Academic journal article Genetics

The Genetic Architecture of Response to Long-Term Artificial Selection for Oil Concentration in the Maize Kernel

Academic journal article Genetics

The Genetic Architecture of Response to Long-Term Artificial Selection for Oil Concentration in the Maize Kernel

Article excerpt

ABSTRACT

In one of the longest-running experiments in biology, researchers at the University of Illinois have selected for altered composition of the maize kernel since 1896. Here we use an association study to infer the genetic basis of dramatic changes that occurred in response to selection for changes in oil concentration. The study population was produced by a cross between the high- and low-selection lines at generation 70, followed by 10 generations of random mating and the derivation of 500 lines by selfing. These lines were genotyped for 488 genetic markers and the oil concentration was evaluated in replicated field trials. Three methods of analysis were tested in simulations for ability to detect quantitative trait loci (QTL). The most effective method was model selection in multiple regression. This method detected ~50 QTL accounting for ~50% of the genetic variance, suggesting that ~50 QTL are involved. The QTL effect estimates are small and largely additive. About 20% of the QTL have negative effects (i.e., not predicted by the parental difference), which is consistent with hitchhiking and small population size during selection. The large number of QTL detected accounts for the smooth and sustained response to selection throughout the twentieth century.

THE genetic architecture of a quantitative trait consists of a set of parameters that explain the genetic component of trait variation within or among populations. These parameters include the number of quantitative trait loci (QTL) affecting the trait, their locations in the genome, the frequencies of alternative genotypes segregating at the QTL, the pattern of linkage disequilibria among QTL, and the magnitudes of additive, dominance, and epistatic effects. Knowledge of genetic architecture has applications in two areas: (1) the identification of genes with utility in agriculture and/or treatment of disease and (2) making inferences about the evolutionary processes that maintain genetic variation and those that cause divergence between populations. Here we report a study of oil variation in maize that has both types of application.

The kernels of a modern maize (Zea mays L.) hybrid typically contain ~4% oil, 9% protein, 73% starch, and 14% other constituents (mostly fiber). The caloric con tent of oil is 2.25 times greater than that of starch on a weight basis and livestock feeding studies have shown a greater rate of weight gain per pound of feed for high-oil (>7%) than for normal maize (reviewed by LAMBERT 1994 and LAMBERT et al. 2004). Therefore, high-oil maize is in demand as a source of animal feed, which is the primary use of maize grown in developed countries. An objective of our study is to identify genes that may be used to increase the oil concentration of maize kernels through plant breeding or genetic engineering.

The experiment reported here originated in 1896 when C. G. Hopkins began the Illinois long-term selection lines, which have become a "textbook" example of the power of artificial selection (see review by DUDLEY and LAMBERT 2004). From an open-pollinated variety of maize, Hopkins started two populations that were selected divergently for the percentage of kernel dry weight that consists of oil ("oil concentration" or "per centage of oil"). These populations are called Illinois high oil (IHO) and Illinois low oil (ILO). In each generation and each population, bulked kernels from each of a number of ears (half-sib families) were analyzed and the highest (or lowest) 20% of ears were selected to be parents of the next generation. This selection was carried on throughout the twentieth century at the University of Illinois and continues to this day for IHO with no sign of a plateau in response. At generation 89, selection in ILO was discontinued because of poor viability an oil concentration so low that it cannot be measured accurately. At this point, the populations had changed from 4.7% oil to 19.3% in IHO and 1. …

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