Biochemical Systematics and Ecology, Vol. 20, No. 4, pp. 325-330, 1992. 0305-1978/92$5.00 + 0.00 Printed in GreatBritain. © 1992PergamonPressLtd. Mitochondrial tRNA and the Phylogenetic Position of Nematoda ROLAND BRANDL,* WOLFGANG MANN*t and MATHIAS SPRINZL$ *Lehrstuhl fL~rTier6kologie, Universit&t Bayreuth, Postfach 101251,D-8580 Bayreuth, F.R.G.; :~Lehrstuhl fSr Biochemie, Universit~it Bayreuth, Postfach 101251,D-8580 Bayreuth, F.R.G. Key Word Index--Nematoda; Pseudocoelomata; coelomate anatomy; molecular systematics; mtDNA; tRNA. Abstract--From sequence information of tRNA genes in mitochondria, a tree of the deep branchings within the animal kingdom was generated to illuminate the systematic position of nematodes. Our results indicate that nematodes are more related to arthropods than to Archicoelomata and Chordata, This implies that the pseudocoelomate body plan of nematodes is derived from the coelomate anatomy. Introduction As early as the Cambrian period, about 600-500 million years ago, all major animal phyla were represented in the fossil record (Tasch, 1973). Fossils tell us little, however, about systematic relationships of the main invertebrate phyla. Anatomy and embryology have been used to infer phylogenetic relationships. For some taxa this task is very difficult. Nematoda, as well as other taxa combined under the term Aschelminthes, are a phylogenetically difficult group. Controversial views about the phylogenetic position of nematodes have been expressed in the literature (see Fig. 1; Clark, 1967). Nematoda and the other Aschelminthes are sometimes termed as Pseudo- coelomata (Remane, 1963), because they generally lack multiple, independent hydro- static systems as well as the septa present in coelomate phyla (Clark, 1967). Thus the question remains as to whether the pseudocoelomate organization of the body cavity is derived from coelomate anatomy (Siewing, 1985) or whether it constitutes a primitive, independent way of evolution. Essential information required to answer this question would be the knowledge of the phylogenetic position of Nematoda within the Metazoa. Some possible evolutionary scenarios are summarized in Fig. 1. Hypothesis A in Fig. 1 makes a clear statement about the pseudocoelomate organization. If one accepts that such a complex structure as the coelom evolved only once during evolution (Siewing, 1985) then Fig. 1A implies that the peculiar organization of the body cavity of nematodes evolved from the coelomate body-plan. Systematists usually use the body cavity as an essential character for classifying invertebrates, but an independent estimate of the relationships on to which we can map the character states of the body cavity without circularity is necessary. Molecular data provide an independent method to infer the phylogenetic position of Nematoda. Recently, the mitochondrial DNA of the parasitic nematode Ascaris suum has been sequenced (Wolstenholme et al., 1987). The tRNAs in the mitochondria show a unique structure in which the T~C arm and variable loop are missing and are replaced by a single loop with four and 12 nucleotides, respectively. Caenorhabditis elegans also shows these unusual mitochondrial tRNAs (Wolstenholme et al., 1987). Furthermore, the anticodon usage differs slightly from other animals (Wolstenholme et tPresent address: Lehrstuhl for Tiergenetik, Technische Universit~t ML~nchen, D-8050 Freising- Weihenstephan, F.R.G. (Received 22 October 1991) 325