Buxus and Tetracentron genomes help resolve eudicot genome history

Ancient whole-genome duplications (WGDs) characterize many large angiosperm lineages, including angiosperms themselves. Prominently, the core eudicot lineage accommodates 70% of all angiosperms and shares ancestral hexaploidy, termed gamma. Gamma arose via two WGDs that occurred early in eudicot history; however, the relative timing of these is unclear, largely due to the lack of high-quality genomes among early-diverging eudicots. Here, we provide complete genomes for Buxus sinica (Buxales) and Tetracentron sinense (Trochodendrales), representing the lineages most closely related to core eudicots. We show that Buxus and Tetracentron are both characterized by independent WGDs, resolve relationships among early-diverging eudicots and their respective genomes, and use the RACCROCHE pipeline to reconstruct ancestral genome structure at three key phylogenetic nodes of eudicot diversification. Our reconstructions indicate genome structure remained relatively stable during early eudicot diversification, and reject hypotheses of gamma arising via inter-lineage hybridization between ancestral eudicot lineages, involving, instead, only stem lineage core eudicot ancestors. Gamma triplication arises via two whole-genome duplications early in eudicot history, but the relative timing of these is unclear. Here, the authors report the genomes of Buxales and Trochodendrales and reject the hypothesis of gamma arising via inter-lineage hybridization between ancestral eudicot lineages.

Automated summary

This study investigates the origin of gamma, a hexaploid state shared by a large number of angiosperms, by analyzing the genomes of Buxus sinica and Tetracentron sinense. The results suggest that gamma arose through two whole-genome duplications early in eudicot history, and reject the hypothesis of inter-lineage hybridization.