Taxonomy and phylogenetics of fish blood flukes using morphology, life history, and molecular markers

Abstract

Fish blood flukes (Digenea: Aporocotylidae) are of rapidly progressing and emerging interest to ecology and evolutionary biologists because some lineages may have coevolved with the major lineages of non-tetrapod vertebrates (= “fishes”). They are relevant to medical researchers because they are the putative immediate ancestors to the blood flukes that cause schistosomiasis (Schistosomatidae), a disease that debilitates millions of people annually. Aquaculture industry personnel regard them as pathogens of high-value fishes in marine aquaculture operations. Taxonomists are interested in them as well because the rate of new species discovery is proportionally high relative to that of other fish trematode groups. Yet, ambiguity regarding fish blood fluke interrelationships obstructs a deeper understanding of the evolutionary origins of flatworm parasitism in craniates, including the origin of schistosomes. In order to address this issue, in my Ph.D. dissertation research, I have employed alpha taxonomy and molecular phylogenetics approaches to explore the taxonomic diversity of fish blood flukes that infect underexplored hosts and generate the most comprehensive phylogeny of Schistosomatoidea sensu lato to date. This work has resulted in publications in the Journal of Parasitology, and Folia Parasitologica comprising the taxonomic characterization of 6 new species of fish blood flukes of 4 genera (i.e., Hyperandrotrema walterboegeri Orélis-Ribeiro and Bullard, 2013 n. sp., Plehniella sabajperezi Orélis-Ribeiro and Bullard, 2015 n. sp., Plehniella armbrusteri Orélis-Ribeiro and Bullard, 2015 n. sp, Cladocaecum tomasscholzi Orélis-Ribeiro and Bullard, 2016 n. gen., n. sp., Elopicola n. sp. 1, and Elopicola n. sp. 2), emended diagnosis of 3 of those genera, and re-description plus erection of a new genus (Kritsky platyrhynchi [Guidelli, Isaac, and Pavanelli, 2002] Orélis-Ribeiro and Bullard n. gen., n. comb.). The specimens described herein were derived from a wide taxonomic scope of fish hosts that included a lamniform shark (shortfin mako shark, Isurus oxyrinchus [Rafinesque]), pimelodid catfishes (long-whiskered catfishes, Pimelodus albofasciatus [Mees], Pimelodus blochii [Valenciennes], Pimelodus grosskopfii [Steindachner]), Hemisorubim platyrhynchos [Valenciennes]), an auchenipterid catfish (driftwood catfish (Ageneiosus inermis [Linnaeus]), and elopiform fishes (Hawaiian ladyfish, Elops hawaiensis [Reagan], and tarpon, Megalops atlanticus [Valenciennes]). Fishes were also collected from diverse sites throughout South and North America, and Asia. Additionally, my collaborators and I published in Advances of Parasitology a synoptic review of all published molecular studies (life history, taxonomy, phylogeny) and summarized all GenBank sequences and primer sets for the fish blood flukes. Further, I lead the analysis of new and all available sequence data for the partial D1–D2 domains of 28S rDNA from 83 blood fluke taxa, and explored the evolutionary expansion of flatworm parasitism in the blood of craniates. In the last chapter of my Ph.D. dissertation research, my collaborators and I substantially improved this previous phylogeny by targeting a denser taxon sampling among underexplored blood fluke lineages that infect chondrichthyan and actinopterygian hosts, and using a combination of 2 nuclear ribosomal genes (18S and 28S rDNA). Based on a dataset that comprises 97 blood fluke taxa, the resultant tree topologies represented the most well-resolved, large-scale phylogeny of blood flukes to date. Although no significant novel relationships were recovered among tetrapod blood flukes (“spirorchiids” and schistosomes), our phylogenetic trees provided meaningful insights about the evolution of fish blood flukes. This is the first phylogenic reconstruction that tested and supported monophyly of chondrichthyan, elopiform, and otophysan blood flukes. The earliest-branching monophyletic group sister to the remaining fish blood flukes comprised an acipenseriform blood fluke, Acipensericola petersoni, plus all chondrichthyan blood flukes. This clade was recovered sister to elopiform blood flukes that, in turn, were sister to a clade comprising blood flukes that infect otophysan plus neoteleost fishes. Such branching order matches that of their hosts. In the context of Schistosomatoidea sensu lato, these results support the notion that blood flukes exhibit co-phyly with their craniate hosts.