Comparative Genomics Suggests That an Ancestral Polyploidy Event Leads to Enhanced Root Nodule Symbiosis in the Papilionoideae

Root nodule symbiosis (RNS) is one of the most efficient biological systems for nitrogen fixation and it occurs in 90% of genera in the Papilionoideae, the largest subfamily of legumes. Most papilionoid species show evidence of a polyploidy event that occurred approximately 58 Ma. Although polyploidy is considered to be an important evolutionary force in plants, the role of this papilionoid polyploidy event, especially its association with RNS, is not understood. In this study, we explored this role using an integrated comparative genomic approach and conducted gene expression comparisons and gene ontology enrichment analyses. The results show the following: 1) Approximately a quarter of the papilionoid-polyploidy-derived duplicate genes are retained; 2) there is a striking divergence in the level of expression of gene duplicate pairs derived from the polyploidy event; and 3) the retained duplicates are frequently involved in the processes crucial for RNS establishment, such as symbiotic signaling, nodule organogenesis, rhizobial infection, and nutrient exchange and transport. Thus, we conclude that the papilionoid polyploidy event might have further refined RNS and induced a more robust and enhanced symbiotic system. This conclusion partly explains the widespread occurrence of the Papilionoideae.