Canning Elizabeth U, Okamura Beth
Department of Biological Sciences, Imperial College London, Sillwood Park Campus, Ascot, Berkshire, SL5 7PY, UK.
Adv Parasitol. 2004;56:43-131. doi: 10.1016/s0065-308x(03)56002-x.
Myxozoans (phylum Myxozoa) are metazoan parasites utilizing invertebrate and (mainly) aquatic vertebrate hosts. They have in common with cnidarians the possession of virtually identical, highly complex organelles, namely the polar capsules in myxozoan spores, serving for attachment to new hosts and the nematocysts in surface epithelia of cnidarians, serving for food capture. Although myxozoan spores are multicellular, the simple trophic body forms of almost all species, reduced to syncytial plasmodia or single cells, reveal no clues to myxozoan ancestry or phylogenetic relationships. The myxozoan genus Buddenbrockia is one of only two known genera belonging to a clade which diverged early in the evolution of the Myxozoa. Today the Myxozoa are represented by two classes, the Myxosporea, containing all the better-known genera, which alternate between fish and annelids, and the Malacosporea, containing Buddenbrockia and Tetracapsuloides, parasitising bryozoans. The latter genus also infects salmonid fish, causing proliferative kidney disease (PKD). The enigmatic Buddenbrockia has retained some of its ancestral features in a body wall of two cell layers and a worm-like shape, maintained by four longitudinally-running muscle blocks, similar to a gutless nematode and suggestive of a bilaterian ancestry. Although some analyses of 18S rDNA sequences tend towards a cnidarian (diploblast) affinity for myxozoans, the majority of these studies place them within, or sister to, the Bilateria. The latter view is supported by their possession of central class Hox genes, so far considered to be synapomorphic for Bilateria. The simple body form is, therefore, an extreme example of simplification due to parasitism. Various hypotheses for the occurrence of identical complex organelles (nematocysts and polar capsules) in diploblast and triploblast phyla are evaluated: common ancestry, convergent evolution, gene transfer and, especially, endosymbiosis. A theory of the evolution of their digenetic life cycles is proposed, with the invertebrate as primary host and secondary acquisition of the vertebrate host serving for asexual population increase.
粘孢子虫（粘孢子虫门）是后生动物寄生虫，寄生于无脊椎动物和（主要是）水生脊椎动物宿主。它们与刺胞动物的共同之处在于拥有几乎相同的高度复杂的细胞器，即粘孢子虫孢子中的极囊，用于附着新宿主，以及刺胞动物体表上皮中的刺丝囊，用于捕获食物。尽管粘孢子虫孢子是多细胞的，但几乎所有物种简单的营养体形式，简化为多核体或单细胞，并未揭示粘孢子虫的祖先或系统发育关系的线索。粘孢子虫属Buddenbrockia是已知仅有的两个属于在粘孢子虫进化早期分化出来的一个分支的属之一。如今，粘孢子虫由两个纲代表，粘孢子虫纲包含所有更知名的属，在鱼类和环节动物之间交替，以及马拉孢子虫纲，包含Buddenbrockia和四囊虫属，寄生于苔藓虫。后一个属也感染鲑科鱼类，引起增殖性肾病（PKD）。神秘的Buddenbrockia在其两层细胞的体壁和蠕虫状形状中保留了一些祖先特征，由四个纵向排列的肌肉块维持，类似于无肠线虫，暗示着两侧对称动物的祖先。尽管对18S rDNA序列的一些分析倾向于粘孢子虫与刺胞动物（双胚层动物）有亲缘关系，但这些研究中的大多数将它们置于两侧对称动物之内或作为其姊妹类群。后一种观点得到了它们拥有中央类Hox基因的支持，迄今为止，这些基因被认为是两侧对称动物的共有衍征。因此，简单的身体形式是寄生导致简化的极端例子。评估了双胚层和三胚层门中出现相同复杂细胞器（刺丝囊和极囊）的各种假说：共同祖先、趋同进化、基因转移，尤其是内共生。提出了它们双宿主生命周期的进化理论，以无脊椎动物为主要宿主，脊椎动物宿主的二次获得用于无性种群增长。
