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Four new species of Albomagister, a genus of Tricholomataceae in the order Agaricales, are described and illustrated from eastern North America. All four are relatively rare or geographically restricted but two have a broad geographical distribution occurring in southeast Canada and in the southern Appalachians. This study increases the number of known species in the genus from three to seven, five of which occur in eastern North America. A broad concept for the genus is discussed. Illustrations and descriptions of the North American taxa are presented, along with a taxonomic key to the known seven species in the genus worldwide. 

Fungus-farming ants cultivate multiple lineages of fungi for food, but, because fungal cultivar relationships are largely unresolved, the history of fungus-ant coevolution remains poorly known. We designed probes targeting >2000 gene regions to generate a dated evolutionary tree for 475 fungi and combined it with a similarly generated tree for 276 ants. We found that fungus-ant agriculture originated ~66 million years ago when the end-of-Cretaceous asteroid impact temporarily interrupted photosynthesis, causing global mass extinctions but favoring the proliferation of fungi. Subsequently, ~27 million years ago, one ancestral fungal cultivar population became domesticated, i.e., obligately mutualistic, when seasonally dry habitats expanded in South America, likely isolating the cultivar population from its free-living, wet forest–dwelling conspecifics. By revealing these and other major transitions in fungus-ant coevolution, our results clarify the historical processes that shaped a model system for nonhuman agriculture.