Co-evolution between Frankia populations and host plants in the family Casuarinaceae and consequent patterns of global dispersal

Symbioses between the root nodule-forming, nitrogen-fixing actinomycete Fra nkia and its angiospermous host plants are important in the nitrogen econom ies of numerous terrestrial ecosystems, Molecular characterization of Frank ia strains using polymerase chain reaction/restriction fragment length poly morphism (PCR/RFLP) analyses of the 16S rRNA-ITS gene and of the nifD-nifK spacer was conducted directly on root nodules collected worldwide from Casu arina and Allocasuarina trees, In their native habitats in Australia, host species contained seven distinctive sets of Frankia in seven different mole cular phylogenetic groups. Where Casuarina and Allocasuarina trees are newl y planted outside Australia, they do not normally nodulate unless Frankia i s introduced with the host seedling. Nodules from Casuarina trees introduce d outside Australia over the last two centuries were found to contain Frank ia from only one of the seven phylogenetic groups associated with the host genus Casuarina in Australia. The phylogenetic group of Frankia found in Ca suarina and Allocasuarina trees introduced outside Australia is the only gr oup that has yielded isolates in pure culture, suggesting a greater ability to survive independently of a host, Furthermore, the Frankia species in th is group are able to nodulate a wider range of host species than those in t he other six groups, In baiting studies, Casuarina spp, are compatible with more Frankia microsymbiont groups than Allocasuarina host spp, adapted to drier soil conditions, and C. equisetifolia has broader microsymbiont compa tibility than other Casuarina spp, Some Frankia associated with the nodular rhizosphere and rhizoplan, but not with the nodular tissue, of Australian hosts were able to nodulate cosmopolitan Myrica plants that have broad micr osymbiont compatibility and, hence, are a potential host of Casuarinaceae-i nfective Frankia outside the hosts' native range. The results are consisten t with the idea that Frankia symbiotic promiscuity and ease of isolation on organic substrates, suggesting saprophytic potential, are associated with increased microsymbiont ability to disperse and adapt to diverse new enviro nments, and that both genetics and environment determine a host's nodular m icrosymbiont.