Australian macropodid (Family Macropodidae) and vombatid (Family Vombatidae) marsupials are parasitised by a diverse array of strongyloid nematodes which are classified in the subfamilies Cloacininae and Phascolostrongylinae (1). The subfamily Cloacininae is found exclusively in the oesophagus and stomach of macropods (kangaroos and wallabies) (2). This subfamily has been the focus of previous morphological and molecular studies due to their extensive diversity, high prevalence and large burden of nematodes present in the hosts (2). Conversely, fewer studies have been conducted on the subfamily Phascolostrongylinae mostly due to the significantly smaller number of species occurring in low abundance and often encountered sporadically (2). The subfamily Phascolostrongylinae is currently comprised of seven genera found in macropodid and vombatid marsupials. Some of the genera possess unusual morphological features which may have contributed to difficulties in previous taxonomic revisions (3, 4).
The first phylogenetic classification of the superfamily Strongyloidea was based on morphological characters in which emphasis on the female ovejector followed by the male copulatory bursa and buccal capsules placed the nematodes according to host groups (3, 4). However, this classification led to the hypothesis that the strongyloid nematodes of Australian marsupials were of polyphyletic origins. The subfamily Phascolostrongylinae was initially characterised by a Y-shaped ovejector and two branches of the dorsal ray of the male bursa, and comprised four genera found in the intestines of macropodid and vombatid marsupials. Although they shared identical ovejector and bursal features as the Phascolostrongylinae, the intestinal parasites of kangaroos and wallabies, genera Hypodontus and Macropicola were placed in in subfamily Strongylinae with other nematodes from horses and elephants due to their uniquely large and globular buccal capsules (3). Corollostrongylus, exclusive to intestine of the musky rat-kangaroo, Hyspiprymnodon moschatus, also possesses a globular buccal capsule. However, because of its J-shaped ovejector, this genus was placed in the subfamily Chabertiinae alongside the nematodes of rodents and domestic ruminants (3).
Subsequently, an alternative classification system was proposed for strongyloid nematodes of Australian marsupials, arguing that greater emphasis on the male reproductive features would result in the monophyly of this group (1). Consequently, the genera Hypodontus, Macropicola and Corollostrongylus were added to the subfamily Phascolostrongylinae and further subdivided into three tribes (1). One tribe, Phascolostrongylinea comprised Phascolostrongylus turleyi and four species of Oesophagostomoides, all occurring within the colon of wombats. Another tribe, Macropostrongylinea, consisting of the genera Macropostrongyloides and Paramacropostrongylus is found in the stomach and large intestines of macropodid hosts. Finally, the tribe Hypodontinea from large intestines of macropods was comprised of Hypodontus, Macropicola and Corollostrongylus (1).
Following Beveridge’s (1) re-classification, several molecular studies have utilised allozyme and DNA sequencing data to detect genetic variation within the genera Hypodontus (5–7), Paramacropostrongylus (8–9), and Macropostrongyloides (10–11). However, phylogenetic studies at the subfamily level have been neglected. One study attempted to examine the relationships within the Phascolostrongylinae based on the second internal transcribed spacer (ITS-2) subunit of the nuclear ribosomal DNA data (12). This technique provided an opportunity to address the gap in research of the strongyloid of Australian marsupials. However, the findings were inconclusive due to the limited number of species analysed within the subfamily Phascolostrongylinae and the analysis of only one internal transcribed spacer (12). Other molecular studies have included both the sequences of the first and second internal transcribed spacers (ITS-1 and ITS-2, respectively [ITS+]) and have found these markers to be extremely useful for assessing phylogenetic relationships among closely related taxa of strongyloid nematodes in Australian marsupials (6–9, 11, 13–16). Although, the relationships between the tribes within the subfamily Phascolostrongylinae proposed by Beveridge (1) still remains untested, analyses of the ITS markers could provide molecular support for Beveridge’s (1) morphological classification.
The current study characterised the ITS + sequences of five genera within the Phascolostrongylinae (i.e., Paramacropostrongylus, Hypodontus, Macropicola, Oesophagostomoides and Phascolostrongylus). Following comparative analyses of the current ITS + sequence data with published sequences of Macropostrongyloides spp., phylogenetic relationships within the Phascolostrongylinae were determined.