Review on the molecular study of the Diplozoidae: analyses of currently available genetic data, what it tells us, and where to go from here

Abstract

The use of molecular tools in the study of parasite taxonomy and systematics have become a substantial and crucial component of parasitology. Having genetic characterisation at the disposal of researchers has produced mostly useful, and arguably more objective conclusions. However, there are several groups for which limited genetic information is available and, coupled with the lack of standardised protocols, renders molecular study of these groups challenging. The Diplozoidae are fascinating and unique monogeneans parasitizing mainly freshwater cyprinid fishes in Europe, Asia and Africa. This group was studied from a molecular aspect since the turn of the century and as such, limitations and variability concerning the use of these techniques have not been clearly defined. In this review, all literature and molecular information, primarily from online databases such as GenBank, were compiled and scrupulously analysed for the Diplozoidae. This was done to review the information, detect possible pitfalls, and provide a "checkpoint" for future molecular studies of the family. Hindrances detected are the availability of sequence data for only a limited number of species, frequently limited to a single sequence per species, and the heavy reliance on one non-coding ribosomal marker (ITS2 rDNA) which is difficult to align objectively and displays massive divergences between taxa. Challenging species identification and limited understanding of diplozoid species diversity and plasticity are also likely restricting factors, all of which hamper the accurate taxonomic and phylogenetic study of this group. Thus, a more integrated taxonomic approach through the inclusion of additional markers, application of more rigorous morphological assessment, more structured barcoding techniques, alongside thorough capturing of species descriptions including genetypes, genophore vouchers and reference collections in open sources are encouraged. The pitfalls highlighted are not singular to the Diplozoidae, and the study of other groups may benefit from the points raised here as well.

Keywords: Afrodiplozoon; Diplozoon; Eudiplozoon; Fish parasites; Genetic characterisation; Inustiatus; Molecular taxonomy; Monogenea; Paradiplozoon; Sindiplozoon.

Fig. 1

Bayesian inference analysis tree based on all available ITS2 rDNA sequences for the Diplozoidae, with Neoheterobothrium hirame Ogawa, 1999 used as the outgroup. Support for BI, ML, and P is indicated at nodes (BI/ML/P), with geographical origin shown in relation to colour coded map and country key. Nodes with less than 50% support for all methods are not annotated, with those that have less than 50% support in certain analyses are indicated with “-”. Nodes not present in either ML or P topologies are indicated with “*”. Branches with thicker formats indicate specific taxonomic groups indicating major clades, species complexes, and current or proposed genera

Fig. 2

Saturation plot of the number of transitions (s, blue) and transversions (v, red) plotted against sequence divergence values (F84 distance) of the alignment of all available ITS2 rDNA sequences for the Diplozoidae