- Volume 57, Issue 10, 2007

The mitochondrial cytochrome-c oxidase subunit 1 (cox1) gene has been proposed as a DNA barcode to identify animal species. To test the applicability of the cox1 gene in identifying ciliates, 75 isolates of the genus Tetrahymena and three non-Tetrahymena ciliates that are close relatives of Tetrahymena, Colpidium campylum, Colpidium colpoda and Glaucoma chattoni, were selected. All tetrahymenines of unproblematic species could be identified to the species level using 689 bp of the cox1 sequence, with about 11 % interspecific sequence divergence. Intraspecific isolates of Tetrahymena borealis, Tetrahymena lwoffi, Tetrahymena patula and Tetrahymena thermophila could be identified by their cox1 sequences, showing <0.65 % intraspecific sequence divergence. In addition, isolates of these species were clustered together on a cox1 neighbour-joining (NJ) tree. However, strains identified as Tetrahymena pyriformis and Tetrahymena tropicalis showed high intraspecific sequence divergence values of 5.01 and 9.07 %, respectively, and did not cluster together on a cox1 NJ tree. This may indicate the presence of cryptic species. The mean interspecific sequence divergence of Tetrahymena was about 11 times greater than the mean intraspecific sequence divergence, and this increased to 58 times when all isolates of species with high intraspecific sequence divergence were excluded. This result is similar to DNA barcoding studies on animals, indicating that congeneric sequence divergences are an order of magnitude greater than conspecific sequence divergences. Our analysis also demonstrated low sequence divergences of <1.0 % between some isolates of T. pyriformis and Tetrahymena setosa on the one hand and some isolates of Tetrahymena furgasoni and T. lwoffi on the other, suggesting that the latter species in each pair is a junior synonym of the former. Overall, our study demonstrates the feasibility of using the mitochondrial cox1 gene as a taxonomic marker for ‘barcoding’ and identifying Tetrahymena species and some other ciliated protists.

16S rRNA gene sequences have been widely used for the identification of prokaryotes. However, the flood of sequences of non-type strains and the lack of a peer-reviewed database for 16S rRNA gene sequences of type strains have made routine identification of isolates difficult and labour-intensive. In the present study, we generated a database containing 16S rRNA gene sequences of all prokaryotic type strains. In addition, a web-based tool, named EzTaxon, for analysis of 16S rRNA gene sequences was constructed to achieve identification of isolates based on pairwise nucleotide similarity values and phylogenetic inference methods. The system developed provides users with a similarity-based search, multiple sequence alignment and various phylogenetic analyses. All of these functions together with the 16S rRNA gene sequence database of type strains can be successfully used for automated and reliable identification of prokaryotic isolates. The EzTaxon server is freely accessible over the Internet at http://www.eztaxon.org/

Following Recommendation 30b of the Bacteriological Code (1990 Revision), a proposal of minimal standards for describing new taxa within the family Halomonadaceae is presented. An effort has been made to evaluate as many different approaches as possible, not only the most conventional ones, to ensure that a rich polyphasic characterization is given. Comments are given on the advantages of each particular technique. The minimal standards are considered as guidelines for authors to prepare descriptions of novel taxa. The proposals presented here have been endorsed by the International Committee on Systematics of Prokaryotes Subcommittee on the Taxonomy of Halomonadaceae.