Novel insights into karyotype evolution and whole genome duplications in legumes

Abstract

Legumes (family Fabaceae) are globally important crops due to their nitrogen fixing ability. Papilionoideae, the best-studied subfamily, have undergone a Whole Genome Duplication (WGD) around 59 million years ago. Recent study found varying WGD ages in subfamilies Mimosoideae and Caesalpinioideae and proposed multiple occurrences of WGD across the family based on gene duplication patterns. Despite that, the genome evolution of legume ancestor into modern legumes after the WGD is not well-understood. We aimed to study genome evolution at the subfamily level using gene-based linkage maps for Acacia auriculiformis and A. mangium (Mimosoideae) and we discovered evidence for a WGD event in Acacia. In additional to synonymous substitution rate (Ks) analysis, we used ancestral karyotype prediction to further corroborate this WGD and elucidate underlying mechanisms of karyotype evolution in Fabaceae. Using publicly available transcriptome resources from 25 species across the family Fabaceae and 2 species from order Fabales, we found that the variations in WGD ages highly correlate (R=0.8606, p-value<0.00001) with the divergence age of Vitis vinifera as an outgroup. If the variation of Ks is corrected, the age of WGDs of the family Fabaceae should be the same and therefore, parsimony would favour a single WGD near the base of Fabaceae over multiple independent WGDs across Fabaceae. In addition, we demonstrated that genome comparison of Papilionoideae with other subfamily provide important insights in understanding genome evolution in legumes.