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SummaryPlasmodium parasites are unicellular eukaryotes that undergo a series of remarkable morphological transformations during the course of a multistage life cycle spanning two hosts (mosquito and human). Relatively little is known about the dynamics of cellular organelles throughout the course of these transformations. Here we describe the morphology of three organelles (endoplasmic reticulum, apicoplast and mitochondrion) through the human blood stages of the parasite life cycle using fluorescent reporter proteins fused to organelle targeting sequences. The endoplasmic reticulum begins as a simple crescentshaped organelle that develops into a perinuclear ring with two small protrusions, followed by transformation into an extensive reticulated network as the parasite enlarges. Similarly, the apicoplast and the mitochondrion grow from single, small, discrete organelles into highly branched structures in laterstage parasites. These branched structures undergo an ordered fission -apicoplast followed by mitochondrion -to create multiple daughter organelles that are apparently linked as pairs for packaging into daughter cells. This is the first in-depth examination of intracellular organelles in live parasites during the asexual life cycle of this important human pathogen.

Apicomplexan parasites harbor a secondary plastid that has lost the ability to photosynthesize yet is essential for the parasite to multiply and cause disease. Bioinformatic analyses predict that 5-10% of all proteins encoded in the parasite genome function within this organelle. However, the mechanisms and molecules that mediate import of such large numbers of cargo proteins across the four membranes surrounding the plastid remain elusive. In this work, we identify a highly diverged member of the Tic20 protein family in Apicomplexa. We demonstrate that Tic20 of Toxoplasma gondii is an integral protein of the innermost plastid membrane. We engineer a conditional null-mutant and show that TgTic20 is essential for parasite growth. To characterize this mutant functionally, we develop several independent biochemical import assays to reveal that loss of TgTic20 leads to severe impairment of apicoplast protein import followed by organelle loss and parasite death. TgTic20 is the first experimentally validated protein import factor identified in apicoplasts. Our studies provide experimental evidence for a common evolutionary origin of import mechanisms across the innermost membranes of primary and secondary plastids.Apicomplexa ͉ plastid ͉ chloroplast

Summary While the signals that control neutrophil migration from the blood to sites of infection have been well characterized, little is known about their migration patterns within lymph nodes, or the strategies that neutrophils use to find their local sites of action. To address these questions, we used two-photon scanning laser microscopy (TPSLM) to examine neutrophil migration in intact lymph nodes during infection with an intracellular parasite, Toxoplasma gondii. We find that neutrophils form both small, transient or large, persistent swarms via a strikingly coordinated migration pattern. We provide evidence that cooperative action of neutrophils and parasite egress from host cells can trigger swarm formation. Neutrophil swarm formation coincides in space and time with the removal of macrophages that line the subcapsular sinus of the lymph node. Our data provide insights into the cellular mechanisms underlying neutrophil swarming and suggest new roles for neutrophils in shaping immune responses.