Minicollagens play the governing role in polar capsule formation in parasitic cnidarians, the Myxozoa

Minicollagens are major structural components in the biogenesis of nematocysts in Cnidaria. Sequence mining and recent proteomic analysis of polar capsules, homologues of cnidarian nematocysts, have confirmed the presence of minicollagens in this evolutionarily ancient cnidarian endoparasitic group. Nonetheless, the role of nematocyst-associated proteins in polar capsule morphogenesis has never been studied in myxozoans. Here, we report the gene expression of three myxozoan minicollagens, ncol-1, ncol-3, and the recently identified ncol-5, during the intrapiscine development of Myxidium lieberkuehni, the myxozoan parasite of Northern pike Esox lucius. Moreover, we determined the abundance and localisation of Ncol-1 and Ncol-5 proteins in the developing myxozoan stages by western blotting and by immunofluorescence and immunogold electron microscopy. We found that expression of minicollagens was spatiotemporally restricted to developing polar capsules in sporogonic stages. Intriguingly, Ncol-1 and Ncol-5 were localised as major components of the polar capsule wall and polar tubule. These results support the common origin of nematocysts and myxozoan polar capsules. Furthermore, our findings have practical implications for a more accurate identification of developmental stages of myxozoan parasites.