Myxozoan Infections in Fish from the Perspective of a Sea Anemone

 1Benjamin E. Americus, 1Nicole Hams, 2Gema Alama-Bermejo, 1Jerri L. Bartholomew, 1Stephen D. Atkinson

ABSTRACT

Myxozoans are parasites of freshwater, marine, and anadromous fishes. Globally, they impact aquaculture stocks and wild fisheries. Myxozoans are members of the phylum Cnidaria, which includes free-living organisms like corals, jellyfish, and anemones. Myxozoans split with their free-living cnidarian relatives ~600 million years ago and have since streamlined their bodies and genomes to become microscopic and parasitic. Despite this reduction, myxozoans have retained stinging organelles called nematocysts. Whereas free-living cnidarians use nematocysts to release venom or entrap prey, myxozoans use them to attach to fish and initiate infection.

In this work, we applied computational techniques adapted from the well-studied anemone, Nematostella vectensis, to better understand myxozoans. Specifically, we asked if, and how they use venom proteins during infection. With Ceratonova shasta, we utilized proteomic data from isolated nematocysts and time-series transcriptomes from infection fish. We screened for venom-like proteins in C. shasta by examining sequence and structure-level similarities to known venoms in other Cnidarians and venomous animals. This yielded six venom candidates of high confidence: three proteases, one inhibitor, one protein related to blood clotting, and one of unknown function. Phylogenetic analyses suggest that these proteins were inherited from the common ancestor of myxozoans and free-living cnidarians, and homologs can be found across Cnidaria.

We then examined the expression of genes for these proteins during infection at 1, 7, 14, and 21 days post-exposure. These data suggested two waves of expression: an initial pulse of proteases at 1 and 7 days, and then an upregulation of genes for the other venom-like proteins and 14 and 21 days. Potentially, the early wave is related to parasite invasion, and the second wave is related to proliferation. These findings suggest that myxozoans inherited venom-like proteins from their free-living ancestors and have since repurposed them to infect fish. We believe the continued adaptation of ‘omics approaches from work with N. vectensis and other cnidarians will further illuminate the interesting biology of Myxozoa.