Those are good questions. I’m mostly a traditional morphological taxonomist; so the differences were morphological some macroscopic, but mostly microscopic. Some are as simple as differences in spore size or stipe decoration. Others have to do with how the cap’s skin (pileipellis) separates from the inner surface of the volva, the form of the tissue supporting the basidia within a gill, etc. I follow my teacher, Dr. Bas, in deciding on species boundaries in Amanita. No one has been able to successfully produce a single-spore isolate of an Amanita. Hence, mating tests are not possible. (The same is true, at present, for most of the mycorrhizal fungi of which I know.) Bas held that differences in three unrelated characters were sufficient to segregate two amanitas at the rank of species. This has turned out to be a good conservative rule. So far it has not been contradicted by molecular work. Dr. Geml’s recent experience with the molecular phylogeny of the muscarioid group shows that three characters was sufficiently conservative for me to separate amerimuscaria and muscaria morphologically and not to run into conflict with molecular results. Cap color as a reason for making a variety (or form) within a species apparently doesn’t go unchallenged by DNA among the same group of organisms. Bas used his experience with hundreds of species to come up with the “three unrelated differences” hypothesis. It has worked for me for 30 years.

I think that the way this has worked (forcing more and more development of microscopic techniques and strategies for searching for differences) can be illustrated by a little key regarding some relatives of A. fulva and A. crocea
here.

Picky, picky, picky. The level of detail is one of the reasons that it takes so long to work out whether a collection represents something old or something new to science.

I don’t know why I can still see through a microscope!

Very best,

Rod

Are the two American A. Crocea lookalikes morphologically distinct from the European species, or is some other character used to differentiate them?

Were mating studies done? While a successful mating attempt demonstrates conspecificity under a biological species concept, it is my understanding that attempts to mate two cultures can fail for a number of reasons. The two cultures could be incompatible strains of the same species, incompatible mating types of the same species, or the necessary environmental factors to encourage anastomosis might be missing from the experimental set up.

Are they associated with different kinds of trees (ecological species concept)?

Phylogenetic analysis of genes (molecular morphological characters)?

If the split is due to macro, micro, or molecular morphological differences, and given that no two specimens of any organism are exactly similar, then how do you determine the level of variation that warrants a new species epithet rather than merely a new variety or sub-species designation?

BTW, are mushrooms in the two American taxa edible?

It looks a bit like Amanita crocea, but I doubt that that is what it is. There are at least two separate crocea look-alikes in North America. One seems to range from WV down to central Mexico (in the neo-volcanic zone) and the other is known from the northern plains at least into the northern part of the Rocky Mtns. (in US). I’m planning to call one of them “americrocea” (this is the one that think is most similar to the European species) and the other “pseudocrocea.” If these names do not reach print within a year or so, I will be documenting them on the Amanita Studies website as provisional names.

Rod