Format: ORAL

Wenjie Mu1, Kexin Li1, Yongzhi Yang1, Adina Breiman2, Jiao Yang1, Ying Wu1, Mingjia Zhu1, Shuai Wang2, Eviatar Nevo3, Jianquan Liu1, Pilar Catalan4

1 State Key Laboratory of Herbage Innovation and Grassland Agro-Ecosystem, College of Ecology, Lanzhou University, Lanzhou 730000, China. 2 University of Tel-Aviv, Tel-Aviv 6997801, Israel. 3 Institute of Evolution, University of Haifa, Mount Carmel, Haifa 3498838, Israel. 4 Escuela Politecnica Superior de Huesca, Universidad de Zaragoza, Ctra. Cuarte km 1, 22071 Huesca, Spain.

Both homeologous exchanges (HEs) and homeologous expression bias (HEB) are generally found in most allopolyploid species. Whether HEs and HEBs differ between repeated allopolyploid speciation events from the same progenitor species remains unknown. Here we detected a third independent and recent allotetraploid origin for the model grass Brachypodium hybridum, discarding alternative scenarios of potential introgressions of early allotetraploids with diploid progenitor species. Our HE with replacement analyses indicated the absence of significant homeologous exchanges in any of the three types of wild allotetraploids, supporting the integrity of their progenitor subgenomes and the immediate creation of the amphidiploids. The absence of HE is probably due to the distinct structure of the homeologous progenitor chromosomes. Specifically, the five B. distachyon chromosomes resulted from four centromeric nested chromosome fusions of the ten B. stacei chromosomes, making homeologous crossovers highly improbable in the B. hybridum nucleus. Further HEB tests did not uncover significant subgenomic dominance in different tissues and conditions of the allotetraploids. This suggests a balanced expression of homeologs under similar or dissimilar ecological conditions in their natural habitats. We observed that the density of transposons around genes was not associated with the initial establishment of subgenome dominance; rather, this feature is inherited from the progenitor genome. We found that drought response genes were highly induced in the two subgenomes, likely contributing to the local adaptation of this species to arid habitats in the third allotetraploid event. These findings provide evidence for the consistency of subgenomic stability of parental genomes across multiple allopolyploidization events that led to the same species at different periods. Our study emphasizes the importance of selecting closely related progenitor species genomes to accurately assess HE with replacement in allopolyploids, thereby avoiding the detection of false HEs when using less related progenitor species genomes.