Evolution of complex life cycles in helminth parasites

The fundamental question of how complex life cycles—where there is typically more than one host—evolve in host–parasite systems remains largely unexplored. We suggest that complex cycles in helminths without penetrative infective stages evolve by two essentially different processes, depending on where in the cycle a new host is inserted. In ‘upward incorporation’, a new definitive host, typically higher up a food web and which preys on the original definitive host, is added. Advantages to the parasite are avoidance of mortality due to the predator, greater body size at maturity and higher fecundity. The original host typically becomes an intermediate host, in which reproduction is suppressed. In ‘downward incorporation’, a new intermediate host is added at a lower trophic level; this reduces mortality and facilitates transmission to the original definitive host. These two processes should also apply in helminths with penetrative infective stages, although the mathematical conditions differ.