Phylogenetic relationships among karyorelictids and heterotrichs inferred from small subunit rRNA sequences: resolution at the base of the ciliate tree.

Ciliate protozoa are among the most diverse and complex cells that have been described. Ciliates are characterized by nuclear dimorphism, possessing a macronucleus and a micronucleus which share the same cytoplasm. An understanding of the evolution of ciliate diversity depends upon knowledge of their phylogeny. In this study we attempted to resolve some of the relationships at the base of the ciliate tree by determining the phylogenetic position of a sample of heterotrich and hypothesized primitive karyorelictid ciliates. Karyorelictids are considered primitive because they possess a "simple" form of nuclear dualism whereby the macronucleus does not divide once it has differentiated from a micronucleus. We micromanipulated cells of two heterotrichs, Spirostomum ambiguum and Gruberia sp., and two karyorelictids, Loxodes magnus and Tracheloraphis sp., and amplified their small subunit (SSU) rDNA using PCR. The primary structure of the SSU rDNA was determined for each species and used to infer their positions in the ciliate phylogenetic tree. The results indicate, with strong support, that the aerobic heterotrichs and the karyorelictids sampled constitute a monophyletic group. The most parsimonious interpretation of the form of nuclear dimorphism in karyorelictids is that it is derived from the general condition as found in its sister group the aerobic heterotrichs. The two anaerobic heterotrichs, Metopus contortus and Metopus palaeformis, comprise a distinct clade, so that the subclass Heterotrichia, as currently conceived, is not a monophyletic group. The complex mouth architecture which characterizes all heterotrichs must be reassessed in light of this finding.