Acc homoeoloci and the evolution of wheat genomes

Abstract

The DNA sequences of wheat Acc-1 and Acc-2 loci, encoding the plastid and cytosolic forms of the enzyme acetyl-CoA carboxylase, were analyzed with a view to understanding the evolution of these genes and the origin of the three genomes in modern hexaploid wheat. Acc-1 and Acc-2 loci from each of the wheats Triticum urartu (A genome), Aegilops tauschii (D genome), Triticum turgidum (AB genome), and Triticum aestivum (ABD genome), as well as two Acc-2-related pseudogenes from T. urartu were sequenced. The 2.3-2.4 Mya divergence time calculated here for the three homoeologous chromosomes, on the basis of coding and intron sequences of the Acc-1 genes, is at the low end of other estimates. Our clock was calibrated by using 60 Mya for the divergence between wheat and maize. On the same time scale, wheat and barley diverged 11.6 Mya, based on sequences of Acc and other genes. The regions flanking the Acc genes are not conserved among the A, B, and D genomes. They are conserved when comparing homoeologous genomes of diploid, tetraploid, and hexaploid wheats. Substitution rates in intergenic regions consisting primarily of repetitive sequences vary substantially along the loci and on average are 3.5-fold higher than the Acc intron substitution rates. The composition of the Acc homoeoloci suggests haplotype divergence exceeding in some cases 0.5 Mya. Such variation might result in a significant overestimate of the time since tetraploid wheat formation, which occurred no more than 0.5 Mya.acetyl-CoA carboxylase ͉ Triticeae ͉ grass

The wheat powdery mildew genome shows the unique evolution of an obligate biotroph

“…hordei diverged 6.3 (± 1.1) million years ago (Supplementary Note). This finding narrows down previous estimates, which ranged from 4.7 to 10 million years ago 5,17 , and indicates that the two formae speciales diverged several million years ago, after the divergence of their hosts 10-15 million years ago 18,19 . As in a previous study 17 , we found gene order to be largely conserved between B. graminis f.sp.…”

Duplication and partitioning in evolution and function of homoeologous Q loci governing domestication characters in polyploid wheat

“…The widespread cultivation of wheat resulted from founder evolutionary events including the acquisition of domestication traits such as a nonbrittle rachis to prevent spikelet shattering and soft glumes and hull-less seed for ease of threshing, as well as allopolyploidization events (12) that resulted in species with better agronomic performance and wide adaptability.…”

“…S1) (13)(14)(15)(16). The tetraploid wheat Triticum turgidum L., (2n = 4x = 28, AABB), or pasta wheat, arose <0.5 million years ago as a result of hybridization between Triticum urartu Tumanian ex Gandylian (2n = 2x = 14, AA) and an unidentified diploid Aegilops species of the section Sitopsis, thought to be Aegilops speltoides Tausch (2n = 2x = 14, SS) or a close relative thereof, as the donor of the B genome (12,13,(17)(18)(19)(20)(21)(22). A spontaneous hybridization that occurred between a primitive domesticated tetraploid T. turgidum subspecies and the diploid goatgrass Aegilops tauschii Coss (2n = 2x = 14, DD) about 10,000 years ago gave rise to T. turgidum (14,15).…”

Single-copy gene fluorescence in situ hybridization and genome analysis: Acc-2 loci mark evolutionary chromosomal rearrangements in wheat

“…The Acc-2 gene should be an easily detectable FISH target because of its large size (transcribed part is 12.3 kb long, with 6.8 kb of coding sequence) (Chalupska et al 2008). PCR primers were designed to produce a pooled FISH probe with a size of 5-6 kb (Table S1).…”

“…Six different sequences of the Acc-2 gene and pseudogene are present in the wheat genome (Podkowinski et al 1996;Huang et al 2002); at least three of the Acc-2 genes are expressed equally in young leaves (Podkowinski et al 2003) and their coding sequences (6.3 kb) have 98% identity (Gornicki et al 1994;Chalupska et al 2008). The genes and pseodogenes are likely arranged in tandem repeats (Faris et al 2001).…”