Genetic diversity and phylogeny analysis of Anabaena azollae based on RFLPs detected in Azolla-Anabaena azollae DNA complexes using nif gene probes

Abstract

The cyanobacterium Anabaena has both symbiotic and free-living forms. The genetic diversity of Anabaena strains symbiotically associated with the aquatic fern Azolla and the evolutionary relationships among these symbionts were evaluated by means of RFLP (restriction fragment length polymorphism) experiments. Three DNA fragments corresponding to nif genes were cloned from the free-living cyanobacterium Anabaena PCC 7120 and used as probes. A mixture of Azolla, Anabaena and bacterial DNA was extracted from Azolla fronds and digested with two restriction enzymes. Single-copy RFLP signals were detected with two of the probes in all Azolla Anabaena examined. Multiple-copy RFLP signals were obtained from the third probe which corresponded to a part of the nif N gene. A total of 46 probe/enzyme combinations were scored as present or absent and used to calculate pairwise Nei's genetic distances among symbiotic Anaebaena strains. Phylogenetic trees summarizing phenetic and cladistic relationships among strains were generated according to three different evolutionary scenarios: parsimony, UPGMA and neighbour joining. All trees revealed identical phylogenetic relationships. Principal component analysis was also used to evaluate genetic similarities and revealed three groups: group one contains the cyanobacteria associated with plants from the Azolla section, group two contains those associated with plants from the pinnata species and group three contains those associated with plants from the nilotica species. The same groups had already been identified earlier in a random amplified polymorphic DNA (RAPD) analysis of Azolla-Anbaena DNA complexes, suggesting that the present Azolla taxonomy should be revised. We now suggest a taxonomy of Anabaena azollae that is parallel to such a revised Azolla taxonomy. An Azolla chloroplast DNA sequence derived from Oryza sativa was also used as an RFLP probe on Azolla DNA to confirm the presence of plant DNA in the total genomic DNA extracted from ferns with or without the symbiont. Our results also suggest that total DNA extracted from the Azolla-Anabaena complexes includes both plant and symbiont DNA and can be used equally well for RFLP analysis of host plant or symbiotic cyanobacteria.