Evolution of ITS1 rDNA in the Digenea (Platyhelminthes: Trematoda): 3′ End Sequence Conservation and Its Phylogenetic Utility

Abstract

A comparison of ribosomal internal transcribed spacer 1 (ITS1) elements of digenetic trematodes (Platyhelminthes) including unidentified digeneans isolated from Cyathura carinata (Crustacea: Isopoda) revealed DNA sequence similarities at more than half of the spacer at its 3' end. Primary sequence similarity was shown to be associated with secondary structure conservation, which suggested that similarity is due to identity by descent and not chance. Using an analysis of apomorphies, the sequence data were shown to produce a distinct phylogenetic signal. This was confirmed by the consistency of results of different tree reconstruction methods such as distance approaches, maximum parsimony, and maximum likelihood. Morphological evidence additionally supported the phylogenetic tree based on ITS1 data and the inferred phylogenetic position of the unidentified digeneans of C. carinata met the expectations from known trematode life-cycle patterns. Although ribosomal ITS1 elements are generally believed to be too variable for phylogenetic analysis above the species or genus level, the overall consistency of the results of this study strongly suggests that this is not the case in digenetic trematodes. Here, 3' end ITS1 sequence data seem to provide a valuable tool for elucidating phylogenetic relationships of a broad range of phylogenetically distinct taxa.

Use of ITS rDNA for discriminating of larval stages of two microphallid (Digenea) species using Hydrobia ulvae (Pennant, 1777) and Corophium volutator (Pallas, 1766) as intermediate hosts

“…We are aware of the possibility that there could be other undescribed microphallid variants in related parasite-host systems showing a nucleotide sequence identical to one of the primers prepared here. As an example, some ITS sequences from DNA extracted from metacercariae hosted by the isopod Cyathura carinata are very similar to the sequences reported here (Schulenburg et al 1999). Therefore, one must test whether a diagnostic primer can distinguish between its target species and a type differing in only two bases at the primerbinding site.…”

Description of Maritrema portucalensis sp. nov. (Digenea, Microphallidae) parasite of Carcinus maenas (Crustacea, Decapoda) from Aveiro estuary, Northern Portugal

“…The 18S and 5.8S rDNA regions were determined by comparison with the sequences reported for other microphallid species in GenBank. The BLAST search of the ITS1 sequence obtained from this study determined it to be highly similar to the available ITS1 sequence AJ001831 from an unidentified digenean type-A1 (96%) and to the ITS1 sequence AJ001833 from another unidentified digenean type-A2 (95%) (Schulenburg et al 1999). The ITS1 sequence obtained in this study was aligned with the corresponding sequences (not available in GenBank) of two microphallid specimens identified as Maritrema subdolum and Microphallidae sp.…”

“…Recently, analysis of nucleotide sequences of the internal tran-*Corresponding author: smpina@icbas.up.pt Susana Pina et al 378 scribed spacer (ITS1) region of the nuclear ribosomal DNA (rDNA) has proven a useful tool for the unequivocal identification of digenean species and knowledge of the diversity of their life cycle patterns (McManus and Bowles 1996, Schulenburg et al 1999, Nolan and Cribb 2005.…”

Phylogeny and Biogeography of Serolid Isopods (Crustacea, Isopoda, Serolidae) and the Use of Ribosomal Expansion Segments in Molecular Systematics

“…In short, molecular synapomorphies can be expected to uniquely identify the monophylum whose last common ancestor developed this evolutionary novelty (symmetrical split supporting position, SSSP) or subsequent events can have degraded this signal to some degree in the outgroup, ingroup, or both (asymmetrical and noisy split supporting positions, ASSP, ASSP ϩ N). For a more detailed explanation of this method, see Schulenburg et al (1999) and Wä gele (1998). The ingroup of the MP scenario included the two Cuspidoserolis species in this study; the ingroup of the ML scenario comprised C. luethjei, A. bromleyana, and S. bouvieri (see Fig.…”