Myxozoan affinities and route to endoparasitism

Abstract

There is now strong evidence that myxozoans have evolved from free-living cnidarians but until recently their higher level relationships have been the subject of considerable controversy. This chapter reviews the morphological and molecular evidence that has contributed to problems in placement and how further collective support has finally resolved their cnidarian affinity. We then consider the inherently difficult but fascinating topic of how myxozoans may have evolved as endoparasitic cnidarians. We first explore how a close association of free-living precursors could have led to the evolution of myxozoans with simple life cycles and the nature of the first myxozoan hosts. We propose that either freshwater bryozoans or fish (or their precursors) were ancestral hosts (in view of the more derived nature of myxozoans that infect annelids and the fact that fish are hosts for most members of all major myxozoan clades) and suggest that the morphological complexity of myxozoans in freshwater bryozoans renders a scenario of fish as first hosts less likely. We then discuss how new hosts may have been adopted subsequently, resulting in the complex life cycles involving invertebrate and vertebrate hosts that now characterise all myxozoans. Cnidarian traits, including life cycle plasticity and a capacity to evolve novel propagative stages, ultimately support many different scenarios regarding the route to endoparasitism.