Fig. 1

Life cycle and microtubule-based structures of apicomplexa. a–e Simplified schematic of the life cycle of Apicomplexa in their different hosts. Apicomplexa replicate either sexually or asexually. Differentiation into gametes and sexual replication occur within definitive hosts. Definitive hosts vary among apicomplexan species; T. gondii replicates sexually within felines, while Plasmodium species do so in mosquitoes. Flagellated forms of Apicomplexa are only found in definitive hosts, where they differentiate into male (micro) and female (macro) gametes. Fusion of gametes gives rise to a zygote which further differentiates into oocysts able to sporulate. Microgametes of different Apicomplexa vary in their number of flagella. T. gondii microgametes, represented here, have two protruding flagella. Plasmodium spp. microgametes emerge with a single flagellum upon terminal differentiation, and are assembled entirely within the cytoplasm of the undifferentiated originating cell. a, b In intermediate hosts, such as humans, apicomplexans grow vegetatively. Distinct replication modes among Apicomplexa allow them to adapt to different host niches. However, they all generate new infective zoites by assembly of daughter cells within the mother cell´s cytosol or at the mother cell surface, and undergo closed mitosis of the nuclear content. c Infective forms of Apicomplexa organize microtubules using functionally and physically distinct MTOCs. subpellicular microtubules, which impart shape and polarity to the cells, are organized by an MTOC localized at the apex, known as the APR. In addition, coccidian species in the phylum contain a specialized tubulin-based structure known as the conoid which has been evolutionarily linked to basal bodies of related flagellated alveolates [51, 52, 56]. Nuclear division occurs by closed mitosis. Chromosomes are organized by an intra-nuclear spindle nucleated by a cytosolic centrosome. Apicomplexa centriole-based centrosomes contain two centrioles of 9+1 singlet microtubule structure, oriented parallel to each other. Malaria-causing parasites (Plasmodium spp.) do not have canonical centrosomes, and organize their mitotic spindle from a “centriolar plaque” which can be identified using anti-centrin antibodies. The centriolar plaque is embedded in the nuclear envelope (not shown). d, e Microgamete flagella and basal body structures. Apicomplexa flagellar axonemes are composed of 9 doublet microtubules and a central pair [15–18]. d Basal bodies in malaria are better characterized, and consist of nine single A-tubules with no central tube, embedded in an electron-dense mass [16]. e Basal body structures are not well characterized in T. gondii. A small number of ultra-structural studies have led researchers to propose multiple alternative microtubule arrangements; a nine singlet microtubules, and a central tubule [20], atypical 9+0 and 9+2 arrangements, or a typical triplet microtubule structure with ninefold symmetry [8, 20, 22–26]