OKAMURA, B.; TOPS, S.; University of Reading: The evolution of life cycle complexity in myxozoans

Myxozoans are extremely morphologically degenerate bilateral animals that are endoparasitic in aquatic invertebrates and fish and present intriguing questions regarding the evolution of life cycle complexity in parasites. The life cycle of myxozoans in the Class Myxosporea entails obligate cycling between worm and fish hosts. The newly-described Malacosporea are parasites of freshwater bryozoans and also occur in fish, but whether fish are true hosts is unresolved. The timing of sexuality implies that invertebrates in both classes are definitive hosts. The recent discovery that the worm-like Buddenbrockia is a malacosporean provides evidence for retention of ancestral features in the Class Malacosporea (with the Myxosporea being a more derived group) or, less likely, an evolutionary atavism. Ultrastructural evidence suggests that the life cycle of Buddenbrockia may entail a facultative polymorphism with primitive, active, worm-like stages developing in certain bryozoan hosts and derived, passive sac-like stages in others via some form of paedomorphosis. The acquisition of secondary hosts in the Myxosporea may have arisen through increased transmission via asexual proliferation in new hosts, increased host longevity and the key innovation of producing resistant, long-lived infective spores. The apparent lack of development of mature malacosporean stages in fish raises the intriguing question of whether fish are in the process of being gained or lost as hosts for the Malacosporea. Consideration of the distribution of apparently primitive and derived traits in the two myxozoan clades provides insights into the evolution of myxozoan life cycles.