Dipartimento di Biologia, Università di Padova, I-35121 Padova, Italy.
J Comp Neurol. 2013 Aug 15;521(12):2756-71. doi: 10.1002/cne.23313.
We analyzed the mouth of three species, representative of the three orders of the class Thaliacea (Tunicata)--Pyrosoma atlanticum (Pyrosomatida), Doliolum nationalis (Doliolida), and Thalia democratica (Salpida)--to verify the presence of mechanoreceptors, particularly hair cells. In vertebrates, hair cells are well-known mechanoreceptors of the inner ear and lateral line, typically exhibiting an apical hair bundle composed of a cilium and stereovilli but lacking an axon. For a long time, hair cells were thought to be exclusive to vertebrates. However, evidence of a mechanosensory organ (the coronal organ) employing hair cells in the mouth of tunicates, considered the sister group of vertebrates, suggests that tunicate and vertebrate hair cells may share a common origin. This study on thaliaceans, a tunicate group not yet investigated, shows that both P. atlanticum and D. nationalis possess a coronal organ, in addition to sensory structures containing peripheral neurons (i.e., cupular organs and triads of sensory cells). In contrast, in T. democratica, we did not recognize any oral multicellular sensory organ. We hypothesize that in T. democratica, hair cells were secondarily lost, concomitantly with the loss of branchial fissures, the acquisition of a feeding mechanism based on muscle activity, and a mechanosensory apparatus based on excitable epithelia. Our data are consistent with the hypothesis that hair cells were present in the common ancestor of tunicates and vertebrates, from which hair cells progressively evolved.
我们分析了三个物种的口部，它们代表了被囊动物门（Tunicata）的三个目——多管水母（Pyrosomatida）、中华哲水蚤（Doliolida）和民主被囊水母（Salpida）——以验证机械感受器的存在，特别是毛细胞。在脊椎动物中，毛细胞是内耳和侧线的知名机械感受器，通常表现为由纤毛和静纤毛组成的顶端毛束，但没有轴突。长期以来，毛细胞被认为是脊椎动物所特有的。然而，在被囊动物的口部发现了一种使用毛细胞的机械感受器器官（冠状器官），被囊动物被认为是脊椎动物的姊妹群，这表明被囊动物和脊椎动物的毛细胞可能具有共同的起源。这项对被囊动物的研究，一个尚未被研究过的被囊动物群体，表明不仅 P. atlanticum 和 D. nationalis 拥有冠状器官，而且还拥有包含外围神经元的感觉结构（即杯状器官和三联体感觉细胞）。相比之下，在 T. democratica 中，我们没有识别出任何口腔多细胞感觉器官。我们假设，在 T. democratica 中，毛细胞是伴随着鳃裂的丧失、基于肌肉活动的摄食机制的获得以及基于可兴奋上皮的机械感受器装置的丧失而被二次丢失的。我们的数据与毛细胞存在于被囊动物和脊椎动物的共同祖先中的假说一致，从共同祖先开始，毛细胞逐渐进化。
