|Hunt, Harriet -|
|Moots, Hannah -|
|Jones, Huw -|
|Parker, Mary -|
|Romanova, Olga -|
|Jones, Martin -|
|Howe, Christopher -|
|Trafford, Kay -|
Submitted to: Molecular Biology and Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 25, 2012
Publication Date: August 30, 2012
Citation: Hunt, H.V., Moots, H.M., Graybosch, R.A., Jones, H., Parker, M., Romanova, O., Jones, M.K., Howe, C.J., Trafford, K. 2012. Waxy-phenotype evolution in the allotetraploid cereal broomcorn millet: Mutations at the GBSSI locus in their functional and phylogenetic context. Molecular Biology and Evolution. DOI: 10.1093/molbev/mss209. Interpretive Summary: Waxy varieties of cereals lack amylose in their endosperm starch, resulting in a characteristic glutinous-textured phenotype. The cultivation of waxy varieties of broomcorn (proso) millet is restricted to east Asia, where there is a culinary preference for glutinous-textured foods that may have developed from ancient food processing traditions. Mutations in a single gene, GBSSI, cause the waxy phenotype. In the polyploid cereal broomcorn millet, this gene is duplicated. By analysing the biochemical properties of different genotypes of broomcorn millet, we showed that one gene copy is responsible for most amylose synthesis, and the second copy has strongly reduced synthesis capacity. However, mutations in both gene copies were needed for the evolution of varieties with the waxy phenotype. We analysed the distribution of known mutations across Eurasia, and their association with genetic groups inferred from a distinct set of genetic markers (microsatellites). Alleles at the two GBSSI genes in broomcorn millet are strongly associated with particular genetic and geographical groups, suggesting that evolution at this locus has played an important role in shaping overall genetic diversity. Intermediately-waxy phenotypes have apparently been selected against, highlighting the limitations of current anthropological understanding of this cultural phenomenon. Results, however, demonstrate that breeders could develop lines with reduced amylose content, which could have application in modern food production systems.
Technical Abstract: Waxy mutants, in which endosperm starch contains ~100% amylopectin rather than the wild-type composition of ~70% amylopectin and ~30% amylose, occur in many domesticated cereals. The cultivation of waxy varieties of broomcorn (proso) millet (Panicum miliaceum L.) is restricted to east Asia, where there is a culinary preference for glutinous-textured foods that may have developed from ancient food processing traditions. The waxy phenotype results from mutations in the GBSSI gene, which catalyses amylose synthesis. Previous work characterised two homeologous GBSSI loci, with multiple alleles at each, but could not determine whether both loci contributed to GBSSI function. We firstly tested the relative contribution of the two GBSSI loci to amylose synthesis, and secondly tested the association between GBSSI alleles and phylogeographic structure inferred from SSRs. We evaluated the phenotype of all known GBSSI genotypes in broomcorn millet by assaying starch composition and protein function. Results showed that the GBSSI-S locus is the major locus controlling amylose content and the GBSSI-L locus has strongly reduced synthesis capacity. We genotyped 178 individuals from landraces from across Eurasia for the 2 GBSSI and 16 SSR loci and analysed phylogeographic structuring and the geographic and phylogenetic distribution of GBSSI alleles. We found that GBSSI alleles have distinct spatial distributions and strong associations with particular genetic clusters defined by SSRs. The combination of alleles that results in a partially-waxy phenotype does not exist in landrace populations.