The complex process of allopolyploid speciation includes various mechanisms ranging from species crosses and hybrid genome doubling to genome alterations and the establishment of new allopolyploids as persisting natural entities. Currently, little is known about the genetic mechanisms that underlie hybrid genome doubling, despite the fact that natural allopolyploid formation is highly dependent on this phenomenon. We examined the genetic basis for the spontaneous genome doubling of triploid F₁ hybrids between the direct ancestors of allohexaploid common wheat (Triticum aestivum L., AABBDD genome), namely Triticum turgidum L. (AABB genome) and Aegilops tauschii Coss. (DD genome). An Ae. tauschii intraspecific lineage that is closely related to the D genome of common wheat was identified by population-based analysis. Two representative accessions, one that produces a high-genome-doubling-frequency hybrid when crossed with a T. turgidum cultivar and the other that produces a low-genome-doubling-frequency hybrid with the same cultivar, were chosen from that lineage for further analyses. A series of investigations including fertility analysis, immunostaining, and quantitative trait locus (QTL) analysis showed that (1) production of functional unreduced gametes through nonreductional meiosis is an early step key to successful hybrid genome doubling, (2) first division restitution is one of the cytological mechanisms that cause meiotic nonreduction during the production of functional male unreduced gametes, and (3) six QTLs in the Ae. tauschii genome, most of which likely regulate nonreductional meiosis and its subsequent gamete production processes, are involved in hybrid genome doubling. Interlineage comparisons of Ae. tauschii’s ability to cause hybrid genome doubling suggested an evolutionary model for the natural variation pattern of the trait in which non-deleterious mutations in six QTLs may have important roles. The findings of this study demonstrated that the genetic mechanisms for hybrid genome doubling could be studied based on the intrinsic natural variation that exists in the parental species.

An X-ray mutant showing an early flowering phenotype has been identified in einkorn wheat (Triticum monococcum L.), for which a major QTL for heading time was previously mapped in the telomeric region on the long arm of chromosome 3A. Recent advances in Triticeae genomics revealed that the gene order in this region is highly conserved between wheat and barley. Thus, we adopted a hypothetical gene order in barley, the so-called GenomeZipper, to develop DNA markers for fine mapping the target gene in wheat. We identified three genes tightly linked to the early heading phenotype. PCR analysis revealed that early-flowering is associated with the deletion of two genes in the mutant. Of the two deleted genes, one is an ortholog of the LUX ARRHYTHMO (LUX)/PHYTOCLOCK 1 (PCL1) gene found in Arabidopsis, which regulates the circadian clock and flowering time. We found distorted expression patterns of two clock genes (TOC1 and LHY) in the einkorn pcl1 deletion mutant as was reported for the Arabidopsis lux mutant. Transcript accumulation levels of photoperiod-response related genes, a photoperiod sensitivity gene (Ppd-1) and two wheat CONSTANS-like genes (WCO1 and TaHd1), were significantly higher in the einkorn wheat mutant. In addition, transcripts of the wheat florigen gene (WFT) accumulated temporally under short-day conditions in the einkorn wheat mutant. These results suggest that deletion of WPCL1 leads to abnormally higher expression of Ppd-1, resulting in the accumulation of WFT transcripts that triggers flowering even under short-day conditions. Our observations from gene mapping, gene deletions, and expression levels of flowering related genes strongly suggest that WPCL1 is the most likely candidate gene for controlling the early flowering phenotype in the einkorn wheat mutant.

A complete genome sequence is an essential tool for the genetic improvement of wheat. Because the wheat genome is large, highly repetitive and complex due to its allohexaploid nature, the International Wheat Genome Sequencing Consortium (IWGSC) chose a strategy that involves constructing bacterial artificial chromosome (BAC)-based physical maps of individual chromosomes and performing BAC-by-BAC sequencing. Here, we report the construction of a physical map of chromosome 6B with the goal of revealing the structural features of the third largest chromosome in wheat. We assembled 689 informative BAC contigs (hereafter reffered to as contigs) representing 91 % of the entire physical length of wheat chromosome 6B. The contigs were integrated into a radiation hybrid (RH) map of chromosome 6B, with one linkage group consisting of 448 loci with 653 markers. The order and direction of 480 contigs, corresponding to 87 % of the total length of 6B, were determined. We also characterized the contigs that contained a part of the nucleolus organizer region or centromere based on their positions on the RH map and the assembled BAC clone sequences. Analysis of the virtual gene order along 6B using the information collected for the integrated map revealed the presence of several chromosomal rearrangements, indicating evolutionary events that occurred on chromosome 6B. We constructed a reliable physical map of chromosome 6B, enabling us to analyze its genomic structure and evolutionary progression. More importantly, the physical map should provide a high-quality and map-based reference sequence that will serve as a resource for wheat chromosome 6B.