Rohde K
Department of Zoology, University of New England, Armidale, N.S.W. Australia.
Int J Parasitol. 1997 Jun;27(6):739-46. doi: 10.1016/s0020-7519(97)00014-3.
A summary is given of the 4 contributions on the origins of parasitism in the Platyhelminthes. Recent ecological literature is used to interpret some of the findings. In particular, recent findings on rugged fitness landscapes, the increasing difficulty of long-jump adaptations, complexity catastrophes, and empty phenotypic space are discussed in order to find an explanation for the large number of Neodermata and the scarcity of parasitic turbellarians; it is concluded that evolutionary stasis of symbiotic turbellarians that are "trapped" in their particular niches is responsible for the small number of symbiotic species of turbellarians, rather than competitive exclusion by the numerous neodermatans. The earliest neodermatans were probably already dependent on the production of many offspring; hence the protoneodermatan was probably a species preadapted to parasitism by high fecundity. Monogenea produce few offspring in spite of their rich and secure food supply; the protomonogenean was either a species with a complex behaviour pattern for habitat selection or, alternatively, a species with high fecundity subsequently reduced to permit the evolution of more complex behaviour patterns (switch from r- to K-strategy). The finding that embryonic replacement of the epidermis is shared by several turbellarian groups, a developmental pattern possibly used and modified in the formation of the neodermis in the Neodermata, can be interpreted as supporting the views that Neodermata and turbellarians with epidermal replacement are all monophyletic, or alternatively, that a similar developmental pattern has arisen several times due to similar selection pressures. However, the possibility of horizontal character transfer should also be considered for explaining similar characters, including similar developmental patterns, in apparently not closely related groups. Horizontal gene transfer and principles for demonstrating horizontal character transfer are discussed.
本文总结了关于扁形动物门寄生起源的4篇论文。运用近期的生态学文献来解读其中一些研究结果。特别讨论了近期关于崎岖适应度景观、跳远式适应难度增加、复杂性灾难和空表型空间的研究结果，以寻找对大量新皮动物和寄生涡虫稀缺现象的解释；得出的结论是，被困在特定生态位的共生涡虫的进化停滞是导致涡虫共生物种数量少的原因，而非众多新皮动物的竞争排斥。最早的新皮动物可能已经依赖于大量后代的产生；因此，原新皮动物可能是一个通过高繁殖力预先适应寄生生活的物种。单殖吸虫尽管食物供应丰富且稳定，但后代数量很少；原单殖吸虫要么是一个具有复杂栖息地选择行为模式的物种，要么是一个随后繁殖力降低以允许更复杂行为模式进化的高繁殖力物种（从r策略转变为K策略）。几个涡虫类群都存在表皮胚胎替代现象，这一发育模式可能在新皮动物新皮层的形成过程中被利用和改变，这一发现可以被解释为支持新皮动物和具有表皮替代的涡虫是单系的观点，或者，由于相似的选择压力，相似的发育模式已经多次出现。然而，在解释明显不密切相关的类群中相似特征（包括相似发育模式）时，也应考虑水平性状转移的可能性。讨论了水平基因转移和证明水平性状转移的原则。
