The highly repetitive genome of Myxobolus sp., a myxozoan parasite of fathead minnows

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Abstract
Background The Myxozoa is a group of at least 2,400 endoparasites within the phylum Cnidaria. All myxozoans have greatly reduced in size and morphology compared to free-living members of the phylum. They are best known for causing disease in economically important fish across the world; for example, Myxobolus cerebralis causes whirling disease, which can kill 90% of infected juvenile salmonid fish. In 2017, a new myxozoan species was identified in Alberta. Myxobolus rasmusseni n. sp. causes distinct lesions in fathead minnows, which are ultimately fatal. Here, we sequenced, assembled and analyzed the genome of M. rasmusseni to understand how the parasite interacts with its fish host and identify potential strategies to counter this emerging threat. Results At 185 Mb, the M. rasmusseni genome is the largest myxozoan genome sequenced so far. This large genome size is, in part, due to the high repetitive content; 68% of the genome was interspersed repeats, with the MULE-MuDR transposon covering 18% of the M. rasmusseni genome. Similar to myxozoan genomes, the M. rasmusseni genome has lost many genes well conserved in other eukaryotes. However, we also identified multiple expansions in gene families—serine proteases, hexokinases, and FLYWCH-domain containing proteins—which suggests their functional importance in the parasite. The mitochondrial genome of M. rasmusseni encodes only five of the thirteen protein-coding genes typically found in animals. We found that the mitochondrial gene atp6 was transferred to the nucleus and acquired a mitochondria-targeting signal in M. rasmusseni. Conclusions Our study provides valuable insights into myxozoan biology and identify promising avenues for future research. We also propose that M. rasmusseni is promising myxozoan model to explore host-parasite interactions in these parasites.

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