Control of flowering time and spike development in

Authors: Maria E Faricelli Miroslav Valárik Jorge Dubcovsky

Publish Date: 2009/10/23

Volume: 10, Issue: 2, Pages: 293-306

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Abstract

The earliness per se gene EpsA m 1 from diploid wheat Triticum monococcum affects heading time spike development and spikelet number In this study the Eps1 orthologous regions from rice Aegilops tauschii and Brachypodium distachyon were compared as part of current efforts to clone this gene A single Brachypodium BAC clone spanned the EpsA m 1 region but a gap was detected in the A tauschii physical map Sequencing of the Brachypodium and A tauschii BAC clones revealed three genes shared by the three species which showed higher identity between wheat and Brachypodium than between them and rice However most of the structural changes were detected in the wheat lineage These included an inversion encompassing the wg241VatpC region and the presence of six unique genes In contrast only one unique gene and one pseudogene was found in Brachypodium and none in rice Three genes were present in both Brachypodium and wheat but were absent in rice Two of these genes Mot1 and FtsH4 were completely linked to the earliness per se phenotype in the T monococcum highdensity genetic map and are candidates for EpsA m 1 Both genes were expressed in apices and developing spikes as expected for EpsA m 1 candidates The predicted MOT1 protein showed amino acid differences between the parental T monococcum lines but its effect is difficult to predict Future steps to clone the EpsA m 1 gene include the generation of mot1 and ftsh4 mutants and the completion of the T monococcum physical map to test for the presence of additional candidate genesPositional cloning of genes in wheat is hampered by its large genome size with abundant repetitive elements and its polyploid nature Common wheat Triticum aestivum L has a 164Gb genome Arumuganathan and Earle 1991 with a large proportion of repetitive elements approximately 80 Moore 1995 and three different genomes 2n = 6x = 42 AABBDD genomes These three genomes originated from the hybridization of diploid Aegilops tauschii Coss 2n = 2x = 14 DD genome and tetraploid Triticum turgidum L 2n = 4x = 28 AABB genomes The AA genome from tetraploid wheat was contributed by Triticum urartu Tumanian ex Gandilyan 2n = 2x = 14 AA genome which is closely related to cultivated diploid wheat Triticum monococcum L 2n = 2x = 14 AmAm genome Dvorak et al 1993 Johnson and Dhaliwal 1976 The BB genome was contributed by a species from the Sitopsis group likely related to Aegilops speltoides Tausch Löve Dvorak and Zhang 1990 Huang et al 2002 Liu et al 2003The use of wheat diploid ancestors or other diploid Triticeae species provides a viable alternative to overcome the complications imposed by polyploidy to positional cloning projects The development of molecular markers and the construction of highdensity genetic maps are considerably simpler in diploid genomes Examples of the use of this strategy are provided by the cloning of the powdery mildew resistance locus Pm3 of hexaploid wheat using both tetraploid and diploid wheat Yahiaoui et al 2004 and by the use of T monococcum to clone the vernalization genes Vrn1 and Vrn2 Yan et al 2003 2004 and the leaf rust resistance locus Lr10 Stein et al 2000The large size of the Triticeae genomes has delayed the sequencing of these species and limited the number of successful positional cloning projects in this economically important group of plants Fortunately there is good colinearity among species of the Poaceae family Devos 2005 Devos and Gale 2000 Feuillet and Keller 2002 Paterson et al 2000 Van Deynze et al 1995 The available genomic sequences of rice Goff et al 2002 Yu et al 2002 and Brachypodium http//wwwbrachypodiumorg provide parallel road maps which are useful to generate markers in the Triticeaetargeted regions and in some cases to identify potential candidate genes Yan et al 2003 2006 This comparative genomics approach has been used in most of the positional cloning projects in wheat Bossolini et al 2006 Feuillet et al 2003 Fu et al 2009 Uauy et al 2006 Yan et al 2003 2004 2006Whereas the initial lowresolution restriction fragment length polymorphism RFLP maps were sufficient to reveal the existence of large colinear chromosome blocks between rice and wheat they were unable to detect small rearrangements affecting this colinear frame Devos 2005 Dubcovsky et al 1996 Guyot and Keller 2004 Linkiewicz et al 2004 Peng et al 2004 Sorrells et al 2003 The development of highdensity maps and the first comparative sequencing of targeted orthologous regions revealed numerous exceptions to the broad RFLP colinearity previously observed along large blocks of rice and wheat chromosomes These alterations were generated by gene duplications deletions and inversions as well as gene insertions associated with transposition of linked retroelements Bennetzen and Ma 2003 Bennetzen and Ramakrishna 2002 Brunner et al 2003 Dubcovsky et al 2001 Guyot et al 2004

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