Novel molecular data for diplozoids reveal similar mitochondrial and ribosomal phylogenies, unexpected geographical structure, and intra-individual mito-nuclear incompatibilities

Diplozoids are parasitic flatworms known for their unique development, whereby two hermaphroditic individuals fuse to form a single adult. They exhibit varying host specificity, with Paradiplozoon homoion having the widest known distribution, making it a suitable model for population genetic studies. In this study, the genetic variability of P. homoion was assessed from 60 cyprinoid fish species at 47 sites in the western Palaearctic using the mitochondrial COI gene. Moreover, novel sequence data of four ribosomal and one mitochondrial marker were used to understand phylogenetic relationships among diplozoids. A total of 29 unique COI haplotypes in six divergent haplogroups among P. homoion populations were identified. The COI fragment showed ≤8.8% intraspecific variability. The Caspian Sea Basin and Anatolio-Mediterranean region had the highest haplotype diversity, while the Danube Basin and western Europe populations had similar haplotypes. Phylogenetic analysis showed congruence between mitochondrial and ribosomal markers, except for Paradiplozoon koubkovae from Iran. The high intraspecific variability highlights the rapid mutation rate in mitochondrial DNA, important for future taxonomic studies. The spatial distribution of mitochondrial DNA haplotypes showed partial geographical structure, suggesting an east-to-west dispersal route for P. homoion into Europe. The presence of different haplotypes within a single individual suggests intraspecific hybridization.