Academic journal article Genetics

Selection Mapping of Loci for Quantitative Disease Resistance in a Diverse Maize Population

Academic journal article Genetics

Selection Mapping of Loci for Quantitative Disease Resistance in a Diverse Maize Population

Article excerpt

ABSTRACT

The selection response of a complex maize population improved primarily for quantitative disease resistance to northern leaf blight (NLB) and secondarily for common rust resistance and agronomic phenotypes was investigated at the molecular genetic level. A tiered marker analysis with 151 simple sequence repeat (SSR) markers in 90 individuals of the population indicated that on average six alleles per locus were available for selection. An improved test statistic for selection mapping was developed, in which quantitative trait loci (QTL) are identified through the analysis of allele-frequency shifts at mapped multiallelic loci over generations of selection. After correcting for the multiple tests performed, 25 SSR loci showed evidence of selection. Many of the putatively selected loci were unlinked and dispersed across the genome, which was consistent with the diffuse distribution of previously published QTL for NLB resistance. Compelling evidence for selection was found on maize chromosome 8, where several putatively selected loci colocalized with published NLB QTL and a race-specific resistance gene. Analysis of F^sub 2^ populations derived from the selection mapping population suggested that multiple linked loci in this chromosomal segment were, in part, responsible for the selection response for quantitative resistance to NLB.

(ProQuest: ... denotes formulae omitted.)

SELECTION mapping (SM) refers to a range of approaches that identifies alleles, loci, and epistatic interactions using populations that have been subjected to iterative cycles of recombination and selection. The effects of selection can be seen as differences in allele frequency, diversity, and/or patterns of recombination, through comparisons of temporally or spatially defined subpopulations. Several studies have been conducted on the principle that significant phenotypic change can be explained by significant changes in allele frequencies (e.g., STUBER and MOLL 1972; LABATE et al. 1999; DE KOEYER et al. 2001; COQUE and GALLAIS 2006). A fundamental challenge in SM, however, is to differentiate the effects of selection from those of genetic drift. Informationonpatterns ofrecombinationandsequence variation can be useful for SM in populations that are highly diverged, when genetic drift confounds the identification of significant changes in allele frequency (e.g., KOHN et al. 2000; POLLINGER et al. 2005; WRIGHT et al. 2005). A combination of allelic frequency data and functional evidence was used as an alternative to population-genetic evidence in the study of LAURIE et al. (2004). In our study, we use temporal shifts in allele frequency to identify putatively selected quantitative trait loci (QTL) for resistance to northern leaf blight (NLB) of maize. Putatively selected QTL are identified with a simulation-based test statistic that compares observed allele-frequency shifts to those expected under genetic drift for the specific maize breeding population under study. We also genetically tested one putatively selected chromosomal region and show that this region does associate with resistance to NLB.

While selection is fundamental to plant breeding and an abundance of selection methodologies have been developed, SM has been a relatively minor element of the rich mapping literature. The authors are aware of only 10 studies that utilized artificially selected plant populations forQTL mapping (STUBER and MOLL 1972; STUBER et al. 1980; SUGHROUE and ROCHEFORD 1994; LABATE et al. 1999; DE KOEYER et al. 2001; SMALLEY et al. 2004; LI et al. 2005; FAN et al. 2006; COQUE and GALLAIS 2006; FALKE et al. 2007), in contrast to the hundreds of studies that have identified QTL on the basis of trait-marker associations in unselected biparental mapping populations. There thus may be potential to take further advantage of the artificial selection exerted in plant or animal improvement programs to detect useful genetic variation.

Selection mapping has several potential advantages in relation to other QTL mapping approaches. …

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