Population Biology

Toxoplasma gondii has a very unusual population structure, consisting of three prominent clonal lineages.  These clonal types are geographically widespread and infect a range of hosts including humans.  Analysis of mutation rates between these strains, relative to ancient, exotic lineages or related species, indicates a common origin of ~10,000 years.  This recent emergence and widespread success is likely due to a change in the life cycle that resulted in more efficient oral transmission (by carnivorous or omnivorous feeding).  The coincident timing of this event with domestication of companion and agricultural animals is intriguing (see figure at left).  We are examining natural populations of Toxoplasma from more widespread geographic regions  to further define this unique population 
structure.  We are also examining clinical isolates to establish if there are correlates between the parasite genotype and severity of  infection. 

Differentiation and Oral Transmission

Toxoplasma has a complex life cycle that involves transmission between its single definitive host (cat) and various intermediate hosts (rodent and many others).  Efficient transmission is accomplished by the differentiation of actively replicating acute stages (known as tachyozites) into more slowly growing forms called bradyzoites that reside within tissue cysts (figure to right).  Tissue cysts of Toxoplasma can be transmitted orally to virtually all warm-blooded hosts.  This trait of “direct oral infectivity” is recently evolved and is likely responsible for the widespread success of a limited number of clonal lineages.  It is also the reason to cook meat thoroughly to avoid infection by tissue cysts that are commonly found in domestic and wild animals.  We are using genetics, developmental models, and animal studies to define the traits of the
tissue cysts that mediate this direct oral infectivity. 

Genetic Mapping

We have recently established a genetic linkage map for Toxoplasma using single nucleotide polymorphisms (SNPs) that were analyzed among the progeny of several genetic crosses.  Toxoplasma has 14 chromosomes that total 65 mb in size, with an average map unit of ~ 100 kb.  The genetic map has been used to assemble scaffolds from the whole genome sequence as shown in the example at the right.  We are using linkage mapping to define genes involved in acute virulence and oral transmission.

Linkage and physical maps of Chromosome VI.

Motility and Cell Invasion

Toxoplasma is a model organism for studying the
unique gliding motility of Apicomplexan parasites (see the video archive for examples).  Gliding propels them across solid surfaces, facilitates migration through tissues, and propels active penetration of mammalian cells.  Actin filaments are both essential for this process and rate limiting for parasite motility and invasion.  Actin serves a substrate for the small myosin TgMyoA, anchored in the inner membrane complex (see expanded view of the cytoskeleton).  Forward motion is driven by the reward translocation of adhesins in a conveyor belt fashion from front (right in the image) to back (left).  We are working on several aspects of this motor complex including the control of actin filament
polymerization and the role of transmembrane
adhesins in connecting the motor with the external
environment.

Multifunctional Adhesins

Traction is a necessary component of gliding motility that is accomplished by transmembrane proteins, known as adhesins.  We study the parasite adhesins MIC2/M2AP that form a complex on the parasite cell surface.  MIC2 contains an integrin A domain (modeled here on mammalian integrin alpha1) and a series of thrombospondin type I repeats that contain an unusual tryptophan-arginine groove (modeled on human TSR-1, 2 domains).  These domains are thought to participate in substrate and host cell recognition.  The cytoplasmic domain links to the cytoskeleton by bridging through aldolase.  Point mutational analyses and forward genetic approaches are being used to explore this model.