在显微镜下，研究人员观察到了戏剧性的一幕：一种叫Pristionchus的蛔虫会咬住另一种线虫，并咬出一个开口，然后吞噬流出的物质。

而这场"战争"的受害者秀丽隐杆线虫（Caenorhabditis elegans）并没有表现出抵御能力。尽管秀丽隐杆线虫可能是Pristionchus的一个近亲，但可惜的是，它并没有同样坚固的"牙齿"。生物学家关注的并不是这种蛔虫令人印象深刻的捕食技巧，而是它们口器的形成机制。

研究人员发现了这种寄生虫嘴巴发育背后的控制机制：如果该寄生虫在细菌充足的环境下生长，也就是说环境中营养素很丰富，那么它们只会形成非常小的牙齿和窄小的口腔。但如果幼虫处在一个缺乏食物且高群体强度的环境下，则会形成一个宽的嘴巴和坚固的牙齿状髓石。

形成宽口和窄口的线虫并没有表现出遗传差异，研究人员表示，环境因素支配着它们的口器形成。

如果幼虫在发育阶段食物短缺，那么几乎所有的寄生虫都会形成宽的，并武装有坚固牙齿的嘴巴。研究表明，该寄生虫的一种信息素同样会触发这种结果。总的来说，生长环境食物短缺和群体过剩，致使这种蛔虫的一个特殊信号通路被激活，最终导致坚固牙齿的形成，从而发生侵略性行为。研究人员表示他们已经识别了控制这种机制的信号通路。

Prisionchus坚固的嘴型结构是否使其更适宜于捕获其他寄生虫或消耗真菌吸取养分，目前科学家还只是推断，但这项研究表明了自然界生物进化的一个重要方式。

原文出处：

Nature  doi:10.1038/nature09164

Co-option of the hormone-signalling module dafachronic acid–DAF-12 in nematode evolution
Gilberto Bento1, Akira Ogawa1 & Ralf J. Sommer1

Department for Evolutionary Biology, Max-Planck-Institute for Developmental Biology, Spemannstrasse 37; D-72076 Tübingen, Germany

Morphological novelties are lineage-specific traits that serve new functions1, 2. Developmental polyphenisms have been proposed to be facilitators of phenotypic evolution, but little is known about the interplay between the associated genetic and environmental factors3, 4, 5, 6, 7, 8, 9, 10, 11. Here, we study two alternative morphologies in the mouth of the nematode Pristionchus pacificus and the formation of teeth-like structures that are associated with bacteriovorous feeding and predatory behaviour on fungi and other worms12, 13, 14, 15, 16. These teeth-like denticles represent an evolutionary novelty, which is restricted to some members of the nematode family Diplogastridae but is absent from Caenorhabditis elegans and related nematodes14. We show that the mouth dimorphism is a polyphenism that is controlled by starvation and the co-option of an endocrine switch mechanism. Mutations in the nuclear hormone receptor DAF-12 and application of its ligand, the sterol hormone dafachronic acid, strongly influence this switch mechanism. The dafachronic acid–DAF-12 module has been shown to control the formation of arrested dauer larvae in both C. elegans and P. pacificus, as well as related life-history decisions in distantly related nematodes17, 18, 19, 20. The comparison of dauer formation and mouth morphology switch reveals that different thresholds of dafachronic acid signalling provide specificity. This study shows how hormonal signalling acts by coupling environmental change and genetic regulation and identifies dafachronic acid as a key hormone in nematode evolution.