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

All 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.

Advances in Legume Systematics 14. Classification of Caesalpinioideae. Part 2: Higher-level classification

“…Although in Caesalpinioideae the ability to nodulate rarely occurs outside tribe Mimoseae , two Cassieae genera, Chamaecrista and Melanoxylum , are known to nodulate ( Sprent 2001 ; Faria et al 2022 ). Not all species of Chamaecrista are known to nodulate and those that do differ in the type of nodule anatomy ( Faria et al 2022 ; Casaes et al 2023 ).…”