Phylogenetic Relationships of the Strongyloid Nematodes of Australasian Marsupials Based on Mitochondrial Protein Sequences

Simple Summary Parasitic strongyloid nematodes endemic to the gastrointestinal tracts of Australasian marsupials are one of the most diverse groups of mammalian parasites. These nematodes are currently placed in the family Chabertiidae comprising two subfamilies, namely the Cloacininae and Phascolostrongylinae. Their current classification relies primarily on morphological features and has not been validated using molecular data. This study aimed to determine the phylogenetic relationships of the Cloacininae and Phascolostrongylinae within the family Chabertiidae and their relationship with other groups of strongyloid nematodes from non-marsupial hosts, using mitochondrial protein sequence datasets. The findings supported the recognition of the family Cloacinidae, containing the Cloacininae and Phascolostrongylinae, as a monophyletic group within the Strongyloidea. However, the subfamily Phascolostrongylinae was paraphyletic, and the relationships of individual genera corresponded with their host families. Genera of the Cloacininae and Phascolostrongylinae occurring in macropod hosts were more closely related compared to genera of the Phascolostrongylinae occurring in wombats. This study suggests an alternative hypothesis for the origin of marsupial strongyloid nematodes in vombatid hosts that should be explored further using molecular approaches and more widespread sampling. Abstract Australasian marsupials harbour a diverse group of gastrointestinal strongyloid nematodes. These nematodes are currently grouped into two subfamilies, namely the Cloacininae and Phascolostrongylinae. Based on morphological criteria, the Cloacininae and Phascolostrongylinae were defined as monophyletic and placed in the family Cloacinidae, but this has not been supported by molecular data and they are currently placed in the Chabertiidae. Although molecular data (internal transcribed spacers of the nuclear ribosomal RNA genes or mitochondrial protein-coding genes) have been used to verify morphological classifications within the Cloacininae and Phascolostrongylinae, the phylogenetic relationships between the subfamilies have not been rigorously tested. This study determined the phylogenetic relationships of the subfamilies Cloacininae and Phascolostrongylinae using amino acid sequences conceptually translated from the twelve concatenated mitochondrial protein-coding genes. The findings demonstrated that the Cloacininae and Phascolostrongylinae formed a well-supported monophyletic assemblage, consistent with their morphological classification as an independent family, Cloacinidae. Unexpectedly, however, the subfamily Phascolostrongylinae was split into two groups comprising the genera from macropodid hosts (kangaroos and wallabies) and those from vombatid hosts (wombats). Genera of the Cloacininae and Phascolostrongylinae occurring in macropodid hosts were more closely related compared to genera of the Phascolostrongylinae occurring in wombats that formed a sister relationship with the remaining genera from macropods. These findings provide molecular evidence supporting the monophyly of the family Cloacinidae and an alternative hypothesis for the origin of marsupial strongyloid nematodes in vombatid hosts that requires further exploration using molecular approaches and additional samples