Avian Schistosome Biodiversity
Adult digenetic trematodes are obligate parasites in most vertebrates. Digeneans have a unique dependence on mollusks as first intermediate hosts in which they reproduce asexually thus immensely increasing the number of infective larvae, one of the most distinctive attributes enabling their success. Representatives of several phyla serve as second intermediate hosts. As a result, these parasites inhabit a number of phyla and have colonized nearly every organ in their hosts. One of the most distinctive, biologically intriguing and medically significant families of digeneans is the Schistosomatidae.
Schistosomatidae includes 96 species in 14 genera and is currently divided into four subfamilies: Bilharziellinae and Gigantobilharzinae in birds, Schistosomatinae in mammals (and a few birds), and Griphobilharzinae in crocodiles. Our molecular studies indicate that Griphobilharzinae nest within the Spirorchiidae so is not a schistosome (Brant & Loker 2005). Over half of schistosome diversity (65%) lies within the BTGD clade that parasitizes birds, which includes Bilharziellinae and Gigantobilharzinae. Until recently, the BTGD clade has received little attention and its taxonomy is in disarray. In the last several years, a new genus and several new lineages in the BTGD clade have been discovered with morphological features quite different and opposite to those for the mammalian schistosomes. These features include: short or absent gynecophoric canal; either flat or thread-like bodies; reduced dimorphism; no pair formation; and the use of both freshwater and marine snail hosts.
The taxonomic focus is a sizeable yet understudied avian schistosome clade. We will provide a robust systematic framework for the globally distributed BTGD clade comprised of subfamilies Bilharziellinae and Gigantobilharzinae. We have assembled a global network of colleagues with whom we will: a) Collect, catalogue, identify, (re)describe, and provide a specimen database for 17 species in hand, plus the estimated 33 newly collected species, and anticipate to produce 19 new species descriptions plus 11 redescriptions; b) Publish the first comprehensive monographic treatments of the four of the six genera in BTGD clade; c) Generate a robust phylogenetic hypothesis of species relationships with morphology and genes; d) Propose a new classification for the species of Bilharziellinae and Gigantobilharzinae; e) Test hypotheses of character evolution and host use; f) Train two graduate students and 4-6 undergraduate students to participate in all aspects of the project; g) Enhance existing museum collections at the Museum of Southwestern Biology Division of Parasites by deposition of newly-collected, quality specimens and; h) Disseminate comprehensive information to the scientific and medical communities and the general public via web-based tools.
Impact on the taxonomy of Schistosomatidae: A critical result of this research will be the establishment of a database and natural classification for the avian schistosomes based on morphology, life cycle biology and DNA. Schistosomes are significant on the world health agenda at a time when taxonomy wanes in expertise. This research will illuminate the life history and diversity of a very understudied schistosome clade by providing baseline data for large and small-scale evolutionary hypothesis testing.
General contribution to host-parasite evolutionary biology: A molecular phylogeny will allow us to undertake revisions and descriptions of genera and species in the avian clade, complete phylogenetic hypotheses of existing and new genera of avian schistosomes and improve our understanding of global biodiversity, provide a framework for progress on evolution and biogeography, host-parasite evolution, and epidemiology, and connectivity with taxonomists, other academic researchers, and government agencies. The widespread availability and accessibility of the Arctos database and our website will make it a useful tool for understanding schistosome diversity and promote advancement of trematode evolution.
Our work since 2004, greatly aided by NSF funding, to discover the biodiversity of schistosomes and their hosts has led to several interesting observations:
1. A majority of the species diversity of schistosomes uses bird definitive hosts.
2. Diversification by host switching into new snail hosts may be one of the major mechanisms, then secondarily schistosome species diversity is correlated with defnitive host ecological diversity and fidelity (in the case of migratory birds).
3. We have found more diversity in the cercarial stages from snails than adults from birds or mammals.
4. The genus Trichobilharzia is the most speciose of the schistosome genera. These parasites occur almost exclusively a few clades of ducks and one species has been found in geese. Thus far they use two families of snails as intermediate hosts, Physidae and Lymnaeidae.
5. Along with our collaborators, to date, we have found 20 distinct lineages of avian schistosomes from North and South America, Kenya, South Africa, Nepal, China, New Zealand, and Iran.
6. These diversity studies alone contributed to a better understanding of the epidemiology of cercarial dermatitis, or swimmer's itch and resulted in a nice review paper as well as the foundational background for development of probes to more quickly assess species and dermatitis risk in waters.
7. Currently there is an effort to use the large amounts of data and diversity to look more specifically at a few species of avian schistosomes using phylogeographic and population genetic approaches to ask how host traits influence parasite evolution and population dynamics.
© copyright 2017 Sara V. Brant, University of New Mexico