Parasitol Res (1995) 81:451-458 9 Springer-Verlag 1995 M. Okamoto - Y. Bessho 9 M. Kamiya 9 T. Kurosawa T. Horii Phylogenetic relationships within Taeniataeniaeformis variants and other taeniid cestodes inferred from the nucleotide sequence of the cytochrome c oxidase subunit I gene Received: 12 December 1994 / Accepted: 30 January 1995 Abstract Nucleotide sequence variations in a region of the mitochondrial cytochrome c oxidase subunit I (COl) gene (391 bp) were examined within seven species of the genus Taenia and two species of the genus Echinococ- cus, including ten isolates of T. taeniaeformis and six isolates of E. multilocularis. More than a 12% rate of nu- cleotide differences between taeniid species was found, allowing the species to be distinguished. In E. multilocu- laris, no sequence variation was observed among iso- lates, regardless of the host (gray red-backed vole, tundra vole, pig, Norway rat) or area (Japan, Alaska) from which each metacestode had been isolated. In contrast, six distinct sequences were detected among the ten T. taeniaeformis isolates examined. The level of nucleotide variation in the COI gene within T. taeniaeformis isolates except for one isolate from the gray red-backed vole (TtACR), which has been proposed as a distinct strain or a different species, was about 0.3%-4.1%, whereas the COI gene sequence for TtACR differed from those of the other isolates, with levels being 9.0%-9.5%. Phylogenet- ic trees were then inferred from these sequence data us- ing two different algorithms. M. Okamoto ( ~ ) 9 T. Kurosawa The Institute of Experimental Animal Sciences, Osaka University Medical School, Suita, Osaka 565, Japan Y. Bessho Laboratory of Molecular Genetics, Department of Biology, School of Science, Nagoya University, Nagoya 464-01, Japan M. Kamiya Department of Parasitology, Faculty of Veterinary Medicine, Hokkaido University, Sapporo 060, Japan T. Horii Department of Molecular Protozoology, Research Institute of Microbial Diseases, Suita, Osaka 565, Japan Introduction Cestodes of the family Taeniidae are parasites of carni- vores and their intermediate hosts are limited to mammali- an hosts. To date, over 100 species and many variants have been described in various parts of the world. A few reports have been published on the phylogeny of taeniid cestodes (Moore 1981; Moore and Brooks 1987). Because of the absence of early fossil records, morphology, host specific- ity, and biological traits such as asexual reproductive po- tential were used for constructing the phylogenetic rela- tionship of the taeniid cestodes in these reports. However, it is difficult to infer reliable phylogenetic relationships from these characters, as not only the phylogenetic rela- tionship between the strains or species but also that be- tween the subfamilies of taeniids remain obscure. With the development of molecular biology, the se- quences of various genes have been used for molecular phylogenetics study. As mitochondrial DNA is known to have a faster evolutionary rate than nuclear DNA, mito- chondrial genes, such as the cytochrome c oxidase sub- unit I and II (COI and COII) genes have been used to study the phylogenetic relationships among related or- ganisms (Crozier etal. 1989; Lecanidou etal. 1994). With respect to the cestode, Bowles et al. (1992) se- quenced a portion of the COI gene for 56 Echinococcus isolates, including 4 Echinococcus species, and classified them into 11 different genotypes. McManus and Bowles (1994) suggested that Asian (Taiwan) Taenia was a strain of T. saginata from comparison of the CO1 gene and nu- clear 28SrDNA sequences of several taeniid cestodes. T. taeniaeformis is one of the species important as a model for taeniid cestodes that cannot readily be main- tained in the laboratory. Recently, intraspecific variation of 4 laboratory-reared isolates of T. taeniaeformis has been examined by morphology, infectivity, comparison of protein composition, and restriction analysis of DNA (Azuma et al. 1995; lwaki et al. 1994; Nonaka et al. 1994). These reports indicated that one isolate from a gray red-backed vole in Japan might be a distinct strain or a new species, but the phylogenetic relationships be-