Utility of complete large and small subunit rRNA genes in resolving the phylogeny of the Neodermata (Platyhelminthes): implications and a review of the cercomer theory

Abstract

We combined nearly complete sequences of large (LSU) and small (SSU) subunit rDNA from 32 flatworm species to estimate the phylogeny of the Platyhelminthes using maximum parsimony, maximum likelihood and Bayesian inference methods. Rooted against the Catenulida, combined evidence trees offered no support for the Revertospermata, which was also rejected by constraint analysis. Generally, nodal support was higher for groupings estimated from the combined data partitions and all methods of analysis provided congruent estimates of phylogeny. The Monogenea and Proseriata were resolved as monophyletic, rejecting previous suggestions of paraphyly based on SSU and partial LSU data sets and thus supporting widely accepted morphological synapomorphies. Monophyly of the Neodermata was supported and its sister group was a clade of neoophoran ‘turbellarians’ to the exclusion of the Proseriata which in turn was more basal. Taxa with similar spermatology to the Neodermata (Ichthyophaga, Notentera, Urastoma and Kronborgia) were the sister group to Tricladida + Prolecithophora, which in turn were sister to the Rhabdocoela. Polycladida + Macrostomida + Lecithoepitheliata was the earliest divergent offshoot of the Rhabditophora. Among the Neodermata, the Cercomeromorphae (Cestoda + Monogenea) was not supported, whereas Cestoda + Trematoda was well supported. Although there is no known synapomorphy for this latter grouping, our data highlight problems associated with the ‘cercomer theory’ and we reject putative homologies regarding neodermatan ‘cercomers’ that have been sustained in the literature without careful scrutiny. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society, 2003, 78, 155–171.

Platyhelminth systematics and the emergence of new characters

“…It would be fair to say that 18S and 28S rRNA genes provide the bedrock of molecular systematics for the parasitic platyhelminths, having been used extensively for revealing interrelationships within and between families and across the phylum (see Table I). Most recent evidence (Lockyer et al, 2003;Waeschenbach et al, 2007) confirms that a combination of complete 18S and 28S rDNA provides added resolution where the more popular complete 18S alone, or the D1-D3 variable regions of 28S alone (or these 18S and 28S fragments in combination), fail to provide stability across the tree, particularly amongst the deeper nodes (Olson & Caira, 1999;Olson & Littlewood, 2002;Olson et al, 2003). Faster rates of evolution and higher variability in the ITSs and IGSs has provided diagnostic markers for species (Blair, 2006), although as might be expected these have been tested mostly on parasites of medical or economic importance.…”

“…However, interpreting the origins of parasitism, requires resolution of neodermatan interrelationships and the identification of the sister group to the Neodermata. Presently, the interrelationships of the Neodermata remain unresolved (Littlewood, 2006), although it seems likely that the Monogenea are paraphyletic and the Cestoda and Trematoda are sister taxa; see Lockyer et al (2003) and citations therein. Each of these new hypotheses rejects previous morphological assessments, and although few morphological synapomorphies supported (Trematoda (Cestoda, Monogenea)) as suggested by Ehlers (1985), instability amongst molecular estimates have not promoted wide acceptance of any single molecular scenario.…”

The phylogeny of the Schistosomatidae based on three genes with emphasis on the interrelationships of Schistosoma Weinland, 1858

“…Previous morphological and molecular phylogenetic estimates of digenean interrelationships have indicated strongly that the Sanguinicolidae are the sister group to the Schistosomatidae within the superfamily Schistosomatoidea (see Cribb et al 2001 (Lockyer, Olson & Littlewood, 2003), added additional information for outgroup rooting. Although the COI fragment could not be amplified from this second outgroup taxon, all ingroup topologies of ssrDNA and lsrDNA trees were identical with one or two outgroups, so the analyses were restricted to rooting against C. trondheimensis alone.…”

Choanocotyle platti sp. nov. from the northern long-necked turtle, Chelodina rugosa (Pleurodira, Chelidae) in Australia

“…PCR reactions were performed according to protocols described by . PCR primers and several internal primers were used in sequencing reactions (for sequences of internal primers see Littlewood and Olson 2001, Platt and Tkach 2003.…”

“…Three different fragments of the nuclear ribosomal ribonucleic acid (rRNA) were amplified by polymerase chain reaction (PCR) and sequenced: the nearly complete 18S gene, the fragment containing internal transcribed spacer 1 (ITS1), 5.8S gene, and ITS2, and an approximately 1,350 bp long fragment at the 5' end of the 28S gene. Primers Worm A (5'-GCGAATGGCT-CATTAAATCAG-3') and Worm B (5'-ACGGAAACCTT-GTTACGACT-3') (Littlewood and Olson 2001), were used to amplify 18S gene fragment. Primers ITSF (5'-CGCCCGT-CGCTACTACCGATTG-3') and 1500R (5'-GCTATCCTGA-GGAAACTTCG-3') were used to amplify a fragment spanning the whole internal transcribed spacer (ITS1 + 5.8S + ITS2) region and the 5' end of the 28S gene.…”