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First published online 14 April 2008
doi: 10.1242/jcs.021089

Journal of Cell Science 121, 1559-1568 (2008)
Published by The Company of Biologists 2008
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Research Article

Organellar dynamics during the cell cycle of Toxoplasma gondii

Manami Nishi, Ke Hu*, John M. Murray and David S. Roos{ddagger}

Departments of Biology, and Cell and Developmental Biology, University of Pennsylvania, Philadelphia PA 19104, USA

{ddagger} Author for correspondence (e-mail: droos{at}sas.upenn.edu)

Accepted 18 February 2008

The protozoan phylum Apicomplexa encompasses ~5000 species of obligate intracellular parasites, including those responsible for malaria and toxoplasmosis. Rather than dividing by binary fission, apicomplexans use a remarkable mechanism for replication, assembling daughters de novo within the cytoplasm. Here, we exploit time-lapse microscopy of fluorescent markers targeted to various subcellular structures in Toxoplasma gondii tachyzoites to determine how these unicellular eukaryotes efficiently package a complete set of organelles, maintaining the highly polarized organization necessary for host cell invasion and pathogenesis. Golgi division and elongation of the apicoplast are among the first morphologically observable events, associated with an unusual pattern of centriolar migration. Daughter parasites are assembled on cytoskeletal scaffolding, whose growth proceeds from the apical end, first encapsulating the divided Golgi. Further extension of the cytoskeletal scaffold results in partitioning of the apicoplast, nucleus, endoplasmic reticulum, and finally the mitochondrion, which enters the developing daughters rapidly, but only very late during the division cycle. The specialized secretory organelles (micronemes and rhoptries) form de novo. This distinctive pattern of replication – in which organellar segregation spans ~75% of the cell cycle, completely encompassing S phase – suggests an unusual mechanism of cell cycle regulation.

Key words: Apicomplexan parasites, Organelle segregation, Apicoplast, Mitochondrion, Endoplasmic reticulum, Golgi, Centrioles, Micronemes, Rhoptries






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