Pepijn W. Kooij
On the origin of mutualisms [link]
Pepijn Kooij, from Kew’s Comparative Plant and Fungal Biology department discusses the importance of understanding the evolution of mutualisms in reference to fungus-growing insects.
Despite what the title might suggest, it was not Charles Darwin but the Belgian researcher Pierre-Joseph van Beneden who first described the term ‘mutualism’ in 1876 in his book Animal Parasites and Messmates.
A mutualism (or mutualistic symbiosis) is a relationship between two or more organisms, where each partner benefits from living with the other. Mutualisms have shaped evolution in many ways, for example, animal and plant cells arose from a mutualism between different bacteria, with one forming the main cells and the other forming organelles such as mitochondria and chloroplasts. Therefore, trying to understand the evolution of mutualisms will help us understand the evolution of life in general.
Fungus-growing termites and leaf-cutting ants
Two famous examples of mutualisms are fungus-growing termites and leaf-cutting ants. These insects have a mutualistic relationship with a fungus that they have domesticated (Mueller et al, 2005). The insects depend on their fungal crops as a reliable food source, and, in return give protection to the fungi and aid their distribution.
n both cases the insects have been farming their fungus for millions of years, which is a great deal longer than humans have practiced agriculture; a system that has been used for around 10,000 years.
This shows that the insects have been able to perfect their way of farming, with millions of years of experience; and we, as humans, might be able to learn something from them. For humans to be able to take advantage of this, it is important to know when it all started. But unfortunately, in both cases – as in many other mutualistic relationships – we have no idea what the domesticated partner’s closest ancestor is.
Why find the origin?
In most cases of mutualistic relationships, especially fungus-growing insects, it is likely that we will know the closet ancestor of only one of the partners (Ward et al, 2015; Legendre et al, 2015). For example, for both the ants and the termites there are well understood, established phylogenies. These phylogenies show us which non-fungus-farming species are related to those that use fungus as their food source. With this in mind, it is possible to make direct comparisons between mutualists and non-mutualists, to determine why the ants and termites evolved to farm fungi as their food source. Genome comparisons can reveal the genetic changes that allowed the insects to evolve in this way, but also why the fungus-growing insects are unable to live without their fungal crops.
The ancestry of the fungal crops, however, is still unknown. In general, it is assumed that the closest relative of the fungi cultivated by termites, Termitomyces, is the free-living species Tephrocybe rancida. However, with its distribution ranging from North America to Northern Europe, it is unlikely to be ancestor of Termitomyces, which, together with the termites has a distribution from South and Central Africa to Southeast Asia.
The fungus cultivated by leaf-cutting ants is known to be related to the fungal genera Leucoagaricus and Leucocoprinus, but which species within these genera is still to be discovered. It is very important to thoroughly investigate this: performing comparison studies using the wrong species as the closest ancestor might lead to scientists drawing the wrong conclusions on the evolution and maintenance of the insect-fungus mutualisms.
Case study using the fungarium at Kew
At Kew we are the proud custodians of between 1 and 1.5 million fungal specimens, comprising a large part of known fungal diversity. In my attempt to find the closest ancestor of the fungus cultivated by leaf-cutting ants, Leucoagaricus gongylophorus, I descended into the fungarium to investigate any species that could possibly be related. By using this rich collection of fungal specimens and modern genetic tools I hope to trace the ancestor of the fungus cultivated by the leaf-cutting ants. I am still analysing data from over 300 specimens that I have been working with from the fungarium. Soon I hope to have a definitive answer as to which species of fungus started it all; and once I have achieved this, the results will open new ways of studying and understanding this intriguing collaboration between ants and their fungi. To be continued…
Van Beneden, P. J. (1876). Animal parasites and messmates – The international scientific series Volume XIX, D. Appleton & Co, New York, 274p.
Legendre, F., Nel, A., Svenson, G. J., Robillard, T., Pellens, R., & Grandcolas, P. (2015). Phylogeny of dictyoptera: dating the origin of cockroaches, praying mantises and termites with molecular data and controlled fossil evidence. PLoS ONE 10 (7) e0130127. DOI: 10.1371/journal.pone.0130127.
Mueller, U. G., Gerardo, N. M., Aanen, D. K., Six, D. L., & Schultz, T. R. (2005). The evolution of agriculture in insects. Annual Review Of Ecology Evolution And Systematics 36: 563–595. DOI: 10.1146/annurev.ecolsys.36.102003.152626.
Ward, P. S., Brady, S. G., Fisher, B. L., & Schultz, T. R. (2014). The evolution of myrmicine ants: phylogeny and biogeography of a hyperdiverse ant clade (Hymenoptera: Formicidae). Systematic Entomology 40(1), 61–81. DOI: 10.1111/syen.12090.
Celebrating the importance of mycological research [link]
Pepijn Kooij, Tuula Niskanen & Laura Martinez-suz
Although Kew is mostly known for its work on plants, a large part of the research is focused on the diversity and importance of fungi. Pepijn Kooij explains how mycologists at Kew are working to understand a wide variety of topics in fungal biology and the importance of fungi for plant diversity.
There have been many estimates of the total number of fungi species, reaching up to 6 million (Taylor et al., 2014). However, the widely accepted range is “at least 1.5, but probably as many as 3 million” (Hawksworth, 2012), far outnumbering flowering plants (Kew’s estimates go as far as 400,000 species of flowering plants).
