Amanita thiersii and Amanita foetens are closely related but genetically and geographically distinct species, leaving the origins of A. thiersii and its range expansion enigmatic

Abstract

Background: The decomposer Amanita thiersii was originally described from a Texas lawn. Over time the species appears to have spread its range, but whether A. thiersii is an introduced and invading fungus or a native expanding its range remains an open question. A striking morphological similarity between A. thiersii and the Argentinian A. foetens led us to question whether the two species are the same. We hypothesized A. thiersii was simply an A. foetens introduced from Argentina. Methods: We first compared the original species descriptions of both taxa. Next, we used databases associated with iNaturalist and Mushroom Observer to plot the global ranges of A. thiersii and A. foetens, revealing new reports of A. thiersii in Mexico and an expanded range in the United States of America. Next, we sequenced three genomes: an A. thiersii specimen from the U.S.A., an isotype of A. foetens, and an Argentinian specimen tentatively identified as A. thiersii . We reconstructed phylogenies using our own and publicly available data of other Amanita species. Because the genetic diversity of A. thiersii in the U.S.A appears to be very low, we also searched for mating type loci. Results: Macroscopic descriptions suggest the two taxa are distinguishable by mushroom stature and the decoration of the stipe. The geographic ranges of the two taxa seem distinct and not overlapping, although the inconsistent names used by database users causes confusion. Phylogenies suggest the genomes of mushroomscollected in U.S.A. are different from Argentinian genomes. We discovered an individual which appears to have a mating type locus present in one nucleus of the dikaryon and absent from the second nucleus. Conclusions: While A. thiersii and A. foetens appear strikingly similar, each is morphologically, geographically and genetically distinct, leaving the question of whether A. thiersii is native or introduced to the U.S.A. unanswered.