DNA probes show genetic variation in cyanobacterial symbionts of the Azolla fern and a closer relationship to free-living Nostoc strains than to free-living Anabaena strains

Abstract

Twenty-two isolates of Anabaena azollae derived from seven Azolla species from various geographic and ecological sources were characterized by DNA-DNA hybridization. Cloned DNA fragments derived from the genomic sequences of three different A. azollae isolates were used to detect restriction fragment length polymorphism among all symbiotic anabaenas. DNA clones were radiolabeled and hybridized against southern blot transfers of genomic DNAs of different isolates of A. azollae digested with restriction endonucleases. Eight DNA probes were selected to identify the Anabaena strains tested. Two were strain specific and hybridized only to A. azollae strains isolated from Azolla microphylla or Azolla caroliniana. One DNA probe was section specific (hybridized only to anabaenas isolated from Azolla ferns representing the section Euazolla), and five other probes gave finer discrimination among anabaenas representing various ecotypes of Azolla species. These cloned genomic DNA probes identified 11 different genotypes of A. azollae isolates. These included three endosymbiotic genotypes within Azolla filiculoides species and two genotypes within both A. caroliniana and Azolla pinnata endosymbionts. Although we were not able to discriminate among anabaenas extracted from different ecotypes of Azolla nilotica, Azolla mexicina, Azolla rubra and Azolla microphylla species, each of the endosymbionts was easily identified as a unique genotype. When total DNA isolated from free-living Anabaena sp. strain PCC7120 was screened, none of the genomic DNA probes gave detectable positive hybridization. Total DNA of Nostoc cycas PCC7422 hybridized with six of eight genomic DNA fragments. These data imply that the dominant symbiotic organism in association with Azolla spp. is more closely related to Nostoc spp. than to free-living Anabaena spp.