Characterisation of the relationship between Spirometra erinaceieuropaei and Diphyllobothrium species using complete cytb and cox1 genes

•All S. erinaceieuropaei isolates from eastern China belong to the same species.•S. erinaceieuropaei isolates from eastern China should be a single population.•Population expansion of S. erinaceieuropaei occurred in the Pleistocene.•S. erinaceieuropaei and Diphyllobothrium started to diverge at the late Miocene. Spirometra erinaceieuropaei is an important diphyllobothriid cestode whose larval plerocercoid (sparganum) can parasitise humans, causing sparganosis. In China, sparganosis is an important foodborne parasitic zoonosis and poses a serious threat to human health. However, our knowledge of the taxonomic classification of S. erinaceieuropaei and its relationship with other diphyllobothriid cestodes, such as Diphyllobothrium, remains fragmentary. In this study, we determined the complete sequences of cytochrome b (cytb) and cytochrome c oxidase subunit I (cox1) genes from 61 isolates of S. erinaceieuropaei collected from 11 geographical locations in eastern China and compared them to the sequences of Diphyllobothrium species from the GenBank database. Then, we evaluated the genetic variation of S. erinaceieuropaei and the phylogenetic relationship between S. erinaceieuropaei and Diphyllobothrium. Both negative Tajima’s D and Fu’s FS values of neutrality tests and unimodal curve of mismatch distribution analyses supported demographic population expansion. Bayesian skyline plot (BSP) analyses indicated that a population expansion occurred in the Pleistocene. Phylogenetic analysis based on cytb, cox1, and cytb+cox1 under all three methods tested (neighbour-joining, maximum parsimony, and maximum likelihood) suggested that the S. erinaceieuropaei isolates from eastern China and the genus Diphyllobothrium are separate monophyletic groups. Furthermore, the uncorrelated log-normal relaxed molecular-clock model suggested that divergence between Spirometra and Diphyllobothrium began in the late Miocene.