Evolution of ITS1 rDNA in the Digenea (Platyhelminthes : Trematoda): 3 ' end sequence conservation and its phylogenetic utility

A comparison of ribosomal internal transcribed spacer 1 (ITS I) elements of digenetic trematodes (Platyhelminthes) including unidentified digeneans is olated from Cyathura carinata (Crustacea: Isopoda) revealed DNA sequence si milarities 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 cha nce. Using an analysis of apomorphies. the sequence data were shown to prod uce a distinct phylogenetic signal. This was confirmed by the consistency o f results of different tree reconstruction methods such as distance approac hes, maximum parsimony, and maximum likelihood. Morphological evidence addi tionally supported the phylogenetic tree based on ITS1 data and the inferre d phylogenetic position of the unidentified digeneans of C. carinata met th e: expectations from known trematode life-cycle patterns. Although ribosoma l ITS1 elements are generally believed to be too variable for phylogenetic analysis above the species or genus level, the overall consistency of the r esults of this study strongly suggests that this is not the case in digenet ic trematodes. Here, 3' end ITS1 sequence data seem to provide a valuable t ool for elucidating phylogenetic relationships of a broad range of phylogen etically distinct taxa.