not-yet-known not-yet-known not-yet-known unknown Trematodes are obligatory parasites that generally must transmit between hosts to complete life cycle. These strategies entail parasitizing diverse intermediate hosts, yet their evolutionary trajectory and ancestral origins remain elusive. We conducted the ancestral state reconstruction of the number of intermediate hosts using mitogenomic (Trematoda) and nuclear-genomic (Neodermata) topologies. Aspidogastrea was identified as the sister-group (basal) to all other Trematoda using a range of approaches, so it is crucial for studying the evolutionary history of trematodes. Unfortunately, mitogenome of this lineage still inaccessible. Herein, we sequenced mitogenomes of two aspidogastreans: Aspidogaster ijimai and Aspidogaster conchicola. As the ancestral state reconstruction analysis is topology-sensitive, we tested multiple phylogenetic strategies, comprising the outgroup selection, phylogenetic models, partitioning strategies, and topological constraints. Based on our analyses, Cestoda was the optimal outgroup choice, and the heterogeneous CAT-GTR model in PhyloBayes was the optimal model choice. We inferred the time tree and conducted ancestral state reconstruction analyses using this optimal topology, as well as constrained mitogenomic and nuclear genomic topologies. Results were ambiguous for some lineages, but scenario that received the strongest support is the direct life cycle (no intermediate hosts) in the ancestors of Trematoda (proto-trematodes) and Aspidogastrea (proto-aspidogastreans), while the ancestor of Digenea (proto-digeneans) had two intermediate hosts. The inferred scenario indicates that host strategies are relatively plastic among trematodes, putatively comprising several independent host gains, and multiple host losses. We propose a timeline for these events and discuss the role played in the evolution of complex parasitic life histories in digeneans.

Gyrodactylus is a lineage of monogenean flatworm ectoparasites exhibiting many features that make them a suitable model to study their co-evolution with fish hosts. Previous co-evolutionary studies of this lineage mainly relied on low-power datasets (a small number of samples and a single molecular marker), and (now) outdated algorithms. To investigate the coevolutionary relationship of gyrodactylids and their fish hosts in high resolution, we used complete mitogenomes (including two newly sequenced Gyrodactylus species), a large number of species in the single-gene dataset, and four different coevolutionary algorithms. The overall co-evolutionary fit between the parasites and hosts was consistently significant. Multiple indicators support gyrodactylids as highly host-specific parasites, but few gyrodactylids can parasitize either multiple (more than 5) or phylogenetically-distant fish hosts. The molecular dating results indicate they tend to evolve towards high host specificity. Speciation by host-switching is a more important speciation mode than co-speciation for them. Assuming the origin on Cypriniformes, we inferred four major host switch events to non-Cypriniformes hosts (mostly Salmoniformes) occurred deep in the evolutionary history. Despite their relative rarity, these events had strong macroevolutionary consequences for gyrodactylid diversity. For example, in our dataset, 57.28% of all studied gyrodactylids parasitised only non-Cypriniformes hosts, which implies that the evolutionary history of more than half of all included lineages could be traced back to these major host switch events. Geographical co-occurrence of fishes and gyrodactylids determined the host use by these gyrodactylids, and geography accounted for most of the phylogenetic signal in host use.