In a manner similar to many other groups of organisms, the tapeworm order Proteocephalidea poses a difficult phylogenetic problem if treated on the basis of single-gene analysis. Since the biogeography and host distribution of proteocephalideans make these tapeworms a potentially interesting model for evolutionary and co-evolutionary studies, we tried to resolve their phylogenetic relationships by applying a multi-gene approach. The ITS2 sequences and V4 hypervariable loop of 18S rRNA were obtained for 43 and 35 proteocephalidean taxa, respectively, and combined with other sequences available in the GenBank. The phylogenetic analysis of the combined DNA set was confronted with characters derived from ITS2 secondary structures. Using this approach, a species-rich Neotropical lineage of proteocephalideans could be reliably resolved. The phylogenetic relationships within this group show a high degree of phylogeny-independent host distribution. The reconstruction of ITS2 secondary structure revealed a universal 4-domain arrangement, which is conserved across a wide range of Neodermata. Several motifs of the secondary structure could be mapped to the phylogenetic tree as possible clade synapomorphies.

Genetic variability in three species of Pimelodidae catfish (Iheringichthys labrosus, Pimelodus maculatus and Pimelodus cf. absconditus), collected at four sites in the Tibagi river-basin: Sertaneja, Londrina (lower Tibagi), Sapopema (middle Tibagi) and Tibagi (upper Tibagi) was analysed using RAPD and isoenzymatic techniques. Iheringichthys labrosus had the greatest genetic variability showing a proportion of mean polymorphic loci (P¯m) of 53.7%. The dendrogram showed that the individuals from the locations were clustered together, supporting the hypothesis that I. labrosus has a sedentary or localized geographical distribution. Pimelodus maculatus is the only species studied that is considered to be highly migratory. The P¯m obtained for this species was 30.2%. Their genetic dendrogram showed that the individuals were not clustered, based on the place of collection. P. cf. absconditus had a P¯m value of 47.4%, and the external morphology of this species is similar to that of I. labrosus, which hindered differentiation of individuals from the two species. RAPD and isoenzyme analysis, however, provided differential patterns for P. cf. absconditus from I. labrosus.