Plastid segregation and cell division in the apicomplexan parasite Sarcocystis neurona.

Apicomplexan parasites harbor a secondary plastid that is essential to their survival. Several metabolic pathways confined to this organelle have emerged as promising parasite-specific drug targets. The maintenance of the organelle and its genome is an equally valuable target. We have studied the replication and segregation of this important organelle using the parasite Sarcocystis neurona as a cell biological model. This model system makes it possible to differentiate and dissect organellar growth, fission and segregation over time, because of the parasite's peculiar mode of cell division. S. neurona undergoes five cycles of chromosomal replication without nuclear division, thus yielding a cell with a 32N nucleus. This nucleus undergoes a sixth replication cycle concurrent with nuclear division and cell budding to give rise to 64 haploid daughter cells. Interestingly, intranuclear spindles persist throughout the cell cycle, thereby providing a potential mechanism to organize chromosomes and organelles in an organism that undergoes dramatic changes in ploidy. The development of the plastid mirrors that of the nucleus, a continuous organelle, which grows throughout the parasite's development and shows association with all centrosomes. Pharmacological ablation of the parasite's multiple spindles demonstrates their essential role in the organization and faithful segregation of the plastid. By using several molecular markers we have timed organelle fission to the last replication cycle and tied it to daughter cell budding. Finally, plastids were labeled by fluorescent protein expression using a newly developed S. neurona transfection system. With these transgenic parasites we have tested our model in living cells employing laser bleaching experiments.