When subjected to environmental stress, such as nitrogen starvation in the case of "Chlamydomonas", cells are induced to form gametes. Gamete formation in single-celled haploid organisms such as yeast is called sporulation, resulting in many cellular changes that increase resistance to stress. Gamete formation in multicellular fungi occurs in the gametangia, an organ specialized for such a process, usually by meiosis. When opposite mating types meet, they are induced to leave the vegetative cycle and enter the mating cycle. In yeast, there are two mating types, a and α. In fungi, there can be two, four, or even up to 10,000 mating types, depending on the species. Mate recognition in the simplest eukaryotes is achieved through pheromone signaling, which induces shmoo formation (a projection of the cell) and begins the process of microtubule organization and migration. Pheromones used in mating type recognition are often peptides, but sometimes trisporic acid or other molecules, recognized by cellular receptors on the opposite cell. Notably, pheromone signaling is absent in higher fungi such as mushrooms.