Effects of sequence alignment on the phylogeny ofSarcocystis deduced from 18S rDNA sequences
The family Sarcocystidae contains a wide variety of parasitic protozoa, some of which are important pathogens of livestock and humans. The taxonomic relationships between two of the genera in this family (Toxoplasma andSarcocystis) have been debated for a number of years and remain controversial. Recent studies, from comparisons of 18S rDNA-sequence data, have suggested thatSarcocystis is paraphyletic, although a hypothesis supporting monophyly ofSarcocystis could not be rejected. The present study shows that the phylogenetically informative nucleotide positions within the 18S rDNA are primarily located in the regions that make up the helices in the secondary structure of the 18S rRNA. A phylogenetic analysis of 18S rDNA-sequence data aligned by secondary structure constraints, or a subset of the data corresponding to all nucleotides found in the helices, provide unambiguous evidence supporting monophyly ofSarcocystis.