Evolution of the microtubular cytoskeleton (flagellar apparatus) in parasitic protists

The microtubular cytoskeleton of most single-celled eukaryotes radiates from an organizing center called the flagellar apparatus, which is essential for locomotion, feeding and reproduction. The structure of the flagellar apparatus tends to be conserved within diverse clades of eukaryotes, and modifications of this overall structure distinguish different clades from each other. Understanding the unity and diversity of the flagellar apparatus provides important insights into the evolutionary history of the eukaryotic cell. Diversification of the flagellar apparatus is particularly apparent during the multiple independent transitions to parasitic lifestyles from free-living ancestors. However, our understanding of these evolutionary transitions is hampered by the lack of detailed comparisons of the microtubular root systems in different lineages of parasitic microbial eukaryotes and those of their closest free-living relatives. Here we help to establish this comparative context by examining the unity and diversity of the flagellar apparatus in six major clades containing both free-living lineages and endobiotic (parasitic and symbiotic) microbial eukaryotes: stramenopiles (e.g., Phytophthora), fornicates (e.g., Giardia), parabasalids (e.g., Trichomonas), preaxostylids (e.g., Monocercomonoides), kinetoplastids (e.g., Trypanosoma), and apicomplexans (e.g., Plasmodium). These comparisons enabled us to address some broader patterns associated with the evolution of parasitism, including a general trend toward a more streamlined flagellar apparatus.