COEVOLUTION OF RHIZOBIA WITH LEGUMES - FACTS AND HYPOTHESES

The ability to form N-2-fixing symbioses has a monophyletic (paraphyle tic) origin in legumes and a polyphyletic origin in rhizobia. Analysis of taxonomic data suggests that in legumes nodulation is a more ancie nt property than symbiotrophic nitrogen nutrition while in rhizobia th e ability to nodulate their hosts is of a more recent origin than the ability to fix N-2. The ancestors of both partners are supposed to hav e possessed the preadaptations which ensured a subsequent evolution of the symbiotic system. The plant preadaptations might have been: spont aneous formation of the nodule-like structures and an ability to permi t a persistence of potential symbionts in them. The bacterial preadapt ations might have been: the ability to fix N-2 and to resist (escape) the plant defense reactions. In rhizobia the nodulation ability is sup posed to evolve mainly via the individual selection, while the ability for symbiotic N-2 fixation - via the kin selection of the clones. Evo lution of nod genes in rhizobia is supposed to elicit formation of the mechanisms for restriction (regulation) of nodulation in hosts. Altho ugh the legume-rhizobia coevolution for nodulation might have the mark ed similarities with the gene-for-gene coevolution, the direct evoluti onary relationships between legume-rhizobia symbioses and the phytopat hogenic systems are not probable. The plant-bacteria coevolution is su ggested to be based on:(i) intensive intergenomic recombination and ho rizontal transfer of genes in rhizobia populations; (ii) increasing th e genetic heterogeneity of these populations via interaction with the hosts.