RE: Mushrooms can have 4,000+ different sexes
I agree that it it definitely very difficult to grasp this fact ;) I also think the mating types function more like varieties than genuine sexes. One thing I completely forgot to include in the post is that the meting type only determines what other individuals the fungi can mate with, and does not affect other attributes (like we are used to in animals and even plants). So the mushroom will look exactly the same no matter which mating type it is.
Answering how many mating types present in your yard is impossible. With several thousand options in Basidiomycota, I don't think it is likely that any are the same mating type unless its a clone from one of the other fungi.
As I mentioned, the mating types is a mechanism to mainly prevent self-fertilization, so I don't think fungi really has a problem when it comes to finding available mates.
Now there's a math problem -- What densities and arrangements of spore distribution would require 40 sexes, 400, or 4000 to create a sufficient level of genetic mixing over the long run?
Does it mean that my yard is so full of fungus spores of all different kinds of fungi, that they have to be able to sort themselves out? I was amazed this fall to see a new mushroom in my yard, the Shaggy Parasol - in 4 widely-spaced locations across my 1.3 acres. Their spores must have been everywhere across the landscape at one time.