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.
Highlights
The Diplozoidae Palombi, 1949 is a fascinating and unique aquatic parasite family, with hermaphroditic monogenean pairs fusing to mature sexually, forming “X”-shaped, “Siamese” organisms, allowing for crossfertilization [1] and supporting genetic diversity
Sequences LC517174 and LC517175 are separated by 3.21% even though they were both identified as E. kamegaii and collected from C. carpio, with LC517174 only separated by 0.24% from LC517173 (E. nipponicum from C. auratus)
Paradiplozoon homoion‐complex From the results presented here (Additional file 2: Table S2, Fig. 1), three species of diplozoids in Clade 1 are very closely related when considering the second internal transcribed spacer region (ITS2) ribosomal DNA (rDNA) fragment
Summary
The Diplozoidae Palombi, 1949 is a fascinating and unique aquatic parasite family, with hermaphroditic monogenean pairs fusing to mature sexually, forming. By its authors [16], as Diplozoon paradoxum Nordmann, 1832, especially when taking into account that the specimens were collected from the type-host of D. paradoxum (captured in Additional file 1: Table S1) If this assumption is made, we can see that the intraspecific range of the combination of 28S rDNA fragments used is 0, with the lowest interspecific distance of 0.98 observed between D. paradoxum and P. bliccae, which have been noted to be closely related but distinct taxa in previous studies [4, 29, 37]. This further supports the possible use of fragments of 28S rDNA for barcoding of diplozoid species. Sequence KU519493 likely represents a diplozoid taxon not yet previously genetically characterised using 28S rDNA, even possibly a new diplozoid taxon as no diplozoid has been recorded from Barilius barila (Hamilton, 1822) or Barilius bendelisis (Hamilton, 1807)
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