Reft Abigail J, Daly Marymegan
Department of Ecology and Evolutionary Biology, Division of Invertebrate Zoology, Natural History Museum, University of Kansas, Lawrence, Kansas 66045, USA.
J Morphol. 2012 Feb;273(2):121-36. doi: 10.1002/jmor.11014. Epub 2011 Sep 30.
Cnidae are complex intracellular capsules made by all cnidarians. The most diverse of these capsules are nematocysts, which are made by all members of the phylum; spirocysts and ptychocysts are made only by members of some lineages, and they show less functional and structural diversity. In nematocysts, the apex has been shown to be either a hinged cap (operculum) or three flaps that flex outward during discharge. The operculum is known only from medusozoan nematocysts; flaps are known only from nematocysts of members of the hexacorallian order Actiniaria, although they have been inferred to be characteristic of Anthozoa, the group to which Actiniaria belongs. Using scanning and transmission electron microscopy, we discover a third apical morphology in nematocysts, an apical cap, which we find in all nonactiniarian anthozoans examined. This apical cap is identical structurally to the apical cap of spirocysts, and it resembles the apical structure of ptychocysts, whose apex is documented here for the first time. Additionally, a full survey of nematocysts from all body structures of two actiniarians demonstrates that a particular type of nematocyst, the microbasic p-mastigophore of the mesenterial filaments, does not have apical flaps. The observed variation does not correspond to conventional categorization of capsule morphology and raises questions about the function and structure of capsules across Cnidaria. Despite some ambiguity in optimization of ancestral states across cnidae, we determine that the apical cap is the plesiomorphic structure for anthozoan cnidae and that apical flaps are a synapomorphy of Actiniaria. At present, the operculum is interpreted as a synapomorphy for Medusozoa, but either it or an apical cap is the ancestral state for nematocysts.
刺丝囊是所有刺胞动物制造的复杂细胞内囊。这些囊中最多样化的是刺丝囊，刺胞动物门的所有成员都会制造；螺旋囊和盘囊仅由某些谱系的成员制造，它们的功能和结构多样性较低。在刺丝囊中，顶端已被证明是一个铰接的帽（盖）或三个在排放时向外弯曲的瓣。盖仅在水母亚门的刺丝囊中已知；瓣仅在六放珊瑚目海葵亚目的成员的刺丝囊中已知，尽管它们被推断为海葵所属的珊瑚虫纲的特征。使用扫描电子显微镜和透射电子显微镜，我们在刺丝囊中发现了第三种顶端形态，即顶端帽，我们在所有检查过的非海葵亚目的珊瑚虫纲动物中都发现了它。这种顶端帽在结构上与螺旋囊的顶端帽相同，并且它类似于盘囊的顶端结构，其顶端在此首次被记录。此外，对两种海葵所有身体结构的刺丝囊进行的全面调查表明，一种特定类型的刺丝囊，即肠系膜丝的微基 p 型鞭毛虫，没有顶端瓣。观察到的变异与囊形态的传统分类不对应，并引发了关于整个刺胞动物门中囊的功能和结构的问题。尽管在整个刺丝囊的祖先状态优化方面存在一些模糊性，但我们确定顶端帽是珊瑚虫纲刺丝囊的原始结构，而顶端瓣是海葵亚目的共有衍征。目前，盖被解释为水母亚门的共有衍征，但它或顶端帽是刺丝囊的祖先状态。
