A likely autotetraploidization event shaped the Chinese mahogany(Toona sinensis)genome

Chinese mahogany(Toona sinensis) is of considerable medical and economic importance, and its genome has been deciphered. However, the process underlying its polyploidy is unclear, and the chromosomal evolutionary trajectory is poorly understood. Here, by reanalysing the T.sinensis genome, we found evidence of a tetraploidization event(T. sinensis special tetraploidization, TST) that occurred approximately 15-17 million years ago(MYA) after the core eudicot-common hexaploidization(ECH or gamma) event. We characterized the synonymous nucleotide substitution rates(Ks values) of collinear genes and found that T. sinensis genes affected by the TST evolve at a slower rate than Acer yangbiense genes. Furthermore, we identified homologous genes related to polyploidization and speciation and constructed multiple alignments with different reference genomes. Notably, the significant balance of gene retention and loss characterized in the two TST-derived subgenomes suggests an autopolyploid nature of the TST. Moreover, we deduced the chromosomal karyotypes of the two subgenomes and identified 7chromosomal fusions that have shaped the T. sinensis genome;more information is available on a newly constructed karyotype platform(http://www.cgrpoee.top/Toona_sinensis/index.html). The T. sinensis genome preserves the ancestral chromosome structure of dicotyledons well and could serve as a good reference for understanding genomic changes in other Meliaceae and Sapindales plants. In addition, we verified that tandem duplication and the ECH have promoted the expansion of terpene synthase(TPS) genes;conversely, the TST seems to have inhibited expansion of these genes. This present effort has clarified the polyploidy events of the T. sinensis genome, filled gaps in the history of karyotype evolution, and laid a solid foundation for further genomic studies in the Meliaceae research community and beyond.