Form species Nostoc commune (Cyanobacteria)

The form species concept for the Cyanobacteria was evaluated using a compre hensive set of Nostoc samples that were collected during the past two centu ries, from all continents, including regions from the Tropics to the Poles. Phylogenies were constructed based upon the conserved regions of tRNA(Leu) (UAA) group I intron DNA sequences. Thirty-four forms contained a tRNA(Leu ) (UAA) intron of 284 nt. These 284-nt introns contained 200 nt of conserve d sequence that, in most cases, shared 100% sequence identity, they had thr ee variable regions (I, II and III) amounting to 84 nt, contained no hyperv ariable region and formed a discrete cluster in phylogenetic analysis. Thes e forms represented 31 independent populations in both hemispheres and cons titute examples of form species Nostoc commune. Multiple introns were obtai ned from several of the populations. Ten populations contained introns of 2 87-340 nt with a hypervariable region, 8 to 59 nt in length, located betwee n variable regions I and If. Alignments identified 15 examples where 5 ' -A AAAUCC-3 ' occurred at the hypervariable region-variable region II boundary ; this sequence is identical to the conserved sequence at the 3 ' intron-ex on boundary (splice site) within the tRNA(Leu) (UAA) gene. The possibility that hypervariable regions were removed from the primary intron through sec ondary splicing was tested in vitro but proved to be negative under the exp erimental conditions used. Shared morphologies of genetically different str ains, dissimilar morphologies in strains that share identical genetic marke rs, incorrect naming of culture collection strains and genetic drift in cul tured strains emphasize that the successful delineation of cyanobacterial s pecies requires the application of multiple taxonomic criteria.