Clarification of the evolution of pasta and bread wheat — Functional diversity of genes induced by allopolyploidization

A research group led by Special Postdoctoral Researcher Akihiro Ezoe and Team Leader Motoaki Seki of the Plant Genomic Network Research Team at the RIKEN Center for Sustainable Resource Science, and Associate Professor Kanako Kawaura of the Kihara Institute for Biological Research at Yokohama City University has announced their research results showing that two new species establishment events, allotetraploidization establishing durum wheat (pasta wheat) and allohexaploidization establishing bread wheat, had different effects on the subsequent functional diversity of the genes. Allopolyploidy in pasta wheat may have produced a higher diversity of gene function than in bread wheat. The findings are expected to contribute to developing improved varieties and new crops. The results were published in the international journal The Plant Journal on October 21.

Individuals having multiplicative numbers of chromosomes in somatic cells compared to the basic number (x) are called polyploids. Allopolyploidization is known to have occurred in hybrids resulting from the crossbreeding of ancestral diploid species. The type of allopolyploidization in which the number of chromosome sets quadruples is called allotetraploidization. In the case of allohexaploidization, the increase is six times. An allopolyploidization event results in significant increases in the number of genes present in a single cell and the likelihood that each gene will have a different function, creating functional diversity of the genes, which drives evolution.

Both pasta and bread wheat resulted from allopolyploidization. Tetraploid pasta wheat occurred through hybridization between diploids and polyploidization. Hybridization of this tetraploid pasta wheat with a different diploid species and polyploidization yielded hexaploid bread wheat. As a result of the diverse functions acquired through the allopolyploidization events, these wheat species are currently grown in the widest range of geographical regions.

Pasta wheat went through a single allopolyploidization event, and bread wheat went through two. The two species have been used in studies to investigate the effects of allopolyploidization on functional diversity because they differ in the number of events they underwent. The genome information on five wheat species involved in the allopolyploidization processes leading to both species has been revealed recently. It has become possible to verify the evolutionary processes in which these species were established through this process.

Previously, the research group had shown in model plants that when genes increased via events such as allopolyploidization, the genes changing their functions differed from one event to another. They have demonstrated the possibility that the first and second allopolyploidization events have different effects on the functional diversity of the genes.

In the present study, they focused on the first allopolyploidization event (about 800,000 years ago) in pasta and bread wheat and the second allopolyploidization event (about 9,000 years ago) in bread wheat and examined the differences. The results showed that their effects on genetic diversity were different. In particular, the allopolyploidization event in pasta wheat could have had a greater impact on the functional diversity of the gene. A closer examination of the genes whose function changed after the first allopolyploidization event revealed that they tended to be the same as the genes whose function changed after allopolyploidization in a completely different allotetraploid cotton species. This finding indicates that functional changes in the same genes may be required in the process from a stable diploid to a tetraploid.

Ezoe said, "Because polyploidization complicates evolution, not much progress has been made in evolutionary research on wheat despite its familiarity because it has gone through several polyploidization events. In this study, we devised a new approach to simplify wheat evolution and showed how wheat polyploidization affected evolution. Our results are expected to be useful for wheat breeding. On top of that, we expect that we can use this approach to unravel the evolutionary mysteries of other polyploid crops one after another."

Journal Information
Publication: The Plant Journal
Title: Decrease in purifying selection pressures on wheat homoeologous genes: tetraploidization versus hexaploidization
DOI: 10.1111/tpj.17047