Table_1_Metatranscriptomic holobiont analysis of carbohydrate-active enzymes in the millipede Telodeinopus aoutii (Diplopoda, Spirostreptida).XLSX

As important decomposers of soil organic matter, millipedes contribute to lignocellulose decomposition and nutrient cycling. The degradation of lignocellulose requires the action of several carbohydrate-active enzymes (CAZymes) and, in most invertebrates, depends on the activity of mutualistic gut microorganisms. To address the question of the importance of the microbiota and endogenous (host) enzymes in digestive processes in millipedes, we analyzed metatranscriptomic data from the tropical millipede Telodeinopus aoutii at the holobiont level. Functional annotation included identification of expressed CAZymes (CAZy families and EC terms) in the host and its intestinal microbiota, foregut, midgut, and hindgut, compared to non-intestinal tissues. Most of the 175 CAZy families were expressed exclusively in the gut microbiota and more than 50% of these microbial families were expressed exclusively in the hindgut. The greatest diversity of expressed endogenous CAZymes from all gut sections was found in the midgut (77 families). Bacteria were the major microbial producers of CAZymes, Proteobacteria dominating in the midgut and Bacteriodetes with Firmicutes in the hindgut. The contribution of the eukaryotic microbiota to CAZymes production was negligible. Functional classification of expressed CAZy families confirmed a broad functional spectrum of CAZymes potentially expressed in the holobiont. Degradation of lignocellulose in the digestive tract of the millipede T. aoutii depends largely on bacterial enzymes expressed in the hindgut. Endogenous cellulases were not detected, except for the potentially cellulolytic family AA15, but an expression of cellulolytic enzymes of this family was not confirmed at the EC-number level. The midgut had the greatest diversity of expressed endogenous CAZymes, mainly amylases, indicating the importance of digesting α-glucosidases for the millipede. In contrast, bacterial lignocellulolytic enzymes are sparsely expressed here. The hindgut was the hotspot of microbial degradation of cellulose and hemicellulases. The gain of the millipede from the microbial lignocellulose degradation in the gut, and consequently the mutualistic status of the relationship between the millipede and its cellulolytic gut bacteria, depends on the ability of the millipede to take up microbial metabolites as nutrients through the hindgut wall. Enzymes expressed in the intestine can degrade all components of lignocellulose except lignin. Assuming that soil microbiota is partially degraded lignin in the millipede diet, T. aoutii can be considered a decomposer of soil organic matter relying primarily on its gut bacteria. The deposition of millipede fecal pellets containing an organic matter modified by the hindgut bacterial community could be of ecological significance.

作为土壤有机质的重要分解者，马陆（millipedes）可参与木质纤维素分解与养分循环过程。木质纤维素的降解需多种碳水化合物活性酶（carbohydrate-active enzymes, CAZymes）参与，且在多数无脊椎动物体内，该过程依赖共生肠道微生物的代谢活性。为解答马陆消化道内微生物群与内源（宿主）酶在消化过程中的重要性这一科学问题，本研究从全共生体（holobiont）层面，对热带马陆*Telodeinopus aoutii*的宏转录组数据（metatranscriptomic data）进行了分析。功能注释涵盖了宿主及其肠道微生物群、前肠、中肠与后肠中表达的CAZymes（包括CAZy家族与酶委员会（EC）编号术语）的鉴定，并以非肠道组织作为对照。在175个CAZy家族中，绝大多数仅在肠道微生物群中表达，其中超过50%的微生物源CAZy家族仅在后肠中被检测到。各肠道区段中，内源表达CAZymes的多样性以中肠最高，共检测到77个家族。细菌是CAZymes的主要微生物来源：中肠以变形菌门（Proteobacteria）为主，后肠则以拟杆菌门（Bacteriodetes）与厚壁菌门（Firmicutes）占优。真核微生物群对CAZymes生成的贡献可忽略不计。对表达的CAZy家族进行功能分类后证实，该全共生体中潜在表达的CAZymes具备广泛的功能谱。热带马陆*T. aoutii*消化道内的木质纤维素降解，在很大程度上依赖于后肠中表达的细菌源酶类。未检测到内源纤维素酶，仅存在潜在具备纤维素水解活性的AA15家族，但该家族的纤维素水解酶表达并未在EC编号层面得到证实。中肠的内源表达CAZymes多样性最高，且以淀粉酶为主，这表明α-葡萄糖苷酶类的消化对于马陆而言至关重要。与之相反，细菌源木质纤维素酶在此处的表达量较低。后肠则是微生物介导的纤维素与半纤维素酶解的热点区域。马陆从肠道内微生物介导的木质纤维素降解中获取的营养收益，以及其与肠道纤维素分解细菌之间的共生关系，均取决于马陆通过后肠壁吸收微生物代谢产物作为养分的能力。肠道内表达的酶类可降解木质纤维素的所有组分，唯独无法分解木质素。假设土壤微生物群可部分降解马陆日粮中的木质素，那么*T. aoutii*可被视为主要依赖肠道细菌的土壤有机质分解者。马陆粪便颗粒中含有经后肠细菌群落修饰的有机质，其沉积过程可能具备重要的生态学意义。