昆虫具有基于几丁质的肠道围食膜，不产几丁质的哺乳动物则具有基于黏蛋白的肠道黏液层。一般认为，无脊椎动物肠道菌群多样性不高且受环境菌群影响很大，而哺乳动物得益于黏液层和黏膜适应性免疫，形成了更为复杂和稳定的肠道菌群。但最新的研究推翻了脊索动物不能合成几丁质的传统观点，发现文昌鱼和鱼类具有几丁质合成酶，并在肠道合成几丁质。这些几丁质可能作为空间区隔、蛋白支架、免疫屏障和共生菌营养源，因此对维护肠黏膜与菌群稳态可能非常重要。然而，作为新生领域，相关机制研究非常欠缺。为此，我们重建了脊索动物几丁质合成酶的演化历程，鉴定并分析了几丁质及一些相关蛋白的表达模式，获得了一个文昌鱼和四个斑马鱼几丁质合成酶的敲除品系。在此基础上，我们希望通过文昌鱼和斑马鱼的对比研究，阐明脊椎动物肠道几丁质的功能，揭示围食膜如何过渡到黏液层，揭示脊椎动物肠道本地菌群的起源及其与肠黏膜免疫的共演化历程，并探索相关应用前景。

The gut of many protostomes such as insects is protected by the gut peritrophic membrane, a structure formed by chitin nanofibers. Mammals cannot produce chitin, but they protect the gut epithelia by using the mucus layers, which is based on gel-forming mucins. It is believed that the gut microbiota of invertebrates is less diverse and highly affected by environmental microbiota; whereas attributed to the mucus layers and the mucosal adaptive immune systems, mammals can form much more complex and stable gut indigenous microbiota. There has been a longstanding belief that chordates do not produce endogenous chitin. But this notion has been revised by recent studies, which reveal that lower chordates, including cephalochordate amphioxus and vertebrate ray-finned fishes, have chitin synthases (CS) and use them to produce chitin in the gut mucosa. Endogenous chitin in the gut of vertebrates could probably contribute to barrier immunity, food for the indigenous microbes, gut lumen matrix and compartmentalization. Therefore, vertebrate endogenous chitin might play an important but delicate role in the regulation of the homeostasis of the gut mucosa and microbiota. However, as a new research field, there is a serious lack of mechanistic studies. So far, we have reconstructed the molecular evolution of the chordate CS gene family, identified several chitin-related proteins, analyzed the in vivo expression pattern of endogenous chitin and some related proteins, and obtained (or about to obtained) gene knock-out models for one amphioxus CS gene and all four zebrafish CS genes. By further comparative analysis using amphioxus and zebrafish, we hope to reveal the function of endogenous chitin in the gut of vertebrates. This study could provide insights into the evolutionary transition from peritrophic membrane to mucus layers, the origins of vertebrate gut microbiota and its coevolution with the gut mucosal immune systems, and the potential applications.