The complete mitochondrial genome of Sindiplozoon coreius: genome structure description, comparative analysis, and phylogenetic implications

Diplozoids are common gill-parasitic monogenean ectoparasites of cyprinid fishes and can adversely affect host respiration. Despite their ecological and pathological significance, genomic information on diplozoids remains limited. Mitochondrial genomes provide valuable resources for species identification and phylogenetic analysis. In this study, we sequenced and analyzed the complete mitochondrial genome of Sindiplozoon coreius to investigate its genomic features and clarify its phylogenetic position within Monogenea. The complete mitochondrial genome of S. coreius is a circular DNA molecule comprising 15,259 bp and contains 36 typical mitochondrial genes. Comparative analysis of Diplozoidae mitogenomes revealed consistently high A + T content (65.6%–73.4%), with a pronounced negative AT skew and positive GC skew. Our analyses indicated the atp6 gene exhibits the highest evolutionary rate among the protein-coding genes in Diplozoidae (Ka/Ks = 1.107, Pi = 0.427), highlighting its potential as a reliable marker for species identification. Gene order was largely conserved among species, except in Paradiplozoon yunnanensis, P. opsariichthydis, and P. hemiculteri, which exhibited rearrangements, providing evidence of lineage-specific evolutionary events. Bayesian phylogenetic analysis revealed two distinct clades within the Polyopisthocotylea and showed that Eudiplozoon forms a sister group to the remaining genera in Diplozoidae (Sindiplozoon, Diplozoon and Paradiplozoon), offering new insights into intrafamilial relationships within Diplozoidae. This study reports the first complete mitochondrial genome of Sindiplozoon coreius and provides a comprehensive comparative mitochondrial genomic analysis within Diplozoidae. Our findings enhance the mitochondrial data available for this family, determine the genomic features of S. coreius and establish its phylogenetic position. The results also confirm the previously reported conservation of cox1, the gene rearrangements that occur in different Paradiplozoon species, and the monophyly of Eudiplozoon. This work provides a valuable foundation for future taxonomic, evolutionary, and population genetic studies of Monogenea.