Project Summary Toxoplasma gondii is the causative agent of toxoplasmosis, a leading cause of death due to foodborne illness that causes serious disease in immunocompromised individuals. The parasite develops asexually and sexually, and asexual parasite development is well-studied. However, the parasite completes its sexual stage only in the cat intestinal epithelium, presenting a significant ethical and logistical barrier to sexual stage research. To remove the need for companion animal research and study the unknown biology of sexual stage T. gondii, our lab developed murine and tissue culture models that support T. gondii sexual development. The polyunsaturated fatty acid linoleic acid is a critical factor for T. gondii to complete sexual development that is uniquely elevated in the feline intestine. The mechanisms by which linoleic acid acts on host or parasite remain unknown. The goal of this proposal is to determine the mechanism of linoleic acid in promoting T. gondii sexual development in cell culture and in vivo. Preliminary data from our group and others suggests that linoleic acid acts on both host and parasite to promote a permissive environment for T. gondii sexual development. I hypothesize that intracellular accumulation of linoleic acid activates T. gondii lipid signaling pathways to promote sexual development. I further hypothesize that sexual development relies on activation of similar signaling pathways in the host. In Aim 1, I will determine the mechanism of linoleic acid action on the parasite by measuring parasite transcriptional responses to linoleic acid treatment. I will also test our new mouse model of linoleic acid accumulation for its ability to promote efficient T. gondii sexual development in vivo. In Aim 2, I will use automated image analysis to determine the importance of host cell type in T. gondii sexual development. siRNA-mediated ablation of linoleic acid-responsive host factors will identify host genes that influence sexual development. Successful completion of these Aims will better define how linoleic acid enables growth of T. gondii sexual stages. The long-term implication of this work is a shareable model of T. gondii sexual development that will help reduce the burden of toxoplasmosis.