Le Mai-Lee Van, Park Seowon, Stach Thomas
Institute of Biology, Comparative Electron Microscopy, Humboldt University Berlin, Berlin, Germany.
J Morphol. 2025 Jul;286(7):e70061. doi: 10.1002/jmor.70061.
Appendicularia comprises about 70 holoplanktonic species traditionally classified in three families: Oikopleuridae, Fritillariidae, Kowalevskiidae. Despite their eminent phylogenetic position and their important role in ocean ecosystems, most research focuses on the model organism Oikopleura dioica while the diversity of appendicularians remains underexplored. Here, we present a comparative morphological analysis of appendicularian endostyles, a pharyngeal gland homologous to the vertebrate thyroid. Based on light- and transmission electron-microscopical investigations in 12 species representing seven (of 15) genera from all three family-level taxa, we describe the 3D-anatomy of endostyles, histologically recognizable cell-types, and discuss our findings in a cladistic framework. We identified seven different cell types arranged in species specific patterns, including the formerly unrecognized 'bright cells'. Two ciliary bands - the peripharyngeal band and the retropharyngeal band - are associated with the endostyles. Outgroup comparison indicates that repeated apomorphic reductions of cell types, rows of cells, the retropharyngeal band, and the complete endostyle occurred within Appendicularia. We propose a phylogenetic hypothesis that suggests that "Oikopleuridae" is a paraphyletic grouping and supports an evolutionary scenario with multiple reductions functionally related to the evolution of the external filter house of appendicularians. While we document the diversity of endostyle anatomy, more detailed cladistic analysis, including other organ systems, is needed to resolve the phylogenetic relationships and to understand the evolution of appendicularian taxa.
尾海鞘纲包括约70种全浮游生物种，传统上分为三个科：住囊虫科、贝氏住囊虫科、科瓦列夫斯基住囊虫科。尽管它们在系统发育中具有显著地位，且在海洋生态系统中发挥着重要作用，但大多数研究集中在模式生物住囊虫上，而尾海鞘纲的多样性仍未得到充分探索。在此，我们对尾海鞘纲内柱进行了比较形态学分析，内柱是一种与脊椎动物甲状腺同源的咽腺。基于对代表所有三个科级分类单元中15个属中的7个属的12个物种进行的光学显微镜和透射电子显微镜研究，我们描述了内柱的三维解剖结构、组织学上可识别的细胞类型，并在系统发育框架中讨论了我们的发现。我们识别出七种以物种特异性模式排列的不同细胞类型，包括以前未被识别的“亮细胞”。两条纤毛带——咽周带和咽后带——与内柱相关。外类群比较表明，尾海鞘纲内发生了细胞类型、细胞排、咽后带和整个内柱的多次趋同简化。我们提出了一个系统发育假说，表明“住囊虫科”是一个并系类群，并支持一种进化情景，即多次简化在功能上与尾海鞘纲外部滤室的进化相关。虽然我们记录了内柱解剖结构的多样性，但需要更详细的系统发育分析，包括其他器官系统，以解决系统发育关系并了解尾海鞘纲分类单元的进化。
