The radiation of nodulatedChamaecristaspecies from the rainforest into more diverse habitats has been accompanied by a reduction in growth form and a shift from fixation threads to symbiosomes

Abstract

AbstractAll non-mimosoid nodulated genera in the legume subfamily Caesalpinioideae confine their rhizobial symbionts within cell wall-bound “fixation threads” (FTs). The exception is the large genusChamaecristain which shrubs and subshrubs house their rhizobial bacteroids more intimately within symbiosomes, whereas large trees have FTs. This study aimed to unravel the evolutionary relationships betweenChamaecristagrowth habit, habitat, nodule bacteroid type, and rhizobial genotype. The growth habit, bacteroid anatomy, and rhizobial symbionts of 30 nodulatedChamaecristaspecies native to different biomes in the Brazilian state of Bahia, a major centre of diversity for the genus, was plotted onto an ITS-TrnL-F-derived phylogeny ofChamaecrista. The bacteroids from most of theChamaecristaspecies examined were enclosed in symbiosomes (SYM-type nodules), but those in arborescent species in the sectionApoucouita, at the base of the genus, were enclosed in cell wall material containing homogalacturonan (HG) and cellulose (FT-type nodules). Most symbionts wereBradyrhizobiumgenotypes grouped according to the growth habits of their hosts, but the tree,C. eitenorum,was nodulated byParaburkholderia.Chamaecristahas a range of growth habits that allow it to occupy several different biomes and to co-evolve with a wide range of (mainly) bradyrhizobial symbionts. FTs represent a less intimate symbiosis linked with nodulation losses, so the evolution of SYM-type nodules by mostChamaecristaspecies may have (a) aided the genus-wide retention of nodulation, and (b) assisted in its rapid speciation and radiation out of the rainforest into more diverse and challenging habitats.