Cophylogeny: insights from fish-parasite systems.

Host-parasite cophylogeny is a topic that has grasped the attention of scientists since the end of the 19th century, but the development of dedicated analytical methods only arose in the last 30 years. Research on host-parasite systems and on the development of more and more sophisticated numerical methods to estimate the degree of cospeciation has thus progressed, permitting the elaboration of evolutionary scenarios. The main outcome of these studies is that the expected clear pattern of cospeciation between many hosts and parasites is often obscure. In practice, much attention has been devoted to few host-parasite systems. Particularly, aquatic host-parasite associations have not been so extensively studied, and, after briefly reviewing the main analytical methods, this paper focuses on host-monogenean systems, because this kind of interaction is expected to be an ideal model for cophylogeny studies. But is it? And what does it tell us about the evolutionary and ecological forces driving cospeciation in the open sea? Biogeography should also be considered when possible, and it has been useful for explaining some patterns of cospeciation. It should thus be more deeply exploited in the future. We need new methods and new biological models that better, if not fully, depict patterns and thereby permit deeper understanding of processes within cophylogenetic patterns.