Aims The large monophyletic genus Mimosa comprises approx. 500 species, most of which are native to the New World, with Central Brazil being the main centre of radiation. All Brazilian Mimosa spp. so far examined are nodulated by rhizobia in the betaproteobacterial genus Burkholderia. Approximately 10 Mya, transoceanic dispersal resulted in the Indian subcontinent hosting up to six endemic Mimosa spp. The nodulation ability and rhizobial symbionts of two of these, M. hamata and M. himalayana, both from north-west India, are here examined, and compared with those of M. pudica, an invasive species. † Methods Nodules were collected from several locations, and examined by light and electron microscopy. Rhizobia isolated from them were characterized in terms of their abilities to nodulate the three Mimosa hosts. The molecular phylogenetic relationships of the rhizobia were determined by analysis of 16S rRNA, nifH and nodA gene sequences. † Key Results Both native Indian Mimosa spp. nodulated effectively in their respective rhizosphere soils. Based on 16S rRNA, nifH and nodA sequences, their symbionts were identified as belonging to the alphaproteobacterial genus Ensifer, and were closest to the 'Old World' Ensifer saheli, E. kostiensis and E. arboris. In contrast, the invasive M. pudica was predominantly nodulated by Betaproteobacteria in the genera Cupriavidus and Burkholderia. All rhizobial strains tested effectively nodulated their original hosts, but the symbionts of the native species could not nodulate M. pudica. † Conclusions The native Mimosa spp. in India are not nodulated by the Burkholderia symbionts of their South American relatives, but by a unique group of alpha-rhizobial microsymbionts that are closely related to the 'local' Old World Ensifer symbionts of other mimosoid legumes in north-west India. They appear not to share symbionts with the invasive M. pudica, symbionts of which are mostly beta-rhizobial.

Ensifer sp. TW10 is a novel N2-fixing bacterium isolated from a root nodule of the perennial legume Tephrosia wallichii Graham (known locally as Biyani) found in the Great Indian (or Thar) desert, a large arid region in the northwestern part of the Indian subcontinent. Strain TW10 is a Gram-negative, rod shaped, aerobic, motile, non-spore forming, species of root nodule bacteria (RNB) that promiscuously nodulates legumes in Thar Desert alkaline soil. It is fast growing, acid-producing, and tolerates up to 2% NaCl and capable of growth at 40oC. In this report we describe for the first time the primary features of this Thar Desert soil saprophyte together with genome sequence information and annotation. The 6,802,256 bp genome has a GC content of 62% and is arranged into 57 scaffolds containing 6,470 protein-coding genes, 73 RNA genes and a single rRNA operon. This genome is one of 100 RNB genomes sequenced as part of the DOE Joint Genome Institute 2010 Genomic Encyclopedia for Bacteria and Archaea-Root Nodule Bacteria (GEBA-RNB) project.