Fungal symbiont transmitted by free-living mice promotes type 2 immunity.

The gut mycobiota is crucial for intestinal homeostasis and immune function. Yet its variability and inconsistent fungal colonization of laboratory mice hinders the study of the evolutionary and immune processes that underpin commensalism. Here, we show that Kazachstania pintolopesii is a fungal commensal in wild urban and rural mice, with an exceptional ability to colonize the mouse gastrointestinal tract and dominate the gut mycobiome.