Construction of a new synthetic allohexaploid wheat and its potential for wheat improvement

Published 23 April, 2025

Researchers have synthesized a new allohexaploid wheat, Triticum kiharae, and demonstrated a feasible route to simultaneously introgress standing genetic variations from both the Timopheevii wheat (T. timopheevii) and goat-grass (Aegilops tauschii), as well as heritable de novo variations that have arisen in T. kiharae, into common wheat.

Common wheat (Triticum aestivum L.) is a staple food crop, providing about 20% of the protein and caloric intake in human diets. T. aestivum is an allohexaploid species (2n = 6x = 42) containing three subgenomes (A, B and D) and stands out as a textbook example of speciation via allopolyploidization. Nonetheless, while modern breeding has achieved remarkable success in boosting wheat production, it has also led to a sharp decline and loss of valuable intraspecific genetic variations.

The introduction of genetic variants from wild relatives through distant hybridization and chromosome engineering has proven to be an effective strategy for sustainable wheat improvement. However, because most of the wild relative species of wheat are diploid or tetraploid species, rendering interspecific hybridization between these species and common wheat invoking different ploidy levels. This limitation makes such hybridization efforts challenging and time-consuming.

Now, researchers from Northeast Normal University and Dalian University of Technology have successfully synthesized a new allohexaploid wheat, T. kiharae, by hybridizing the tetraploid Timopheevii wheat, T. timopheevii Zhuk. (genome GGAA) and goat-grass Aegilops tauschii (genome DD).

The researchers demonstrated that T. kiharae, being of the same ploidy level as common wheat, can be easily crossed with the latter, and more importantly, the G-subgenome of T. kiharae can readily recombine with chromosomes of the B-subgenome of common wheat in the resulting backcrossed progenies, largely circumventing the intrinsic hindrances associated with inter-ploidy distant hybridizations. Notably, the study has confirmed that T. kiharae indeed merits the status as an independent biological species—a proposal raised but not confirmed in previous studies.

“We conducted reciprocal crossing experiments between common wheat (cv. Chinese Spring, CS) and T. kiharae, and found both reciprocal F₁ hybrids (BGAADD and GBAADD) displayed complete sterility, demonstrating the existence of intrinsic postzygotic reproductive isolation berries between them,” shares the study’s co-corresponding author Ruili Lyu (吕睿丽), a postdoctoral researcher at Dalian University of Technology.

These results confirm that T. kiharae constitutes a distinct biological species, with the observed sterility likely stemming from genomic incompatibilities between the B and G subgenomes—a finding consistent with phylogenomic analyses indicating their divergence approximately 1.5 million years ago.

“Fortunately, the sterility of F₁ hybrids is uni-gametal, that is, only the pollen is defective (male sterility), while the egg gamete is fertile (female fertility), enabling successful backcrossing of the F₁ hybrids to common wheat to generate largely fertile BC₁F₁ plants,” adds Lyu.

T. kiharae could serve as an intermediate to allow simultaneous, high-throughput introgression of useful segments or genes from T. timopheevii and Ae. tauschii, as well as de novo heritable variations that have occurred in T. kiharae, into common wheat, thereby promising to dramatically enhance the increasingly diminishing intraspecific genomic diversities of common wheat.

In addition, the researchers have also addressed a long-standing question in the field of basic allopolyploidy research, that is, whether a homoeologous pairing suppressor gene that is evolved and functions in a lower ploidy allotetraploid can retain full functionality when the ploidy level rises accompanied with the addition of a new subgenome.

“The Timopheevii wheat, like its tetraploid emmer wheat (T. turgidum L.) counterpart, has always been suspected to also contain the major homoeologous pairing suppressor gene called Ph1 based on classical cytogenetic analyses of chromosome pairing during meiosis,” explains Lyu. “Thanks to the recent release of the genome sequence of Timopheevii, we retrieved the homolog of the known Ph1 gene of emmer and common wheat, called Zip4-2B, and did comparative analyses.”

Together with the findings of abnormal meiotic chromosome pairing in T. kiharae and both numerical and structural chromosomal variations detected in selfed progenies of the synthetic allohexaploid, the researchers posit that the Timophevii Ph1 gene is likely hypo-functional in T. kiharae.

“We retrieved and aligned the protein sequences of all homologous members of the Zip4-2B gene (the Ph1 gene), including TaZip4-5B, TdZip4-5B, and TtZip4-5G, and identified six amino acid substitutions, which happened in critical functional domains” says Lyu. “This suggests that the Ph1 gene might have originated from the last common ancestor of the two diploid Aegilops species with the BB and GG genomes, respectively, which are the direct donors to the B and G subgenomes of tetraploid emmer wheat and Timopheevii wheat, but functional divergence of this gene may have occurred during evolution for still unknown reasons.”

The researchers also found T. kiharae shows broadly similar global gene expression patterns as previously documented in common wheat, but it also manifests some distinct features.

“Going forward, we will conduct more detailed molecular characterization of T. kiharae, but may prioritize our efforts to the practical aspect, such as assessing the environmental resilience and adaptability of T. kiharae, and test whether these features (if present in T. kiharae) can also be transferred to common wheat together with known desirable traits in T. kiharae, like disease resistance”, says Lyu.

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Media contact:

Name: Guangming Yang
Email address: yangguangming@caas.cn 
State of Origin: China
Tel: 13683670916

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