Evolution and biological radiation of trematodes: A synopsis of ideas and opinions

The chapter is devoted to the evolution and biological radiation of digeneans (class Trematoda, subclass Digenea), a central taxon of parasitic flatworms (Platyhelminthes). A system of adaptations at various stages of the digenean life cycle is analysed and the crucial adaptations promoting the circulation of these parasites in various terrestrial, marine and freshwater ecosystems are identified. It is shown that the key to the success of this flourishing group of parasites was the acquisition of the ability of parthenogenetic reproduction of sporocysts and rediae, the life cycle phases parasitizing the molluscan host, and the emergence of the cercaria, free-living larva of the hermaphroditic generation, in the life cycle. The chapter also presents a comparative overview of theories concerning the early stages of digenean evolution and the formation of their life cycles: the sequence of host colonisation, the formation of the primary twohost life cycle, the emergence of the cercaria and the involvement of the second intermediate host into the life cycle. The role of different categories of hosts in the evolution of digeneans and in their geographic expansion and host radiation is considered in detail. Special attention is given to the coadaptation processes in the systems “mollusc–parthenitae”, the formation of the genetic variation of parthenitae and cercariae as well as the assessment of their role in the microevolution and speciation of the digeneans. It is emphasised that the macroevolution of these parasites is closely associated with the phenomenon of host switching. This process was the most intensive during ecosystem transformations, especially in the course of global biosphere changes such as periodic glaciations of the late Pliocene–Pleistocene. In the light of this, an attempt is made to forecast the influence of the present-day climatic changes on the geographic and host expansion of the digeneans and the potential evolutionary consequences of these events.