THE EVOLUTIONARY HISTORY OF THE GENUS ACANTHAMOEBA AND THE IDENTIFICATION OF 8 NEW 18S RIBOSOMAL-RNA GENE SEQUENCE TYPES

The 18S rRNA gene (Rns) phylogeny of Acanthamoeba is being investigate d as a basis for improvements in the nomenclature and taxonomy of the genus. We previously analyzed Rns sequences from 18 isolates from morp hological groups 2 and 3 and found that they fell into four distinct e volutionary lineages we called sequence types T1-T4. Here, we analyzed sequences from 53 isolates representing 16 species and including 35 n ew strains. Eight additional lineages (sequence types T5-T12) were ide ntified. Four of the 12 sequence types included strains from more than one nominal species. Thus, sequence types could be equated with speci es in some cases or with complexes of closely related species in other s. The largest complex, sequence type T4, which contained six closely related nominal species, included 24 of 25 keratitis isolates. Rns seq uence variation was insufficient for full phylogenetic resolution of b ranching orders within this complex, but the mixing of species observe d at terminal nodes confirmed that traditional classification of isola tes has been inconsistent. One solution to this problem would be to eq uate sequence types and single species. Alternatively, additional mole cular information will be required to reliably differentiate species w ithin the complexes. Three sequence types of morphological group 1 spe cies represented the earliest divergence in the history of the genus a nd, based on their genetic distinctiveness, are candidates for reclass ification as one or more novel genera.