Abstract

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.