Marine Biological Section, Department of Biology, University of Copenhagen, Universitetsparken 4, 2100 Copenhagen Ø, Denmark.
Zoological Lett. 2016 Sep 22;2:21. doi: 10.1186/s40851-016-0054-3. eCollection 2016.
Diuronotus is one of the most recently described genera of Paucitubulatina, one of the three major clades in Gastrotricha. Its morphology suggests that Diuronotus is an early branch of Paucitubulatina, making it a key taxon for understanding the evolution of this morphologically understudied group. Here we test its phylogenetic position employing molecular data, and provide detailed descriptions of the muscular, nervous, and ciliary systems of Diuronotus aspetos, using immunohistochemistry and confocal laser scanning microscopy.
We confirm the proposed position of D. aspetos within Muselliferidae, and find this family to be the sister group to Xenotrichulidae. The muscular system, revealed by F-actin staining, shows a simple, but unique organization of the trunk musculature with a reduction to three pairs of longitudinal muscles and addition of up to five pairs of dorso-ventral muscles, versus the six longitudinal and two dorso-ventral pairs found in most Paucitubulatina. Using acetylated α-tubulin immunoreactivity, we describe the pharynx in detail, including new nervous structures, two pairs of sensory cilia, and a unique canal system. The central nervous system, as revealed by immunohistochemistry, shows the general pattern of Gastrotricha having a bilobed brain and a pair of ventro-longitudinal nerve cords. However, in addition are found an anterior nerve ring, several anterior longitudinal nerves, and four ventral commissures (pharyngeal, trunk, pre-anal, and terminal). Two pairs of protonephridia are documented, while other Paucitubulatina have one. Moreover, the precise arrangement of multiciliated cells is unraveled, yielding a pattern of possibly systematic importance.
Several neural structures of Diuronotus resemble those found in Xenotrichula (Xenotrichulidae) and may constitute new apomorphies of Paucitubulatina, or even Gastrotricha. In order to test these new evolutionary hypotheses, comparable morphological data from other understudied gastrotrich branches and a better resolution of the basal nodes of the gastrotrich phylogeny are warranted. Nonetheless, the present study offers new insights into the evolution of organ systems and systematic importance of so-far neglected characters in Gastrotricha.
Diuronotus 是 Paucitubulatina 中最近描述的属之一，Paucitubulatina 是 Gastrotricha 的三个主要分支之一。其形态表明 Diuronotus 是 Paucitubulatina 的早期分支，因此是了解这个形态研究不足的群体进化的关键分类群。在这里，我们使用分子数据测试其系统发育位置，并通过免疫组织化学和共聚焦激光扫描显微镜对 Diuronotus aspetos 的肌肉、神经和纤毛系统进行详细描述。
我们证实了 D. aspetos 在 Muselliferidae 中的提出位置，并发现该科是 Xenotrichulidae 的姐妹群。通过 F-肌动蛋白染色显示的肌肉系统显示出简单但独特的躯干肌肉组织的组织，减少到三对纵向肌肉和增加多达五对背-腹肌肉，而在大多数 Paucitubulatina 中发现六对纵向和两对背-腹肌肉。使用乙酰化 α-微管蛋白免疫反应性，我们详细描述了咽，包括新的神经结构、两对感觉纤毛和独特的管系统。通过免疫组织化学显示的中枢神经系统显示出 Gastrotricha 的一般模式，具有双叶脑和一对腹-纵神经索。然而，还发现了一个前神经环、几对前纵神经和四个腹侧神经吻合（咽、躯干、前肛和末端）。记录了两对原肾管，而其他 Paucitubulatina 有一对。此外，揭示了多纤毛细胞的精确排列，产生了可能具有系统重要性的模式。
Diuronotus 的一些神经结构与 Xenotrichula（Xenotrichulidae）中的神经结构相似，可能构成 Paucitubulatina 甚至 Gastrotricha 的新的进化特征。为了测试这些新的进化假设，需要来自其他研究不足的 Gastrotricha 分支的可比形态数据和更好地解决 Gastrotricha 的系统发育的基础节点。尽管如此，本研究为器官系统的进化和迄今被忽视的 Gastrotricha 特征的系统重要性提供了新的见解。
