Adaptive radiation and evolution within the myxozoa

Abstract

Myxozoans are endoparasites characterized by extensive morphological simplification and complex life cycles. Their definitive hosts are bryozoans—in the case of the more primitive and species-poor Malacosporea, or annelids—in the case of the more derived and speciose Myxosporea. This chapter reviews patterns of adaptive radiation within the Myxozoa and explores the drivers that may have promoted evolutionary change. Topics covered include: multiple transitions between worm-like and sac-like forms in the Malacosporea; undersampling that likely limits our appreciation of malacosporean diversity; and multiple shifts between marine and freshwater environments in the Myxosporea. We also examine morphological simplification that is observed in stages that produce spores and associated changes in the size of these sporogonic stages. This contrasts with the evolution of morphological complexity in spores. Features proposed to be involved in diversification include the acquisition of hardened spores and plasmodia, a high diversity of definitive hosts and invasion of novel hosts and host environments in the Myxosporea. The evolution towards higher complexity in spores can, in some cases, be linked with increasing transmission success. Convergence in spore form suggests that certain morphologies are highly adaptive. The significance of many features of spores, however, remains poorly known.