De Ley Paul
Department of Nematology, University of California-Riverside, Riverside, CA 92521, USA.
WormBook. 2006 Jan 25:1-8. doi: 10.1895/wormbook.1.41.1.
Contrary to textbook dogma, nematodes are not only highly diverse, but often also complex and biologically specialized metazoans. Just a few of the many fascinating adaptations are reviewed in this chapter, as a prelude to a quick tour through phylogenetic relationships within the phylum. Small Subunit rDNA sequences have confirmed several controversial prior hypotheses, as well as revealing some unexpected relationships, resulting in a recent proposal for revised classification. Three major lineages exist within the phylum: Chromadoria, Enoplia and Dorylaimia. The exact order of appearance of these lineages is not yet resolved, which also leaves room for uncertainty about the biology and morphology of the exclusive common ancestor of nematodes. Enoplia and Dorylaimia differ considerably in many respects from C. elegans, which is a member of Chromadoria. The latter group is extremely diverse in its own right, for example in ecological range, in properties of the cuticle and in structure of the pharynx. The formerly relatively widely accepted class Secernentea is deeply nested within Chromadoria, and has therefore recently been relegated to the rank and name of order Rhabditida. Within this order, closer relatives of C. elegans include strongylids, diplogasterids and bunonematids. Tylenchs, cephalobs and panagrolaimids are also members of Rhabditida, albeit probably more distantly related to C. elegans.
与教科书上的教条相反，线虫不仅种类极其多样，而且往往是复杂且具有生物学特异性的后生动物。本章将回顾众多迷人适应性中的一部分，作为快速浏览该门内系统发育关系的前奏。小亚基核糖体DNA序列证实了几个先前有争议的假说，同时也揭示了一些意想不到的关系，从而产生了最近关于修订分类的提议。该门内存在三个主要谱系：色矛纲、刺嘴纲和矛线纲。这些谱系出现的确切顺序尚未确定，这也为线虫唯一共同祖先的生物学和形态学留下了不确定性的空间。刺嘴纲和矛线纲在许多方面与秀丽隐杆线虫（色矛纲的一员）有很大不同。后一组本身就极其多样，例如在生态范围、角质层特性和咽部结构方面。以前相对广泛接受的尾感器纲深深地嵌套在色矛纲内，因此最近被降级为小杆目。在这个目内，秀丽隐杆线虫的近亲包括圆线科线虫、双胃线虫和布氏线虫。垫刃线虫、头叶线虫和小杆线虫也是小杆目的成员，尽管可能与秀丽隐杆线虫的关系更远。
