A major symbiont shift supports a major niche shift in a clade of tree-killing bark beetles

1. One small clade of bark beetles, out of thousands of species worldwide, has shifted from using phloem to using a combination of phloem and outer bark, or to completely using outer bark. 2. The shift to outer bark has been accompanied by a shift to non-typical bark beetle mutualist fungi in Entomocorticium (Basidiomycota) and Ceratocystiopsis (Ascomycota). 3. This study compared the growth and metabolic capabilities of fungi associated with a nearly phloem-independent species, Dendroctonus brevicomis, with those of mutualist fungi of Dendroctonus ponderosae, a completely phloem-colonising beetle in a sister clade associated with more typical Ascomycota in Grosmannia and Ophiostoma. 4. Only the basidiomycete associated with D. brevicomis could degrade cellulose and lignin, whereas both the ascomycete and basidiomycete could grow in outer bark. Ascomycetes associated with D. ponderosae could not degrade cellulose or lignin or grow in outer bark. 5. Beetles and fungi in these mutualisms may best be considered as co-niche constructors. For niche construction, one organism must modify a resource in a way that enhances its fitness while also influencing fitness of other organisms using the resource. Here, the beetles kill the tree, transport the fungi into the tree, and modify the woody substrate for use by the fungi. They have also evolved mycangia to ensure vertical dissemination of the fungi. In turn, the fungi modify tree tissues and provision nutrients to the host and have evolved traits that support their acquisition and transport by the beetle host in mycangia.