Fungal sexual reproduction has more than two mating types, as well, which is apparently made possible by handling the mitochondria differently from animals. In

Fungal sexual reproduction has more than two mating types, as well, which is apparently made possible by handling the mitochondria differently from animals. In animals, the two parent cells fuse and the male's mitochondria is destroyed. In fungi, the nuclear material is transferred between cells, and incompatible mitochondria never occupy the same cell. So a species could have one type or thousands of mating types, and may be homothallic or heterothallic. Meaning that individuals of the same type could be compatible, or not, respectively. In a dimorphic animal species, if you wander the wild and encounter a random member of your species, it is about 50% likely you could reproduce. For some species of fungus, a random encounter could be as high as 99.999% likely to be compatible. We don't even need to imagine aliens for this. We have species right here on Earth that are incredibly different from us, and also available to be studied.