Blaxter Mark L
Institute of Cell, Animal and Population Biology, University of Edinburgh, King's Buildings, Edinburgh, EH9 3JT, UK.
Adv Parasitol. 2003;54:101-95. doi: 10.1016/s0065-308x(03)54003-9.
Nematodes are remarkably successful, both as free-living organisms and as parasites. The diversity of parasitic lifestyles displayed by nematodes, and the diversity of hosts used, reflects both a propensity towards parasitism in the phylum, and an adaptability to new and challenging environments. Parasitism of plants and animals has evolved many times independently within the Nematoda. Analysis of these origins of parasitism using a molecular phylogeny highlights the diversity underlying the parasitic mode of life. Many vertebrate parasites have arthropod-associated sister taxa, and most invade their hosts as third stage larvae: these features co-occur across the tree and thus suggest that this may have been a shared route to parasitism. Analysis of nematode genes and genomes has been greatly facilitated by the Caenorhabditis elegans project. However, the availability of the whole genome sequence from this free-living rhabditid does not simply permit definition of 'parasitism' genes; each nematode genome is a mosaic of conserved features and evolutionary novelties. The rapid progress of parasitic nematode genome projects focussing on species from across the diversity of the phylum has defined sets of genes that have patterns of evolution that suggest their involvement with various facets of parasitism, in particular the problems of acquisition of nutrients in new hosts and the evasion of host immune defences. With the advent of functional genomics techniques in parasites, and in particular the possibility of gene knockout using RNA interference, the roles of many putative parasitism genes call now be tested.
线虫无论是作为自由生活的生物还是寄生虫都非常成功。线虫所表现出的寄生生活方式的多样性以及所利用宿主的多样性，既反映了该门生物对寄生生活的倾向，也反映了其对新的具有挑战性环境的适应能力。植物和动物的寄生现象在线虫纲中已多次独立进化。利用分子系统发育分析这些寄生起源，突出了寄生生活方式背后的多样性。许多脊椎动物寄生虫都有与节肢动物相关的姐妹分类群，并且大多数以第三期幼虫的形式侵入宿主：这些特征在整个进化树中同时出现，因此表明这可能是一条共同的寄生途径。秀丽隐杆线虫项目极大地促进了对线虫基因和基因组的分析。然而，这种自由生活的杆形线虫全基因组序列的可用性并不能简单地定义“寄生”基因；每个线虫基因组都是保守特征和进化新奇性的镶嵌体。专注于该门生物多样性中不同物种的寄生线虫基因组项目的快速进展，已经确定了一些基因集，这些基因的进化模式表明它们参与了寄生的各个方面，特别是在新宿主中获取营养以及逃避宿主免疫防御的问题。随着寄生虫功能基因组学技术的出现，特别是利用RNA干扰进行基因敲除的可能性，现在可以测试许多假定的寄生基因的作用。