As only around 100,000 species of fungi have so far been described worldwide, one of the goals for the mycologists at Kew is to document the missing diversity, but also to investigate their importance for ecosystems as a whole. Key to this task is the Fungarium, containing over 1.25 million specimens of dried fungi, the largest collection of fungal specimens in the world today.
How to investigate fungal diversity
As part of the fungal diversity research at Kew, Tuula Niskanen, one of Kew’s mycologists, studies the diversity and evolution of mushrooms, with a special interest in webcaps (Cortinarius). Webcaps are the most species-rich genus of the Agaricales, the gilled mushrooms, with a worldwide distribution. They are important ectomycorrhizal fungi and play a significant role in the nutrient economy of forest trees. However, they are still very poorly known and many species are not yet described and named. Even in Britain many new species remain to be discovered.
Knowledge of the evolutionary history of fungi provides the means for a better understanding of the current diversity and distribution of species. It also sets a baseline for further studies of diverse evolutionary questions, which cannot advance without the fungal specimens. The vast collections of fungi in Kew and other institutes world-wide provide a significant resource for these studies, which in turn help new expeditions to be targeted to previously unexplored areas.
Plant-fungal symbiosis and environmental change
Globally, plant-fungal partnerships underpin terrestrial ecosystems. "Fungus-roots", or mycorrhizas (myco= fungus, rhiza=root), are ancient, obligate and ubiquitous mutualisms between the vast majority of plants and members of several fungal phyla to exchange carbon derived from photosynthesis for fungal-acquired soil nutrients (Field et al., 2015). We can say that most plants don’t have roots, they have mycorrhizas! For example, tree roots in boreal, temperate and some tropical forests form ectomycorrhizas (ecto=outside), which envelop root tips like gloves, and play crucial ecological roles by determining the nutrient acquisition and drought tolerance of trees. Due to their distinctive ecological niche, mycorrhizal fungi are at particular risk to changes in either their soil environment or host carbon allocation.
Global change is one of the biggest threats to organismal and functional diversity, yet little is known about its potential impacts on plant-fungal interactions. Fungi with different soil exploration types (such as those specialised for long-, medium or short-distance water and nutrient transport) respond strongly to pollution causing eutrophication and acidification in European forests (Suz et al., 2014). Kew researchers Laura Martinez-Suz, Martin I. Bidartondo, Sietse van der Linde and William Rimington (Imperial College London) study the evolution, diversity, ecology and distribution of mycorrhizal fungi and their environmental drivers in different ecosystems. Research on this functional guild of fungi is important because, even though they are still largely neglected when it comes to conservation, they are likely to determine the resilience of ecosystems to environmental change (Suz et al., 2015).
Humans have been domesticating crops for approximately 10,000 years. Ants, however, have been growing fungal crops for 50 million years and are considered to be the oldest farmers in the world. Much like humans did, for instance with bananas, the most recently derived group of these ants, the leaf-cutting ants, created a polyploid (with more than two sets of chromosomes) fungal lineage, while maintaining this crop without sexual reproduction or mushroom growth (Kooij et al., 2015). This polyploidisation may enhance traits the ants benefit from, such as increased nutrition or resistance to disease.
My research focuses on the mechanisms that lie behind the maintenance of the asexuality of these fungi. Even though there doesn’t seem to be any genetic mixing, the fungi cultivated by ants have a high diversity with a still unknown number of species. By investigating fungarium specimens from free-living fungi closely related to the ant crops in the genera Leucoagaricus and Leucocoprinus, it will be possible to find the closest wild relatives from which the ants derived their cultivars. This will in turn help to understand the evolution of this enigmatic mutualism of ants and fungi, and potentially lend insight into our own agricultural symbioses.
Field, K.J., Pressel, S., Duckett, J.G., Rimington, W.R., & Bidartondo, M.I. (2015). Symbiotic options for the conquest of land. Trends in Ecology and Evolution 30: 477-486.
Kooij, P.W., Aanen, D.K., Schiøtt, M., & Boomsma, J.J. (2015). Evolutionarily advanced ant farmers rear polyploid crops. Journal of Evolutionary Biology, DOI: 10.1111/jeb.12718.
Suz, L.M., Barsoum, N., Benham, S., Dietrich, H.P., Fetzer, K.D., Fischer, R., García, P., Gehrman, J., Kristöfel, F., Manninger, M., Neagu, S., Nicolas, M., Oldenburger, J., Raspe, S., Sánchez, G., Schröck, H.W., Schubert, A., Verheyen, K., Verstraeten, A., & Bidartondo, M.I. (2014). Environmental drivers of ectomycorrhizal communities in Europe's temperate oak forests. Molecular Ecology 23(22): 5628-5644.
Suz, L.M., Barsoum, N., Benham, S., Cheffings, C., Cox, F., Hackett, L., Jones, A.G., Mueller, G.M., Orme, D., Seidling, W., Van der Linde, S., & Bidartondo M.I. (2015). Monitoring ectomycorrhizal fungi at large scales for science, forest management, fungal conservation and environmental policy. Annals of Forest Science. DOI 10.1007/s13595-014-0447-4.
Taylor, D.L., Hollingsworth, T.N., McFarland, J.W., Lennon, N.J., Nusbaum, C., & Ruess, R.W. (2014). A first comprehensive census of fungi in soil reveals both hyperdiversity and fine-scale niche partitioning. Ecological Monographs 84(1): 3-20.
©Copyright 2017 Pepijn Kooij