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

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.

Keywords: Cloacininae; Phascolostrongylinae; Strongyloidea; mitochondrial protein-coding genes; phylogenetics.

Figure 1

Nucleotide diversity (Pi) across twelve concatenated mitochondrial protein-coding genes (y-axis) of the Cloacininae, Phascolostrongylinae, Strongylidae, Oesophagostominae, Chabertiinae and Syngamidae included in the alignment. Nucleotide diversity was calculated in the software DnaSP version 6 using a window of 100 bp and 25 bp-steps. The nucleotide position (base pairs) is indicated on the X-axis next to the boundaries between mitochondrial protein-coding genes.

Figure 2

Topology of the Bayesian Inference phylogenetic analyses inferred from the concatenated alignment of the amino acid sequence of 12 mitochondrial genes of the genera of Cloacininae, Phascolostrongylinae, Oesophagostominae and Chabertiinae (Chabertiidae) and Strongylidae. Nodal support is indicated as posterior probabilities of the Bayesian Inference analysis. Syngamus trachea from the family Syngamidae was used as the outgroup. The host families (Macropodidae or Vombatidae) in which the species of Cloacininae and Phascolostrongylinae occur are represented by icons. The scale bar indicates the number of inferred substitutions per site.