A Molecular Test of Platyhelminth Phylogeny: Inferences from Partial 28S rDNA Sequences

Abstract

Nucleotide sequences of the region extending from the D3 to the D6 expansion segments of the 28S rDNA gene were used to reconstruct evolutionary relationships within the Platyhelminthes. Neighbor-joining and parsimony analyses of representatives of most major platyhelminth taxa revealed a basal Catenulida, a monophyletic Acoelomorpha, a sister-group relationship of Macrostomorpha and Polycladida (59% of bootstrap replications), and monophyletic Tricladida. We found no evidence of a taxon Seriata (p=.0001); however, the paraphyletic status of the Proseriata needs further investigation. Although Neodermata appeared as a monophyletic group, Monogenea was paraphyletic (p<.0001), thus not supporting the taxon Cercomeromorpha. Monopisthocotylean Monogenea was the most basal taxon among the Neodermata, and Udonella sp. consistently grouped with the Monopisthocotylea. Other relationships within the Neodermata showed Trematoda as a monophyletic taxon and, among its members, Rugogaster hydrolagi was identified as an aspidogastrean. The taxa Cestodaria (Amphilinidae and Gyrocotylidae) and Eucestoda were both supported; however, constraining these 2 clades into a monophyletic Cestoda resulted in a significantly longer tree (p=.0303). We tentatively conclude that the immediate sister group of Neodermata is Fecampiida. But because of contradictory results of the constraint analyses, we cannot exclude the possibility that the sister group to Neodermata is a taxon containing many neoophoran turbellarians (e.g., Rhabdocoela and Tricladida). Additional key words: Acoelomorpha, Neodermata, Udonellida, molecular systematics Although the Platyhelminthes, and especially the order Acoela, hold a key position in several theories of metazoan origins (Hadzi 1963; Steinbock 1963; Salvini-Plawen 1978; Ehlers 1985; Ax 1987; Nielsen 1987), their within-group relationships are poorly understood. Attempts at classifying the platyhelminths have produced a wealth of ultrastructural information, but none of these studies have resulted in a definitive phylogenetic tree (Smith et al. 1986). However, two major points regarding platyhelminth systematics have emerged from these ultrastructural studies. First, Turbellaria may be a paraphyletic group, and the term should probably be used with caution (Ax 1987). Secondly, 3 clearly defined monophyletic lineages have been identified within the phylum (Fig. 1): the Catenulida; the Acoelomorpha, consisting of the orders Acoela and Nemertodermatida; and the Rhabditophora, which comprises the remaining orders and the parasitic Neodermata (Ehlers 1985, 1986). To determine a Author for correspondence. E-mail: mkll @cisunix.unh.edu relationships among these 3 groups, however, is difficult because of a lack of convincing synapomorphies. Proposed characters such as frontal organ/frontal glands or epidermal replacement are either not homologous across the taxa (Klauser et al. 1986; Smith & Tyler 1986) or have only an extremely low probability of homology (Smith et al. 1986), respectively. Therefore, monophyly for the platyhelminths, intuitive though it may be, will be difficult to test using traditional morphological characters. In the last decade, molecular phylogenies at any taxonomic level have gained great popularity. Nucleotide sequences provide an alternative data set for the resolution of evolutionary relationships in cases where morphologists may have reached an impasse regarding a specific phylogeny, or where the number of species and structural characters needed to reliably resolve a phylogeny may be prohibitively high. Molecular systematics has attempted to resolve just such issues by providing large and novel data sets (Turbeville et al. 1991; Turbeville et al. 1992). Some of the most popThis content downloaded from 207.46.13.164 on Sun, 26 Jun 2016 07:38:12 UTC All use subject to http://about.jstor.org/terms Molecular platyhelminth phylogeny MonoI,uI pI lal Monopisthocotylea genea Neo-(incl. Udonella sp.) dermat; L Digenea [-EAspidogastrea Trematoda (incl. R. hydrolagi) Dalyellioida (incl. Fecampiida) RhabdoKalyptorhynchia coela Tricladida Seriata Proseriata