Pröts Philipp, Novotny-Diermayr Veronica, Ott Jörg A
Department of Functional and Evolutionary Ecology, University of Vienna, Vienna, Austria.
Experimental Drug Development Centre (EDDC), A*Star, Singapore, Singapore.
Org Divers Evol. 2024;24(3):353-373. doi: 10.1007/s13127-024-00643-0. Epub 2024 Jul 5.
Stilbonematinae are nematodes commonly found in shallow marine sands. They are overgrown by a genus- and species-specific coat of chemoautotrophic sulphur-oxidizing ectosymbiotic bacteria which profit from the vertical migration of their hosts through the chemocline by alternately gaining access to oxidizing and reducing chemical species, while in return, the host feeds on its symbionts. The subfamily exhibits a large morphological variability; e.g. the anterior pharynx is cylindrical in genera possessing a voluminous coat, but species with a bacterial monolayer possess a distinctly swollen corpus and therefore a tripartite pharynx. Since 18S-based phylogenetic analyses do not show close relationships between corpus-bearing species, we investigated the pharynx morphology using phalloidin staining in combination with confocal laser scanning microscopy, transmission electron microscopy and light microscopy in order to assess an independent evolution. The class-wide stable position of the subventral pharynx ampullae was used as a morphological marker. Ampullae are positioned at the anterior-most end of the isthmus in , further posterior in and and inside the corpus in . We therefore conclude an independent evolution of corpus enlargements within Stilbonematinae. This further suggests that pharynx morphology is driven by the volume of the symbiotic bacterial coat rather than phylogeny. Based on an existing mathematical model, an enlarged corpus should enable its bearer to ingest food in smaller quantities, in gourmet style, whereas a cylindrical pharynx would restrict its bearer to ancestral gourmand feeding. A review of pharynx types of Nematoda showed that the Stilbonematinae pharynx is substantially different compared to other tripartite pharynges. The lack of pharyngeal tubes and valves, the undivided corpus and evenly distributed nuclei in the isthmus warrant the definition of the "stilbonematoid" three-part pharynx.
细杆线虫亚科是常见于浅海砂质中的线虫。它们被一层属特异性和种特异性的化能自养硫氧化外共生细菌覆盖，这些细菌通过宿主在化学跃层中的垂直迁移，交替接触氧化态和还原态化学物质而获益，作为回报，宿主以其共生体为食。该亚科表现出很大的形态变异性；例如，在前咽呈圆柱形的属中，其体表有大量覆盖物，但具有细菌单层的物种则有明显肿胀的体部，因此有一个三段式咽。由于基于18S的系统发育分析未显示有体部的物种之间存在密切关系，我们使用鬼笔环肽染色结合共聚焦激光扫描显微镜、透射电子显微镜和光学显微镜来研究咽的形态，以评估其独立进化。腹侧下咽壶腹在全纲中的稳定位置被用作形态学标记。壶腹在[具体属1]中位于峡部的最前端，在[具体属2]和[具体属3]中更靠后，在[具体属4]中位于体部内部。因此，我们得出细杆线虫亚科内体部增大是独立进化的结论。这进一步表明，咽的形态是由共生细菌覆盖物的体积而非系统发育驱动的。基于现有的数学模型，增大的体部应使其拥有者能够以精致的方式少量摄取食物，而圆柱形咽则会限制其拥有者采用祖先式的大量进食方式。对线虫纲咽类型的综述表明，细杆线虫亚科的咽与其他三段式咽有很大不同。缺乏咽管和瓣膜、未分隔的体部以及峡部中均匀分布的细胞核，使得可以定义“细杆线虫样”三段式咽。
