MBE Advance Access published online on April 24, 2009
Molecular Biology and Evolution, doi:10.1093/molbev/msp076
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Research Article |
Sixty million years in evolution of soft grain trait in grasses: emergence of the softness locus in the common ancestor of Pooideae and Ehrhartoideae, after their divergence from Panicoideae
* URGV (UMR INRA 1165 - CNRS 8114 – UEVE) Organization and evolution of Plant Genomes, 91057 Evry Cedex, France
Plant Genome Mapping Laboratory University of Georgia, Athens, GA 30602, USA
Department of Molecular Genetics and Cell Biology University of Chicago, Chicago, IL60615, USA
1 Corresponding author: Boulos Chalhoub, UMR INRA 1165 - CNRS 8114 – UEVE, Unité de Recherches en Génomique Végétale (URGV), Organization and evolution of Plant Genomes, 2 rue Gaston Crémieux, 91057 Évry cedex, France, Tel: 33 1 60 87 45 03 Fax: 33 1 60 87 45 10, E-mail: chalhoub{at}evry.inra.fr
Received for publication February 6, 2009. Revision received April 2, 2009. Accepted for publication April 13, 2009.
Together maize, Sorghum, rice and wheat grass (Poaceae) species are the most important cereal crops in the world and exhibit different ‘Grain endosperm texture’. This trait has been studied extensively in wheat because of its pivotal role in determining quality of products obtained from wheat grain. Grain Softness protein-1 and Puroindolines A and B (grain storage proteins), encoded by Ha-like genes: Gsp-1, Pina and Pinb, of the Hardness (Ha) locus, are the main determinants of the grain softness/hardness trait in wheat. The origin and evolution of grain endosperm texture in grasses was addressed by comparing genomic sequences of the Ha orthologous region of wheat, Brachypodium, rice and Sorghum. Results show that the Ha-like genes are present in wheat and Brachypodium but are absent from Sorghum bicolor. A truncated remnant of a Ha-like gene is present in rice. Synteny analysis of the genomes of these grass species shows that only one of the paralogous Ha regions, created 70 MYA by whole-genome duplication, contained Ha-like genes. The comparative genome analysis and evolutionary comparison with genes encoding grain reserve proteins of grasses suggest that an ancestral Ha-like gene emerged, as a new member of the prolamin gene family, in a common ancestor of the Pooideae (Triticeae and Brachypoidieae tribes) and Ehrhartoideae (rice), between 60-50 MYA, after their divergence from Panicoideae (Sorghum). It was subsequently lost in Ehrhartoideae. Recurring duplications, deletions and/or truncations occurred independently and appear to characterize Ha-like gene evolution in the grass species. The Ha-like genes gained a new function in Triticeae, such as wheat, underlying the soft grain phenotype. Loss of these genes in some wheat species leads, in turn, to hard endosperm seeds.
Key Words: Poaceae • Evolution • Comparative-Genomics • Grain-endosperm-softness