* kentaro.shimizu@uzh.ch

Polyploidization is a common phenomenon found in both plants and animals, and is considered an important evolutionary driver. However, the contribution of polyploidization to biological evolution and environmental adaptation is still debated. Wheat serves as a typical example of how polyploidy expands habitats compared with their parental species.

In this study, to reveal how the expression patterns of each homoeologous gene located on the A, B, and D genomes were altered by allopolyploidization, we conducted RNA-seq analysis of leaf samples from three hexaploid wheat (Triticum aestivum L. cv. 'Chinese Spring' (CS), 'Arina LrFor' (Ar), and 'Norin 61' (N61)), one tetraploid wheat (Triticum turgidum ssp. durum Desf.) cv. 'Langdon' (Ldn), an artificial tetraploid obtained by removing the D genome from CS (Tetra-Chinese Spring, TCS), two accessions of Aegilops tauschii Coss., and two synthetic hexaploid wheat lines obtained by crossing between Ldn and the two Ae. tauschii under both control and cold conditions.

The seeds of N61, Ldn, and the two accessions of Ae. tauschii were obtained by the National BioResource Project-Wheat (Kyoto University, Japan). TCS, Ar and synthetics were supplied by Dr. Hisashi Tsujimoto, Dr. Beat Keller and Dr. Yoshihiro Matsuoka, respectively.

We examined the change in the ratio of maternal (AB) to paternal (D) expression. For accurate homoeologous expression analysis, we used EAGLE-RC software (Kuo et al. 2020) to distinguish the RNA-seq reads into the A, B, and D homoeologs. First, we evaluated how the expression ratio of the AB and D genomes changed after the cold treatment.

Homoeologs that showed differential expression ratios (differentially expressed homoeologs, DEHs) accounted for approximately 1% of all genes and included cold-responsive genes. Second, we compared the expression ratios between synthetic and virtual synthetic wheat created by combining the count data of Ldn and Ae. tauschii. Virtual synthetics have more DEHs than synthetics, indicating that the regulation of gene expression occurs through the interaction of three homoeologous genes of ABD in the hexaploid background.

To evaluate the direct effect of allopolyploidization on homoeologous expression patterns, we compared DEHs among virtual synthetics, synthetics, and cultivars. The number of DEHs decreases with each passing generation.

In summary, our findings suggest that allopolyploidization allows allopolyploid species to adapt to novel environments, not only by combining their partial expression patterns, but also by obtaining new expression patterns of genes that are necessary for a particular situation.