Evolution and diversity in the legume-rhizobium symbiosis: chaos theory?

EVOLUTION AND DIVERSITY IN THE LEGUME-RHIZOBIUM SYMBIOSIS: CHAOS THEORY? CONTENTS * Abstract * Citation * Links ABSTRACT Diversity in both legumes and rhizobia is discussed, in the light of

evolution of nodulation. An hypothesis is developed that two separate nodulation events occurred in the humid tropics during the evolution of legumes in the late Cretaceous. One of these

involved an ancestor of Rhizobium and a root infection. This was initially 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 climates 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. CITATION Plant and Soil (1994) 161 (1) 1-10 [doi: 10.1007/BF02183080] LINKS Evolution and diversity in the

legume-rhizobium symbiosis: chaos theory? UPDATES TO THIS PAGE Published 1 January 1994 Contents