Centrioles and kinetosomes: Form, function, and evolution

we review the literature on centrioles, kinetosomes, and other microtubule organizing centers (MTOCs) in animal, plant, and protist cells in the conte xt of the henneguy-Lenhossek theory of 1899 This 100-year-old cytological t heory, valid today, defines centrioles and kinetosomes as identical, homolo gous but developmentally distinguishable structures. Centrioles (paired con stituents of mitotic centrosomes in animal cells) become kinetosomes (cilia ry basal bodies) when their 9(2) + 2 microtubular axonemes grow outward. Du ring mitosis in Chlamydomonas, the kinetosomes are segregated at the poles of the mitotic spindle. Mitotic centrioles function as organelles of motili ty in many protists, though nowhere is this centriole-kinetosome relation m ore clearly seen than in the karyomastigont structure (kinetosome-nucleus-G olgi complex organellar system) of the trichomonads and other amitochondria te parabasalids. Constituent sequences of mitotic spindle-centriole-kinetos ome proteins (gamma -tubulin, pericentrin, and the cyclin-dependent kinases Cdc2 and Cdc3 members of the centrin family) are conserved across taxa, oc curring in animal and protist centrioles, plant MTOCs, and fungal spindle p ole bodies. We review ultrastructural and molecular data on these and other important MTOC proteins, and present a model whereby the cytological arran gement of centrioles (i.e., orthogonal pairs as in centrosomes) may have or iginated. We compare and contrast endogenous and exogenous (bacterial symbi ont integration) models for the evolution of centriole-kinetosomes (c-ks), with illustrative examples from Kingdom Protoctista.