP slide-window covering 2160 kb on average) revealed much higher genetic diversity than the 10 kb-window and gene-window haplotypes. The polymorphic information content values revealed by the haplotypes (0.436–0.566) were generally much higher than individual SNPs (0.247–0.259). Cluster analysis classified the 447 maize lines into two major groups, corresponding to temperate and tropical types. The level of genetic diversity and subpopulation structure were associated with the germplasm origin and post-domestication selection. Compared to temperate lines, the tropical lines had a much higher level of genetic diversity with no significant subpopulation structure identified. Significant variation in LD decay distance (2–100 kb) was found across the genome, chromosomal regions and germplasm groups. The average of LD decay distance (10–100 kb) in the temperate germplasm was two to ten times larger than that in the tropical germplasm (5–10 kb). In conclusion, tropical maize not only host high genetic diversity that can be exploited for future plant breeding, but also show rapid LD decay that provides more opportunity for selection.

An Analytical Model of Farmers' Demand for Replacement Seed

Abstract: The purchase of new seed of a variety that a farmer is already growing, varietal replacement in general, and varietal choice during periods of rapid technological change are examples of seed replacement choices. A seed replacement choice differs from decisions about other inputs, such as fertilizer or labor, in that seeds can be repro- duced for the next crop season. Benefits from the purchase of new seed can continue for sev- eral years, and purchased seed is a self-sustain- ing input, although it is subject to depreciation. Replacement seed is, therefore, in some ways analogous to a capital item rather than a variable input. Agricultural economists, in general, have considered only the analysis of varietal choice with rapid technological change. Theoretical (Nowshirvani) and empirical (Herath, Har- daker, and Anderson; Gafsi and Roe) studies of varietal choice tend to model the farmer's de- cision as a single period portfolio allocation of land between modern and traditional varieties. Thus, the attributes of interest are yield, mo- ments of the yield distribution, and occasionally other varietal characteristics. Other approaches to analyzing farmers' choice of seed technology emphasize how a related input (fertilizer, tube- wells) affects the level and riskiness of return (Feder 1980, 1982). Prices, risk, the lumpiness of the associated inputs, and possible credit con- straints determine the portfolio choice over time in these models. In none of these approaches is the more general case of demand for replace- ment seed explicitly recognized. This paper proposes an analytical model of the demand for replacement seed. The model describes the factors that induce a farmer to re- place seed and the optimal frequency of new seed purchase given myopic and infinite time hori- zons. The model is applied to wheat seed de- mand in Pakistan. Conclusions, limitations, and possible extensions of the model are also con- sidered.