Comparative transcriptomics reveal stage-dependent parasitic adaptations in the myxozoan Sphaerospora molnari

In order to address the threat posed by myxozoans to both cultured fish species and wild stocks, it is imperative to deepen our understanding of their genetics. The analysis of genomic and transcriptomic datasets of myxozoans over the past decade revealed significantly reduced and highly derived genomes, which negatively affects gene expression studies within their typically polyploid fish hosts. Consequently, information on genes essential for myxozoan intrapiscine survival is scarce. Here we employ new parasite isolation protocols in combination with transcriptomics to compare early proliferative with spore-forming stages of S. molnari, thereby unveiling the most crucial life strategies of each stage. We discovered several innovations, from genes transferred for improved host cell adhesion, to proteins for internalization of previously unknown key nutrients, and we identified diverse immune evasion factors that explain the long-term survival of these parasite in their hosts. We explore genes unique to S. molnari for their evolutionary novelty and as adaptations to specific host niches. We further elucidate the pathogenicity-associated genetic toolbox of S. molnari and discuss the implications of our discoveries in the context of disease control, shedding light on specific targets for intervention strategies.