Nematode Small Subunit Phylogeny Correlates with Alignment Parameters

The number of nuclear small subunit (SSU) ribosomal RNA (rRNA) sequences for Nematoda has increased dramatically in recent years, and although their use in constructing phylogenies has also increased, relatively little attention has been given to their alignment. Here we examined the sensitivity of the nematode SSU data set to different alignment parameters and to the removal of alignment ambiguous regions. Ten alignments were created with CLUSTAL W using different sets of alignment parameters (10 full alignments), and each alignment was examined by eye and alignment ambiguous regions were removed (creating 10 reduced alignments). These alignment ambiguous regions were analyzed as a third type of data set, culled alignments. Maximum parsimony, neighbor-joining, and parsimony bootstrap analyses were performed. The resulting phylogenies were compared to each other by the symmetric difference distance tree comparison metric (SymD). The correlation of the phylogenies with the alignment parameters was tested by comparing matrices from SymD with corresponding matrices of Manhattan distances representing the alignment parameters. Differences among individual parsimony trees from the full alignments were frequently correlated with the differences among alignment parameters (580/1000 tests), as were trees from the culled alignments (403/1000 tests). Differences among individual parsimony trees from the reduced alignments were less frequently correlated with the differences among alignment parameters (230/1000 tests). Differences among majority-rule consensus trees (50%) from the parsimony analysis of the full alignments were significantly correlated with the differences among alignment parameters, whereas consensus trees from the reduced and culled analyses were not correlated with the alignment parameters. These patterns of correlation confirm that choice of alignment parameters has the potential to bias the resultant phylogenies for the nematode SSU data set, and suggest that the removal of alignment ambiguous regions reduces this effect. Finally, we discuss the implications of conservative phylogenetic hypotheses for Nematoda produced by exploring alignment space and removing alignment ambiguous regions for SSU rDNA.