Maximum Ages of Ciliate Lineages Estimated Using a Small Subunit rRNA Molecular Clock: Crown Eukaryotes Date Back to the Paleoproterozoic

Abstract

We estimated the rate of nucleotide substitution for the obligate freshwater fish ectoparasite, Ichthyophthirius, and its closest free-living relative, Ophryoglena, using an independently-timed event — the origin of freshwater fish in the fossil record. Based on this information, the rate of nucleotide substitution per site, per year, per lineage is 1.25 to 1.4 × 10⁻⁸ or 1% divergence per 72 to 80 million years (My). Using this rate, we determined that the origin of the ciliates (i.e. crown eukaryotes) is much older than previously speculated, dating back to the Paleoproterozoic some 1980 to 2200 million years ago (Ma). We also determined that the wellestablished lineages recognized as classes today (e.g. Spirotrichea, Oligohymenophorea, Nassophorea, Colpodea, Heterotrichea, Karyorelictea, and Litostomatea) diverged within 600-My of the ciliate-like ancestor diverging from the main eukaryotic line.