Nested Association Mapping Recominant Inbred Lines in maize

The August 7 issue of Science includes a report on quantitative trait loci (QTLs) affecting flowering time in maize. It is important because it presents the first results with an important new technique known as nested association mapping (NAM), which utilizes a large set of recombinant inbred lines derived from diverse founder lines. The article is accompanied by an excellent piece by Trudy Mackay that puts this method into perspective. "Linkage mapping can readily detect chromosomal regions containing one or more QTLs that affect a trait ... but it is difficult to precisely localize the QTLs. This approach usually relies on crosses between two strains, thus capturing only a tiny fraction of genetic diversity in the population. By contrast, association mapping widely samples genetic diversity and requires fewer individuals, but has less power to detect QTLs when alleles are not common." The new method combines advantages of these earlier approaches. It also provides surprising results, different from what has been found in other systems. In particular, the authors find numerous genes of small effect with few genetic or environmental interactions, so that "a simple additive model accurately predicts flowering time for maize." The authors argue that their data supports "common genes with uncommon variants." Moving forward, I look forward to seeing this system applied to other traits, and to the discovery of specific genes involved in this and other traits.

Relevant links:

McMullen et al.. 2009 "Genetic Properties of the Maize Nested Association Mapping Population" Science 325: 737.
This is the main paper presented here. It describes QTLs for flowering time.

Mackay. 2009 "A-maize-ing Diversity" Science 325: 688.
A very nice summary of how NAM compares to other methods for finding QTLs and how these results compare with those from other systems.

Mackay. 2001. "The genetic architecture of quantitative traits." Annu. Rev. Genet. 35: 303.

Yu et al. "Genetic Design and Statistical Power of Nested Association Mapping in Maize" Genetics 178: 539.
An earlier paper from some of the same authors describing the NAM method.