Diversification in Mycorrhizal Fungi_ Spr24 BIOL 010B #35557 THE DIVERSITY OF LIFE ON EARTH-Hybrid.pdf

Diversification in Mycorrhizal Fungi The global species richness of ectomycorrhizal (ECM) fungi is estimated to be between 20,000 and 25,000 species; these fungi belong predominantly to Basidiomycota and also to Ascomycota and nonDikarya fungi. These fungi are associated with many dominant trees in...

Diversification in Mycorrhizal Fungi The global species richness of ectomycorrhizal (ECM) fungi is estimated to be between 20,000 and 25,000 species; these fungi belong predominantly to Basidiomycota and also to Ascomycota and nonDikarya fungi. These fungi are associated with many dominant trees in both temperate and tropical forests worldwide, including those belonging to the families Fagaceae, Betulaceae, Salicaceae, Dipterocarpaceae, Myrtaceae and Pinaceae. Although some species are generalists (they associated with multiple hosts), many specialize and have strong host preferences at the host genus or family level. Let's take a closer look at how this diversification can happen! How do mycorrhizal fungi associate themselves with plants in the first place? Healthy soil is teeming with microbes, some of which are beneficial for plants (e.g. nitrogen fixing bacteria), while some are parasitic and pathogenic. Plant roots, much like our bodies, have a layer of cells that act as a barrier to external invaders. However, plants need to be able to discern between beneficial and harmful species. They do this biochemically. ECM fungi respond to chemical signals emitted by plant roots and when they are in close proximity to the root, permeability needs to change in order to allow contact with the ECM fungi. Although we will not get into the biochemical details, this "crosstalk" evolves through time and is essential for successful establishment of the mutualism. Most plants that have ECM associations have roots with a modified lateral branching pattern. This pattern, which is called heterorhizy, consists of short mycorrhizal lateral roots (called short roots) supported by a network of long roots. Short roots normally grow much more slowly than long roots and the restricted growth of short roots may be necessary to allow ECM fungi time to form an association. As plants move to new niches or mycorrhizal networks expand to broader ranges, the fungi encounter different environmental conditions, which increase ecological opportunities and possibly create conditions for speciation. Indeed, the paper: Sato, H., Tanabe, A. S., & Toju, H. (2017). Host shifts enhance diversification of ectomycorrhizal fungi: diversification rate analysis of the ectomycorrhizal fungal genera Strobilomyces and Afroboletus with an 80‐gene phylogeny. New Phytologist, 214(1), 443-454. provides some evidence that ECM diversification rates (speciation minus extinction) increased during hostshift events within the Strobilomyces– Afroboletus clade. Strobilomyces sp. Afroboletus sp. Host plants of these fungi were divided into four host categories: 1. Fagaceae/Pinaceae 2. Dipterocarpoideae, 3. Nothofagaceae/ Eucalyptus and 4. Caesalpinioideae/Monotoideae When you look at a phylogenetic tree like this one below, rapid diversification is indicated by the appearance of many species across a short horizontal interval on the tree. Can you identify the region that indicated the highest diversification rate of ECM fungi? The study indicates that lineages of the genera Strobilomyces and Afroboletus were initially associated with African ECM hosts (i.e. Caesalpinioideae/Monotoideae), and subsequently began to be associated with Dipterocarpoideae in tropical Asia and then switched to the Eurasian (i.e. Fagaceae/Pinaceae) and Australian (i.e. Nothofagaceae/Eucalyptus) temperate trees. The study also suggests that a burst in the diversification of ECM genera was driven by major host-shift events. Host shifts to temperate host trees in Eurasian regions, particularly shifts to fagaceous trees, would have caused an expansion of the distribution range, leading to a rapid radiation of the fungi.