Evaluation of ITS rDNA as a complement to mitochondrial gene sequences for phylogenetic studies in freshwater mussels: an example using Unionidae from north‐western Europe

Abstract

Mitochondrial inheritance in the Unionidae is complex since mitochondria can be inherited from both parents. An increased rate of recombination could lead to erroneous homology assessments, which could cause problems for phylogenetic reconstruction. For this reason we investigated the possibility of using a nuclear marker, the ribosomal internal transcribed spacer region, for phylogenetic studies in the Unionidae, as a complement and comparison to two of the most widely used mitochondrial genes today. The nuclear ribosomal internal transcribed spacer region (ITS1, 5.8S, ITS2) was sequenced from 72 specimens representing six of the seven species of Unionidae mussels occurring in NW Europe: Unio pictorum, U. tumidus, U. crassus, Anodonta anatina, A. cygnea and Pseudanodonta complanata. Sequences from Margaritifera margaritifera were used as an outgroup. The ITS sequences of all species were found to have low intragenomic and infraspecific variation. Compared to mitochondrial genes (16S and COI) they show an intermediate genetic diversity. Phylogenetic analyses produce tree topologies that are congruent with those resulting from analyses of the mitochondrial sequences. Likewise, an incongruence length difference (ILD) test showed no significant incongruence between data sets, indicating that if recombination has occurred it has not produced any conflicting patterns. Best‐resolved and supported trees are obtained when gaps are treated as a fifth character state. A combined analysis of the three gene regions shows that Unio crassus and U. pictorum are more closely related than either to U. tumidus. Pseudanodonta is nested within Anodonta as sister taxon to A. cygnea. Advice on how to best preserve mussel material for DNA studies is provided.