In many non-cultivated angiosperm species, seed dispersal is facilitated by the shattering of the seed head at maturity; in the Triticeae tribe, to which several of the world's most important cereals belong, shattering takes the form of a disarticulation of the rachis. The products of the genes Btr1 and Btr2 are both required for disarticulation to occur above the rachis nodes within the genera Hordeum (barley) and Triticum/Aegilops (wheat). Here, it has been shown that both Btr1 and Btr2 are specific to the Triticeae tribe, although likely paralogs (Btr1-like and Btr2-like) are carried by the family Poaceae including Triticeae. Aegilops tauschii (the donor of the bread wheat D genome) lacks a copy of Btr1 and disarticulation in this species occurs below, rather than above the rachis node; thus, the product of Btr1 appears to be required for disarticulation to occur above the rachis node.

Background and Aims The brittle rachis trait is a feature of many wild grasses, particularly within the Triticeae tribe. Wild Hordeum and Triticum species form a disarticulation layer above the rachis node, resulting in the production of wedge type dispersal units. In Aegilops longissima, only one or two of the nodes in the central portion of its rachis are brittle. In the Triticeae species, the formation of a disarticulation layer above the rachis node requires the co-transcription of the two dominant and complementary genes Btr1 and Btr2. This study aims to establish whether homologs of Btr1 and/or Btr2 underlie the unusual brittle rachis phenotype observed in Ae. longissima. Methods Scanning electron microscopy was used to examine the disarticulation surfaces. Quantitative RT-PCR and RNA in situ hybridization experiments were employed to identify the gene expression in the immature inflorescence. Key Results An analysis based on scanning electron microscopy was able to demonstrate that the disarticulation surfaces formed in the Ae. longissima rachis are morphologically indistinguishable from those formed in the rachises of wild Hordeum and the Triticum species. An RNA in situ hybridization experiment was able to identify that in the immature Ae. longissima inflorescence, the intensity of Btr1 transcription varied from high at the rachis base to low at its apex, while that of Btr2 was limited to the nodes in the central to distal portion of the rachis. Conclusions The disarticulation pattern shown by Ae. longissima results from the limitation of Btr1 and Btr2 co-expression to nodes lying in the center of the rachis.

This study provides a link between a de novo gene and novel phenotype in wheat-rye hybrids that can be used as a model for induced de novo genetic variation. Wide hybridization can produce de novo DNA variation that may cause novel phenotypes. However, there is still a lack of specific links between changed genes and novel phenotypes in wide hybrids. The well-studied high-molecular-weight glutenin subunit (HMW-GS) genes in tribe Triticeae provide a useful model for addressing this issue. In this study, we investigated the feasibility of a wheat-rye hybridization method for inducing de novo phenotypes using the Glu-1Dx2.2 subunit as an example. We developed three hexaploid wheat lines with normal fertility and a Glu-1Dx2.2 variant, named Glu-1Dx2.2 (v) , derived from three F1 hybrids. The wild-type Glu-1Dx2.2 has two direct repeats of 295 bp length separated by an intervening 101 bp in its central repetitive region. In the mutant Glu-1Dx2.2 (v) , one copy of the repeats and the intervening sequence were deleted, probably through homology-dependent illegitimate recombination (IR). This study provides a direct link between a de novo allele and novel phenotype. Our results indicate that the wheat-rye method may be a useful tool to induce de novo genetic variations that broaden the genetic diversity for wheat improvement.