It was of interest to perform a phylogenetic analysis of
the AOB based on the ISR sequences, as the 16s
rRNA gene- based phylogeny has previously been
shown to have a very low resolution, owing to highsequence similarity. Phylogenetic trees constructed on
the basis of the ISR sequences (Fig. la) were topologically
quite similar to the trees constructed on the
basis of 16s rDNA sequences (Fig. 1 b). The branching
of the ISR trees was the same regardless of the method
was markedly improved compared with the 16s rRNA
gene-based phylogeny. The clustering of the trees
constructed on the basis of 16s rRNA gene sequences
has been shown to vary slightly when different
algorithms are used for the phylogenetic analysis
(Utiker et al., 1995), probably because of the small
overall sequence differences between the 16s rRNA
gene sequences. The trees in Fig. 1 were both made
using the maximum-likelihood method (Felsenstein,
198 1) and show the same branching patterns for 10 out
of the 12 AOB examined. Nitrosospira spp. F3 and
N11 3T cluster differently in the two phylogenetic trees
shown in Fig. 1. In a bootstrapped parsimony analysis
with 100 replicates, the branching points occur with a
considerably higher frequency for the ISR sequences
than in the corresponding tree based on 16s rRNA
gene sequences (not shown). The only exception is the
node dividing Nitrosospira multiformis spp. Nl1 3T and
F3 from the other Nitrosospiras, which occurs with a
frequency of 0.53. Clear conclusions on the phylogenetic
position of these two AOB cannot therefore be
drawn at present.
used for tree construction. The resolution, however,