Abstract Background Phylogenetic relationships among members in the order Diplostomida remain contentious, with mitochondrial (mt) and nuclear genomic data often yielding conflicting topologies. A major limitation is the availability of only a few mt genomes from the type genus Strigea , hindering a robust test of the monophyly of the family Strigeidae and the order Diplostomida. Results The mt genome of S. falconis was completely sequenced for the first time, which was a circular molecule of 16,872 bp in length, encoding the typical set of 36 mt genes and six duplicate tRNA-Glu genes. Notably, there were seven identical and consecutive tandem repeat units each consist of a 169 bp non-coding region followed by a trnE gene in the newly assembled genome. Phylogenomic analyses based on concatenated predicted amino acid sequences of 12 proteins robustly placed S. falconis in the same clade as Apharyngostrigea pipientis . Crucially, the family Strigeidae was not recovered as monophyletic. Instead, two species within Strigeidae, Cardiocephaloides medioconiger and Cotylurus marcogliesei , clustered with representatives of Diplostomidae, providing mt evidence for the paraphyly of Strigeidae under the current sampling. Conclusions The newly sequenced mt genome of S. falconis reveals a previously unreported six-copy tandem repeat of trnE -containing units among currently available diplostomoid mt genomes. Phylogenetic analyses based on mt protein-coding genes provide additional mt evidence that the family Strigeidae was not recovered as monophyletic under the present taxon sampling. However, because mt genomes represent a single maternally inherited linkage group, broader taxon sampling, independent nuclear phylogenomic data, and explicit sensitivity analyses will be required to confirm these relationships and guide any formal systematic revision.
ZHANG et al. (Sat,) studied this question.