A Tale of Two Nuclei

Mushrooms may look mundane, but they've got a lot going on underneath the surface.  In animals, each cell in a body contains one nucleus, and each nucleus has 2 copies of the genome — one from the mother, one from the father — which fused at fertilization. Unlike in animals, where the nuclei of the egg and sperm quickly join after the cells combine, the nuclei in mushroom cells stay separate. The reason for the difference boils down to the particular way fungi have sex.

Frisky fungi creep through the soil with long filaments. These moldy structures occupy the spaces between dirt, and allow the organisms to digest organic matter. They're also great for mating. Fungi spend much of their lives with only a single nucleus. Except, that is, when two filaments cross paths.

When two lonely filaments find each other, the cells at the tip of the filaments fuse, and form new structures that have two nuclei per cell. This cell with two nuclei takes on a life of its own and divides many times to form a mushroom. Each mushroom cell contains a copy of each parent nucleus. The nuclei only fuse in the mushroom gills (pictured lower right), just prior to the formation of mushrooms spores, which are then carried away by the breeze, off to seed the next generation of fungi.