The Medicago genome provides insight into the evolution of rhizobial symbioses

TLDR

Legumes uniquely fix atmospheric nitrogen in root nodules, and Medicago truncatula, a model legume derived from a common ancestor 60 Myr ago, has been widely used to study this symbiosis. The study reports a draft euchromatic genome sequence of Medicago truncatula, covering approximately 94 % of its genes. The genome was assembled using BAC sequencing complemented by Illumina short‑read data. Whole‑genome duplication ~58 Myr ago and subsequent extensive rearrangements shaped the Medicago truncatula genome, and its sequenced genome offers a valuable resource for improving alfalfa genetics.

Abstract

Legumes (Fabaceae or Leguminosae) are unique among cultivated plants for their ability to carry out endosymbiotic nitrogen fixation with rhizobial bacteria, a process that takes place in a specialized structure known as the nodule. Legumes belong to one of the two main groups of eurosids, the Fabidae, which includes most species capable of endosymbiotic nitrogen fixation. Legumes comprise several evolutionary lineages derived from a common ancestor 60 million years ago (Myr ago). Papilionoids are the largest clade, dating nearly to the origin of legumes and containing most cultivated species. Medicago truncatula is a long-established model for the study of legume biology. Here we describe the draft sequence of the M. truncatula euchromatin based on a recently completed BAC assembly supplemented with Illumina shotgun sequence, together capturing ∼94% of all M. truncatula genes. A whole-genome duplication (WGD) approximately 58 Myr ago had a major role in shaping the M. truncatula genome and thereby contributed to the evolution of endosymbiotic nitrogen fixation. Subsequent to the WGD, the M. truncatula genome experienced higher levels of rearrangement than two other sequenced legumes, Glycine max and Lotus japonicus. M. truncatula is a close relative of alfalfa (Medicago sativa), a widely cultivated crop with limited genomics tools and complex autotetraploid genetics. As such, the M. truncatula genome sequence provides significant opportunities to expand alfalfa's genomic toolbox.