Rapid evolutionary adaptations to parasitology: Insights from basal metazoans belonging to the Myxozoa (Cnidaria)

The complexity of biological phenomena can often be attributed to dynamic interactions among individuals within and across taxa. In parasitology, such interactions are commonly expressed as coevolutionary arms races between hosts and parasites, characterized by the necessity of continuous adaptation to maintain evolutionary fitness. Myxozoans represent a remarkable group of cnidarians that have undergone extreme morphological and genomic reduction, resulting in minute body sizes and highly compact genomes. These obligate parasites alternate between vertebrate and invertebrate aquatic hosts and are responsible for a variety of diseases affecting both cultured and wild fish populations. Despite their streamlined genomes, myxozoans retain a core set of conserved genes, while extensive gene loss and rapid, lineage-specific gene gains have driven their diversification and host specialization. These genomic innovations underpin several striking biological adaptations, including the complete loss of mitochondrial respiration in one lineage and the evolution of specialized surface proteins that facilitate blood-feeding and immune evasion in others. Here, we trace a decade-long journey of discovery that began with the sequencing of the first myxozoan genome in 2014. Subsequent research has profoundly expanded our understanding of the molecular and evolutionary mechanisms underlying parasitism in this lineage, shedding light on how myxozoans have achieved successful and stable genomic integration of new and transferred tools to adapt to their hosts—an extraordinary example of parasitic adaptation in one of the oldest groups of metazoan parasites on Earth.