Blair D, Davis G M, Wu B
School of Tropical Biology, James Cook University, Townsville, Qld, Australia.
Parasitology. 2001;123 Suppl:S229-43. doi: 10.1017/s003118200100837x.
Snails and digeneans have been associated for at least 200 million years. Their inter-relationships over such a time-span must have been complex and varied. Few studies have attempted to explore these relationships in the light of knowledge of the phylogeny of both host and parasite groups. Here we focus on two important families of digeneans, the Schistosomatidae and the Paragonimidae, for which molecular phylogenies are available. We investigate the types of evolutionary relationships between host and parasite, operating at different phylogenetic depths, that might explain current host specificity and distributions of both associates. Both families of parasites utilise a number of highly diverged gastropod families, indicating that host extensions have featured in their histories. However, schistosomatids and paragonimids show different patterns of association with their snail hosts. As befits the apparently more ancient group, schistosomatids utilise snails from across a wide phylogenetic range within the Gastropoda. The genus Schistosoma itself has experienced one long-range host switch between pulmonates and caenogastropods. By contrast, paragonimids are restricted to two superfamilies of caenogastropods. Despite these differences, modern schistosomatid species appear to be more host specific than are paragonimids and host additions, at the level of host family, are far less common among species of schistosomatids than among paragonimids. Some species of Paragonimus exhibit remarkably low levels of host specificity, with different populations utilising snails of different families. Existing knowledge relating to the phenomenon will be presented in the context of phylogenies of schistosomatids, paragonimids, and their snail hosts. Discussion focuses on the usefulness of current theories of snail-digenean coevolution for interpreting these findings. In the past, much emphasis has been placed on the idea that digeneans engage in a one-to-one arms race with their snail host. We consider that phylogenetic tracking rather than an arms-race relationship might be a common alternative. Not being bound by the restrictions imposed by an arms race, some digeneans might be able to extend to new host species more easily than the literature suggests. Switches into related host taxa are most likely. However, ecologically equivalent but unrelated gastropod hosts may also be exploited. Given the right ecological setting, digeneans are able to switch across considerable phylogenetic distances. Examples from the Paragonimidae and Schistosomatidae are given.
蜗牛和复殖吸虫的联系至少已有2亿年。在如此漫长的时间跨度里，它们之间的相互关系必定复杂多样。很少有研究尝试依据宿主和寄生虫类群的系统发育知识来探究这些关系。在此，我们聚焦于两个重要的复殖吸虫科，即裂体吸虫科和并殖吸虫科，它们都有分子系统发育研究可供参考。我们研究了在不同系统发育深度下宿主与寄生虫之间的进化关系类型，这些关系或许可以解释当前宿主特异性以及这两类共生生物的分布情况。这两个寄生虫科都利用了一些高度分化的腹足纲动物科，这表明宿主扩展在它们的演化历史中发挥了作用。然而，裂体吸虫和并殖吸虫与它们的蜗牛宿主呈现出不同的关联模式。作为明显更为古老的类群，裂体吸虫利用腹足纲内广泛系统发育范围内的蜗牛。裂体吸虫属自身在肺螺亚纲和新腹足目之间经历了一次远距离宿主转换。相比之下，并殖吸虫局限于新腹足目的两个超科。尽管存在这些差异，但现代裂体吸虫物种似乎比并殖吸虫更具宿主特异性，而且在宿主科的层面上，裂体吸虫物种中的宿主增加情况远比并殖吸虫中少见。一些并殖吸虫物种表现出极低的宿主特异性水平，不同种群利用不同科的蜗牛。关于这一现象的现有知识将在裂体吸虫、并殖吸虫及其蜗牛宿主的系统发育背景下呈现。讨论聚焦于当前蜗牛 - 复殖吸虫协同进化理论在解释这些发现方面的实用性。过去，人们非常强调复殖吸虫与它们的蜗牛宿主进行一对一军备竞赛的观点。我们认为系统发育追踪而非军备竞赛关系可能是一种常见的替代情况。不受军备竞赛所带来限制的束缚，一些复殖吸虫可能比文献所表明的更容易扩展到新的宿主物种。最有可能转向相关的宿主类群。然而，生态上等效但不相关的腹足纲宿主也可能被利用。在合适的生态环境下，复殖吸虫能够跨越相当大的系统发育距离进行转换。文中给出了并殖吸虫科和裂体吸虫科的例子。
