What we are reading in January, 2018
Some news from the Neotropics! While investigating ectomycorrhizal (ECM) associations in white sand forests of the Colombian Amazon, Vasco-Palacios et al. identified 48 species of ECM fungi out of which seven are new to science. Several species had been previously reported from geographically distant forests. Long-distance dispersal and low host specificity potentially contribute to maintain gene flow between these distant populations of ECM fungi. This study also pointed how the mantle of ECM root tips was reduced and not clearly visible in some tropical plants.
Further investigation taking into account the diversity of potential host plants across the Amazon region is needed to clarify the diversity and distribution patterns of ECM associations in the Neotropics.
Do all ectomycorrhizal (ECM) fungi contribute to liberate nitrogen from soil organic matter (SOM) in forest ecosystems? Because ECM fungi evolved independently from various saprophytic ancestors, the degree to which they have retained genes involved in SOM degradation is also variable. Although it might be true for some Agaricomycetes, it is very unlikely in ascomycetes. Even if ECM fungi possessed these genes, several questions need to be fully addressed: Are these genes expressed when the fungus is forming the mycorrhizal symbiosis? Is the function of these genes to release nitrogen from SOM? If yes, is the nitrogen transferred to the plant? Abiotic factors (pH, availability in nitrogen) potentially influence these interactions and more experimental studies are needed. Among the few cases that have been addressed, Cortinarius species have been shown to be potential nitrogen regulators in boreal forests. An important fact for those working in Nothofagaceae forests, where Cortinarius is both highly diverse and abundant!
More than 90% of plants form mycorrhizas yet the evolutionary history of the mycorrhizal symbiosis remains largely unknown. By reviewing the current mycorrhizal status of flowering plants, the authors highlight several major trends: The mycorrhizal symbiosis emerged in early land plants with arbuscular mycorrhizas (AM), evolving later in the Cretaceous into other forms – ectomycorrhizal (ECM), ericoid and orchid – or reversing to non-mycorrhizal (NM) plants. A third, ongoing and complex diversification, with switches between ECM and AM mycorrhizas within families of angiosperms, is potentially linked to the rapid plant diversification of biodiversity hotspots. Despite some omissions, such as the potential evolution of ECM symbiosis in earliest groups of land plants in parallel or preceding AM associations (Bidartondo et al. 2011), this study highlights the diversity and the complex evolutionary history of mycorrhizal plants. The ECM status of many ECM plants still await discovery: 2 % of plants (335 genera) are currently known to form ECM associations, but at least 22 plant genera require further investigation.
Can mycorrhizal symbiosis be involved in plant speciation? Several studies have already shown that mycorrhizal symbiosis can modify, expand and help to differentiate nutritional niches for plants, and thus alter their survival, expansion and distribution (see Gerz et al 2018 for a recent one). But can mycorrhiza aid beyond adaptive plant strategies into evolutionary processes? Osborne and collaborators presented an exciting study and one of the first providing some clear indication that this can be happening. Choosing a unique system with two endemic sister palm trees: Howea forsteriana and H. belmoreana from an Australian island (Lord Howe Island) they present several independence lines of hard and soft evidences. The study suggests that sympatric speciation through adaptation to a different soil can be facilitated by AMF. They hypothesized, analyzing soil biota, seedling survival and root colonization, that AMF may play a critical role in ancestral Howea colonizing a calcareous soil, and differentiating from the ancestral species. Ultimately, Howea species might have evolved into two species with abiotic and biotic different requirements. This complex study opens the door to recognize and analyze the role of mycorrhizal symbiosis in plant speciation.
Osborne, O. G., De-Kayne, R., Bidartondo, M. I., Hutton, I., Baker, W. J., Turnbull, C. G. N. and Savolainen, V. (2018). Arbuscular mycorrhizal fungi promote coexistence and niche divergence of sympatric palm species on a remote oceanic island. New Phytol, 217: 1254–1266.
For the role of mycorrhizal symbiosis in plant niche see: Gerz M, Guillermo Bueno C, Ozinga WA, Zobel M, Moora M. Niche differentiation and expansion of plant species are associated with mycorrhizal symbiosis. J Ecol. 2018;106:254–264.
How much of our knowledge bias the way we look at the results? In other words, how difficult is to think outside the box in our own field and expertise? In a recent commentary Selosse and coworkers stress the need to rethink the fungal ecology with accumulated old and new empirical evidences. They target the concept of duality in fungal ecology, whether a fungal species can behave differently depending on their interactive partner and the context. For example Fusarium graminearum a well-known pathogenic fungal species, causing billions of dollars of losses ruining wheat and barley production, is at the same time a positive endophytic fungal of native north American grass species (see Lofgren et al. 2018 for details). They smartly hypothesized that the fungal duality of niches should have an evolutionary explanation. It has been observed that fungi can easily change their ecological niches (and functional roles). While some fungi can retain the old and new functional roles, others can lose the ancestral one or never completely adopt the new role. They illustrate how, for example, a saprotrophic fungus can turn directly into mycorrhizal role losing many of its saprotrophic genes. Alternatively, saprotrophic fungus can adopt mycorrhizal niche from an intermediate organism able to act as an endophyte. Unveiling the evolution and spread of duality of fungal niches is on their way, let’s be ready to rethink what we know!!
Fig. 1 Some evolutionary trajectories (grey arrows) between classical ecological niches (in bold), with some factors promoting the evolution in the direction of the arrow (italics). (a) Trajectories along the soil saprotrophy- mycorrhizal continuum, which mainly concern soil fungi. These trajectories and factors are not meant to be exhaustive but simply illustrate the way large, dual niches can arise in evolution. The cited work in the arrows refer to examples of fungi (also in Table 1) that occupy the two linked classical ecological niches, because they keep the previous and acquired niche in their evolution.
Lofgren, L. A., LeBlanc, N. R., Certano, A. K., Nachtigall, J., LaBine, K. M., Riddle, J., Broz, K., Dong, Y., Bethan, B., Kafer, C. W. and Kistler, H. C. (2018) Fusarium graminearum: pathogen or endophyte of North American grasses? New Phytologist 217:3: 1203–1212. doi: 10.1111/nph.14894