Active mode of excretion across digestive tissues predates the origin of excretory organs

Most bilaterian animals excrete toxic metabolites through specialized organs, such as nephridia and kidneys, which share morphological and functional correspondences. In contrast, excretion in non-nephrozoans is largely unknown, and therefore the reconstruction of ancestral excretory mechanisms is problematic. Here, we investigated the excretory mode of members of the Xenacoelomorpha, the sister group to Nephrozoa, and Cnidaria, the sister group to Bilateria. By combining gene expression, inhibitor experiments, and exposure to varying environmental ammonia conditions, we show that both Xenacoelomorpha and Cnidaria are able to excrete across digestive-associated tissues. However, although the cnidarian Nematostella vectensis seems to use diffusion as its main excretory mode, the two xenacoelomorphs use both active transport and diffusion mechanisms. Based on these results, we propose that digestive-associated tissues functioned as excretory sites before the evolution of specialized organs in nephrozoans. We conclude that the emergence of a compact, multiple-layered bilaterian body plan necessitated the evolution of active transport mechanisms, which were later recruited into the specialized excretory organs.绝大多数两侧对称动物（bilaterian animals）通过特化器官排泄有毒代谢物，例如肾管（nephridia）与肾脏（kidneys），这类器官在形态与功能上均存在对应性。与之相对，非肾动物类群（non-nephrozoans）的排泄机制大多尚不明晰，因此对其祖先排泄机制的重构存在难点。本研究针对作为肾动物类群（Nephrozoa）姊妹群的异涡动物门（Xenacoelomorpha）成员，以及作为两侧对称动物姊妹群的刺胞动物门（Cnidaria）的排泄模式展开了探究。本研究结合基因表达分析、抑制剂实验以及不同环境氨浓度暴露实验，证实异涡动物门与刺胞动物门均可通过与消化相关的组织完成排泄过程。不过，尽管刺胞动物中的艾氏美海葵（Nematostella vectensis）似乎以扩散作为主要排泄方式，两种异涡动物则同时采用主动运输与扩散两种机制。基于上述结果，本研究提出：在肾动物类群演化出特化排泄器官之前，与消化相关的组织便已承担了排泄位点的功能。本研究最终得出结论：紧凑的多层级两侧对称动物体构型的出现，促成了主动运输机制的演化，而该机制后续被整合至特化的排泄器官中。