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Open Access Research article

New insights into the origin of the B genome of hexaploid wheat: Evolutionary relationships at the SPA genomic region with the S genome of the diploid relative Aegilops speltoides

Jérome Salse12, Véronique Chagué1, Stéphanie Bolot2, Ghislaine Magdelenat3, Cécile Huneau1, Caroline Pont2, Harry Belcram1, Arnaud Couloux3, Soazic Gardais1, Aurélie Evrard1, Béatrice Segurens3, Mathieu Charles1, Catherine Ravel2, Sylvie Samain3, Gilles Charmet2, Nathalie Boudet1 and Boulos Chalhoub1*

Author Affiliations

1 UMR INRA 1165 – CNRS 8114 UEVE – Unité de Recherche en Génomique Végétale (URGV), 2, rue Gaston Crémieux, CP5708, 91057 Evry cedex, France

2 UMR 1095 INRA – Université Blaise Pascal – Génétique Diversité Ecophysiologie de Céréales (GDEC), Domaine de Crouelle, 234, avenue du Brézet, F-63100, Clermont-Ferrand, France

3 CEA: Institut de génomique – GENOSCOPE, 2, rue Gaston Crémieux, CP 5706, 91057, EVRY Cedex, France

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BMC Genomics 2008, 9:555  doi:10.1186/1471-2164-9-555

Published: 25 November 2008

Abstract

Background

Several studies suggested that the diploid ancestor of the B genome of tetraploid and hexaploid wheat species belongs to the Sitopsis section, having Aegilops speltoides (SS, 2n = 14) as the closest identified relative. However molecular relationships based on genomic sequence comparison, including both coding and non-coding DNA, have never been investigated. In an attempt to clarify these relationships, we compared, in this study, sequences of the Storage Protein Activator (SPA) locus region of the S genome of Ae. speltoides (2n = 14) to that of the A, B and D genomes co-resident in the hexaploid wheat species (Triticum aestivum, AABBDD, 2n = 42).

Results

Four BAC clones, spanning the SPA locus of respectively the A, B, D and S genomes, were isolated and sequenced. Orthologous genomic regions were identified as delimited by shared non-transposable elements and non-coding sequences surrounding the SPA gene and correspond to 35 268, 22 739, 43 397 and 53 919 bp for the A, B, D and S genomes, respectively. Sequence length discrepancies within and outside the SPA orthologous regions are the result of non-shared transposable elements (TE) insertions, all of which inserted after the progenitors of the four genomes divergence.

Conclusion

On the basis of conserved sequence length as well as identity of the shared non-TE regions and the SPA coding sequence, Ae speltoides appears to be more evolutionary related to the B genome of T. aestivum than the A and D genomes. However, the differential insertions of TEs, none of which are conserved between the two genomes led to the conclusion that the S genome of Ae. speltoides has diverged very early from the progenitor of the B genome which remains to be identified.