Nodulation in the endemic Australian Papilionoideae tribes Mirbelieae and Bossiaeeae

The closely related papilionoid tribes Mirbelieae and Bossiaeeae are a large (ca. 750 species) group of endemic Australasian legumes. Typically they are ericoid shrubs that are conspicuous understorey members of sclerophyll communities growing on poor soils of the southwest and southeast temperate biomes (Crisp et al. 2004). The centre of diversity is in Southwest Australia, a biodiversity hotspot, unglaciated since the Permian (ca. 250 Mya) and dominated by old landscapes with nutrient-deficient soils (Hopper and Gioia 2004). The crown clade age for the mirbelioids and their sister tribe Hypocalypteae is estimated to be ca. 55 Mya, suggesting that mirbelioids evolved in isolation, shortly after Australia separated from the Antarctic continent, having lost contact with other Gondwanan land masses (Sprent et al. 2013). Molecular-dated phylogenies indicate that the group radiated rapidly during the Mid-Cenozoic (ca. 25–10 Mya) period of climatic cooling and drying (Crisp et al. 2004). In common with most papilionoid legumes, Mirbelieae and Bossiaeeae species form nitrogen-fixing associations with rhizobia, usually with strains of Bradyrhizobium (Lafay and Burdon 1998; Stepkowski et al. 2012). Nodule morphology and structure has not yet been studied. This character trait has previously been found to be a useful phylogenetic and taxonomic marker (Sprent et al. 2013). Phylogenetic analyses place the mirbelioid legumes between the Dalbergioid and Indigoferoid clades, two groups that have quite different nodule structures (Sprent et al. 2013). This study aimed to characterise rhizobial microsymbionts of diverse mirbelioid species collected from sites across southwest Australia and to determine the mirbelioid nodule morphology and structure.