The origins of parasitism in the platyhelminthes: a summary interpreted on the basis of recent literature.

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.