Genetic diversity and selection in the maize starch pathway - PubMed (original) (raw)

Genetic diversity and selection in the maize starch pathway

Sherry R Whitt et al. Proc Natl Acad Sci U S A. 2002.

Abstract

Maize is both phenotypically and genetically diverse. Sequence studies generally confirm the extensive genetic variability in modern maize is consistent with a lack of selection. For more than 6,000 years, Native Americans and modern breeders have exploited the tremendous genetic diversity of maize (Zea mays ssp. mays) to create the highest yielding grain crop in the world. Nonetheless, some loci have relatively low levels of genetic variation, particularly loci that have been the target of artificial selection, like c1 and tb1. However, there is limited information on how selection may affect an agronomically important pathway for any crop. These pathways may retain the signature of artificial selection and may lack genetic variation in contrast to the rest of the genome. To evaluate the impact of selection across an agronomically important pathway, we surveyed nucleotide diversity at six major genes involved in starch metabolism and found unusually low genetic diversity and strong evidence of selection. Low diversity in these critical genes suggests that a paradigm shift may be required for future maize breeding. Rather than relying solely on the diversity within maize or on transgenics, future maize breeding would perhaps benefit from the incorporation of alleles from maize's wild relatives.

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Figures

Figure 1

Figure 1

A simplified pathway of starch production in maize and the position of the six sampled genes in the pathway. *, loci with strong evidence of selection (either HKA or Tajima's D test).

Figure 2

Figure 2

Comparison of silent diversity in maize (white bars) and its wild relative Z. mays ssp. parviglumis (black bars). For each locus, 500–2,700 silent bases were sampled. The numbers above the bars indicate the fold reduction in diversity between maize and Z. m. ssp. parviglumis. Neutral refers to the average of six nonselected genes (adh1, adh2, glb1, hm1, hm2, and te1); tb1 is the only cloned domestication gene from maize (only the highly selected promoter region is shown).

Figure 3

Figure 3

Comparison of nucleotide diversity in maize and various grass crops. Maize data are the average diversity at random loci from Tenaillon et al. (2), the maize starch average is from the six genes examined here. The other grasses are diversity averages from published studies (see review in ref. 1); however, they are often based on a limited number of loci.

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