Spezielle Zoologie, Fachbereich 5, Universita ¨t Osnabru ¨ck, Osnabru ¨ck, Germany The phylogenetic position of the Aeolosomatidae and Parergodrilidae, two enigmatic oligochaete-like taxa of the ‘Polychaeta’, based on molecular data from 18S rDNA sequences T. Struck, R. Hessling and G. Purschke Abstract The Aeolosomatidae and the Parergodrilidae are meiofaunal Annelida showing different combinations of clitellate-like and non-clitellate character states. Their phylogenetic positions and their systematic status within the Annelida are still in debate. Here we attempt to infer their systematic position using 18S rDNA sequences of the aeolosomatid Aeolosoma sp. and the parergodrilid Stygocapitella subterranea and several other meiofaunal taxa such as the Dinophilidae, Polygordiidae and Saccocirridae. The data matrix was complemented by sequences from several annelid, arthropod and molluscan species. After evaluation of the phylogenetic signal the data set was analysed with maximum-parsimony, distance and maximum-likelihood algorithms. Sequences from selected arthropods or molluscs were chosen for outgroup comparison. The resolution of the resulting phylogenies is discussed in comparison to previous studies. The results do not unequivocally support a sister-group relationship of Aeolosoma sp. and the Clitellata. Instead, depending on the algorithms applied, Aeolosoma clusters in various clades within the polychaetes, for instance, together with eunicidan species, the Dinophilidae, Harmothoe ¨ impar or Nereis limbata. The position of Aeolosoma sp. thus cannot be resolved on the basis of the data available. S. subterranea always falls close to a cluster comprising Scoloplos armiger, Questa paucibranchiata and Magelona mirabilis, all of which were resolved as not closely related to both Aeolosoma sp. and the Clitellata. Therefore, convergent evolution of clitellate-like characters in S. subterranea and hence in the Parergodrilidae is suggested by our phylogenetic analysis. Moreover, the Clitellata form a monophyletic clade within the paraphyletic polychaetes. Key words: Annelida – Clitellata – Polychaeta – phylogeny – phylogenetic signal – molecular systematics – evolution – 18S rDNA Introduction The Aeolosomatidae and the Parergodrilidae (Fig. 1a,b) are meiofaunal Annelida showing different combinations of cli- tellate-like and non-clitellate characters (Purschke 1999; Hess- ling and Purschke 2000; Purschke and Hessling 2002). Although both taxa are of apparently simple organization and small body size, they are clearly coelomate and possess rather complex sensory structures, nervous and reproductive systems as revealed in previous ultrastructural analyses (see Purschke 1999, 2002; Hessling and Purschke 2000). The phylogenetic position of both taxa within the Annelida is more or less unresolved and still in debate (Rota et al. 2001). The Aeolosomatidae have variously been considered as a subtaxon of the ‘Oligochaeta’, as the sister group of the Clitellata or as a group with no close affinity to the Clitellata (see Hessling and Purschke 2000). Hermaphroditism, lack of parapodia, the possession of simple chaetae and the predom- inant occurrence in limnetic habitats were the main arguments for placing this taxon within the ‘Oligochaeta’, probably as their most primitive group (e.g. Michaelsen 1929). Alternat- ively, Stephensen (1930) regarded the Aeolosomatidae as a highly derived taxon within the Clitellata. Bunke (1967) was the first to question their relationship to the Clitellata, which finally resulted in their exclusion from this taxon (Brinkhurst and Jamieson 1971). Although this view initially found broad acceptance, it was later modified and the Aeolosomatidae (together with the Potamodrilidae) were placed as the sister group of the Clitellata, both having possibly evolved from a common freshwater ancestor (Timm 1981; Brinkhurst and Nemec 1987). This hypothesis, however, could not be corro- borated by recent ultrastructural and immunohistochemical investigations (Bunke 1985, 1986, 1994; Hessling and Purschke 2000; Purschke and Hessling 2002). Furthermore, an aquatic origin of the Clitellata was questioned by Westheide (1997) and Purschke (1997, 1999). In their cladistic analyses Rouse and Fauchald (1997) found the Aeolosomatidae, Potamodril- idae and Parergodrilidae to cluster within the polychaetes although their exact placement remained uncertain. Prelimin- ary molecular studies based upon 18S rDNA sequences including the Aeolosomatidae and only a few other annelid taxa have given some support to a sister-group relationship of the Aeolosomatidae and Clitellata contradicting these recent morphological analyses (Moon et al. 1996; Winnepenninckx et al. 1998). However, this finding was not corroborated by the analyses of Martin (2000) and Rota et al. (2001) using the same gene. Similarities between the Parergodrilidae and the Clitellata include a prostomium lacking appendages, absence of para- podia, simple chaetae, epidermis without kinocilia, direct transfer of sperm, modified spermatozoa, eggs deposited in cocoons, direct development as well as circumoesophageal connectives which are not split into dorsal and ventral roots (Purschke et al. 2000). However, these similarities have widely been regarded as the result of convergent evolution due to similar selection pressures (Erse´us and Rota 1998; Rota 1998; Purschke 1999, 2002). Other morphological characters which clearly support the exclusion of the Parergodrilidae from the Clitellata are the presence of nuchal organs, absence of a true clitellum and a completely different reproductive system as was shown in previous ultrastructural analyses. However, their systematic position within the polychaetes remains unresolved (e.g. Rouse and Fauchald 1997). This study attempts to infer the systematic position of the Aeolosomatidae and Parergodrilidae on the basis of 18S rDNA sequences including new data from the parergodrilid Stygocapitella subterranea and other meiofaunal taxa such as Dinophilidae, Polygordiidae and Saccocirridae. The data set J. Zool. Syst. Evol. Research 40 (2002) 155–163 Ó 2002 Blackwell Verlag, Berlin ISSN 0947–5745 Received on 3 December 2001 U.S. Copyright Clearance Center Code Statement: 0947–5745/02/4003–0155$15.00/0 www.blackwell.de/synergy