Phylogenetic perspectives on nodulation: evolving views of plants and symbiotic bacteria

Phylogenetic studies are contributing greatly to our knowledge of relations hips on both sides of the plant-bacteria nodulation symbiosis. Multiple ori gins of nodulation (perhaps even within the legume family) appear likely. H owever, all nodulating flowering plants are more closely related than previ ously suspected, suggesting that the predisposition to nodulate might have arisen only once. Phylogenies of 16S rRNA genes highlight the evolutionary diversity of symbiotic bacteria and appear to rule out any broad coevolutio n with their plant hosts, but high levels of gene transfer might obscure th e relevant pattern. The origins of nodulation, and the extent to which deve lopmental programs are conserved in nodules remain unclear, but an improved understanding of the relationships between nodulin genes is providing some clues.