last update October 28, 2000
Diversity in both legumes and rhizobia is discussed, in the light of evolution of nodulation. An hypothesis is developed that two separate nodulation events occured in the humid topics during the evolution of legumes in the late Cretaceous. One of these involved an ancestor of Rhizobium and a root infection. This was parasitic and provided little benefit until bacteria were released from infection threads as in modern crop species. The other involved a photosynthetic ancestor of Bradyrhizobium with a wound infection on stems, and has never involved infection threads. As continents moved and changed to a seasonal type, involving either rainfall or temperature extremes, further constraints were imposed. The argument is pursued for the case of acacias and their rhizobia in arid regions. Here selection pressures on rhizobia led to the evolution of stress tolerant forms, not all of which are capable of symbiosis, and where symbiotic genes may be an expensive encumbrance. Lateral transfer of material on megaplasmids led to a wide range of symbiotic and non-symbiotic forms in response to local pressures. When environmental constraints are superimposed on initial evolutionary developments, the result is an apparently chaotic situation where there is no obvious pattern of co-evolution between hosts and rhizobia. Evidence of such coevolution may still be buried in this chaos and may be amenable to molecular analysis.