Department of Microbiology, Oregon State University, Corvallis, OR, USA.
Department of Marine Biology, The Leon H.Charney School of Marine Sciences, University of Haifa, Haifa, Israel.
Int J Parasitol. 2020 Sep;50(10-11):763-769. doi: 10.1016/j.ijpara.2020.04.012. Epub 2020 Jul 21.
Myxozoans are obligate parasites that have complex life cycles requiring alternate vertebrate and invertebrate hosts, with transmission via microscopic waterborne spores. Unusually for parasites, they belong to the phylum Cnidaria, alongside thousands of free-living corals, sea anemones, jellyfish and hydrozoans. Their cnidarian affinity is affirmed by genetic relatedness and the presence of nematocysts, historically called "polar capsules" in myxozoan research. Free-living cnidarians utilise this cellular weaponry for defence, predation and adhesion, whereas myxozoans use it to anchor to their hosts as the first step in infection. Despite the ~650 million years of divergence between free-living cnidarians and myxozoans, their nematocysts retain many shared morphological and molecular characters. Both are intra-cellular capsules with a single opening, and contain a coiled, evertable tubule. They are composed of unique nematocyst proteins, nematogalectin and minicollagen, and both likely contain an internal matrix of metal cations covalently bound to the anionic polymer poly-gamma glutamate. The rapid dissociation of this matrix and the resulting increase in internal osmotic potential is the driving force behind tubule elongation during discharge. In this review, we compare the structure and function of nematocysts in Myxozoa and free-living Cnidaria, incorporating recent molecular characterizations. We propose that terminology for homologous myxozoan structures be synonymized with those from other Cnidaria, hence, "polar capsule" as a taxon-specific nematocyst morphotype and "polar filament" as "tubule." Despite taxonomic divergence, genome reduction and an evolution to parasitism, myxozoans maintain nematocysts that are structurally and functionally homologous to those of their free-living cnidarian relatives.
粘孢子虫是专性寄生虫，具有复杂的生命周期，需要交替的脊椎动物和无脊椎动物宿主，并通过微小的水生孢子传播。与寄生虫不同的是，它们属于刺胞动物门，与数以千计的自由生活的珊瑚、海葵、水母和水螅门动物一起。它们的刺胞动物亲缘关系通过遗传关系和刺丝囊的存在得到证实，刺丝囊在粘孢子虫研究中历史上被称为“极囊”。自由生活的刺胞动物利用这种细胞武器进行防御、捕食和黏附，而粘孢子虫则利用它来感染宿主，这是感染的第一步。尽管自由生活的刺胞动物和粘孢子虫之间有大约 6.5 亿年的分化，但它们的刺丝囊保留了许多共同的形态和分子特征。两者都是具有单个开口的细胞内囊，包含一个卷曲的、可外翻的小管。它们由独特的刺丝囊蛋白、刺丝囊凝集素和微胶原蛋白组成，两者可能都含有一个内部基质，该基质由与阴离子聚合物聚-γ谷氨酸共价结合的金属阳离子组成。这种基质的快速解离和内部渗透压的增加是小管在放电过程中伸长的驱动力。在这篇综述中，我们比较了粘孢子虫和自由生活的刺胞动物的刺丝囊的结构和功能，并结合了最近的分子特征。我们提出，同源粘孢子虫结构的术语应与其他刺胞动物的术语同义化，因此，“极囊”作为一个分类群特有的刺丝囊形态类型，“极丝”作为“小管”。尽管粘孢子虫经历了分类学分化、基因组减少和向寄生的进化，但它们仍然保持着结构和功能上与自由生活的刺胞动物亲缘关系相似的刺丝囊。
