Date of Award
Master of Science
One of the most diverse groups of parasitic nematodes includes the nematodes of the suborder Trichostrongylina. Trichostrongyle systematic classification is based on the study of the anterior end, the female reproductive system, the male copulatory bursa and cuticular ridges, which make up the synlophe. These morphological characters also assist taxonomists to characterize species and assign them into one of the three superfamilies. Heligmosomoidea is the most diverse superfamily, including Viannaiidae. This family represents one of the major evolutionary radiations of trichostrongyles in South America. Members of Viannaiidae parasitize a variety of mammals endemic to the Neotropics including, but not limited to, the opossums. Viannaiids have great morphological variation in the reproductive organs of the females and the secondary sexual structures of the males (e.g., rays in the bursa). Consequently, the placement of these species in classification schemes has drastically changed over time. The prevailing taxonomy relies on the host used by the parasites as well as their geographic distribution. Viannaiidae has always included monodelphic nematodes with simple synlophes, yet it eventually included species in the genus Travassostrongylus, which the morphology is very distinct from the rest of the members of the family. Differences pertain to a greater number of ridges of the synlophe and the didelphic female reproductive system. These features are more similar to trichostrongyles in Herpetostrongylidae and Nicollinidae, which infect Australian vertebrates, the majority of which are marsupials. I herein document the diversity of this group by reporting the presence of a putative new species and use five gene regions to reconstruct the phylogeny of Viannaiidae exclusive of didelphid marsupials. I used the resulting phylogeny to test the monophyly of Travassostrongylus and Viannaia and to reconstruct the character evolution of the monodelphic/didelphic condition and the ornamentation of the cuticle. The phylogeny indicates that Viannaiidae is not monophyletic, recovering a clade with the Travassostrongylus species and trichostrongyles from Australian fauna. The tests for character reconstruction assist in determining that the didelphic condition and the presence of dorsal synlophe ridges in Travassostrongylus and Austrostrongylus may be traits inherited from a common ancestor. Furthermore, it was apparent that a character change from didelphic to monodelphic occurred in the common ancestor of the Viannaia species. This ancestor also underwent a change from a dorsal cuticle with ridges to a smooth cuticle. Though the study suggests that Viannaiidae is not monophyletic, the inclusion of more species from these genera and the viannaiids found in hystricognaths from the New World rodents will conclusively determine the affinities of the members of the family. Finally, I suggest that the relationship between the species in Travassostrongylus and the Australian parasites dates to the Gondwana landmass and that these trichostrongyles, or their ancestors, were present in the marsupials of that time period. The examination of microbiotheriids could provide more information and illuminate the factors that led to the evolutionary relationship between the parasites of America and Australia.
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