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Dryad

Parasitic cnidarians (Myxozoa) do not retain key oxygen-sensing and homeostasis tool-kit genes: Transcriptome assemblies

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Sep 22, 2021 version files 1.13 GB
Sep 09, 2022 version files 1.13 GB

Abstract

For aerobic organisms, both the Hypoxia-Inducible Factor (HIF) pathway and the mitochondrial genomes are key players in regulating oxygen homeostasis. However, recent work has suggested that these mechanisms are not as highly conserved as previously thought, prompting more thorough surveys across animal higher taxonomic levels, which would in turn permit testing of hypotheses about the ecological conditions that may have facilitated evolutionary loss of such genes. The phylum Cnidaria is known to harbor wide variation in mitochondrial genome morphology, from typical single circular chromosomes to fragmented linear chromosomes. More recently, members of the cnidarian clade Myxozoa, comprising obligate endoparasites, were shown to have lost their mitochondrial genome, suggesting that variation in environmental oxygen availability may be a key determinant in the evolution of metabolic gene networks. Here, we surveyed genomes and transcriptomes across 42 cnidarian species for the presence of HIF pathway members (HIFa, EGLN, VHL), as well as for an assortment of hypoxia, mitochondrial, and stress-response toolkit genes. We find that presence of the HIF pathway, as well as number of genes associated with mitochondria, hypoxia, and stress response, do not vary based on mitochondrial genome morphology. More interestingly, we uncover evidence that myxozoans have lost the canonical HIF pathway repression machinery, potentially altering HIF pathway functionality to work under the specific conditions of their parasitic lifestyles. In addition, relative to other cnidarians, myxozoans show loss of large proportions of genes associated with the mitochondrion (~39%), and involved in response to hypoxia (~27.5%) and general stress (~32%). Our results provide additional evidence that the HIF regulatory machinery is evolutionarily labile and that variations in the canonical system have evolved in many animal groups.