Laccaria bicolor is a stunningly beautiful species that has been used as a model to study mycorrhizal relationships. Along with other closely related Laccaria species, Laccaria bicolor forms ectomycorrhizal associations with pines, and many times, shows up in the early stages of forest succession. These fungi are not the easiest to identify because their habitat and genome can grant a wide variation of coloration. Microscopic features usually are required for proper identification in older specimens, but in younger ones, looking at the network of hyphae from the base of the mushroom (basal mycelium) can help distinguish these similar looking species. Every time I have encountered this species I have been pleasantly surprised. Inspecting the mushroom closely, I expect to find this tan-brown coloration throughout, but as I turn the specimen over, I find gills with a splendid lavender color. You are most likely to encounter this species in temperate ecosystems supporting pines on the West Coast and in the Great Lakes region, but it has been found throughout the Eastern seaboard and Europe.
What is most interesting is not what color the surface of these mushrooms is, but what these organisms are comprised of genomically. The amino acids that make up Laccaria bicolor paint a vivid evolutionary and ecological picture. In 2008, Martin et al. sequenced this species genome and found that it contains around 65 million base pairs. This is considerably smaller than our own genome which hovers around 3.2 billion base pairs. But remember, many species have highly repeated sections of their DNA that doesn’t have much function. So, it is not the size of the genome, but what functional proteins and enzymes that genome encodes for.
L. bicolor invading pine sapling roots. Martin et al. 2008.
By looking at these building blocks all life on Earth are made up of, Martin and his sizeable team found that Laccaria bicolor lacks genes that encode for enzymes the directly break down dead plant material. Even still, they identified other genes associated with the breakdown of non-plant cell wall polysaccharides. This shows us that this species ancestors were most likely saprotrophic, feeding solely on dead organic matter. Over time, it has shifted to more of a mycorrhizal strategy, but still has maintained the ability to break down sugars present in its ecosystem. This dual ecology broadens its niche width, allowing it to persist in forests when one possible source becomes less available.
Additionally, this same study identified that much of the species genes are activated throughout most of its tissues, yet some specialized genes are only active in specific tissues. For example, genes that encode for small secreted proteins (SSP’s) are only found in tissues associated directly with symbiosis. The highest concentration of SSP’s are found in the hyphae colonizing the roots of their host tree. These SSP’s probably play integral roles in mycorrhizal development, but not much is known about the specific function of these molecules. In 2011, Jonathan M. Plett and Francis Martin dove head first into these ectomycorrhizal interactions. Through their work, they hypothesized that some of these SSP’s released into plant roots disrupt the plants defenses, allowing colonization and the successful trading of resources.
A close look at this fungal and plant interface. SSP's most likely disrupt the plant from initiating a invasion response. Plett and Martin 2011.
In 2016, Andrew Wilson and his scientific crew extended this ecological outlook to most of the Laccaria genus. Using really complex molecular dating techniques, we now know that Laccaria and its closest related genus (Mythicomyces) diverged nearly 80 million years ago. About 16 million years after this divergence, Laccaria ‘figured out’ this mycorrhizal strategy. Like the other 80 times mycorrhizae independently evolved within the fungal kingdom, again we see that their ancestral condition is a saprotrophic one. With this newly evolved ecology, Laccaria species dispersed throughout the world, since suitable tree hosts were already established.
A phylogenetic analysis of the Laccaria genus. Wilson et al. 2016.
With this in-depth Wilson et al. 2016 study, I have learned that Laccaria bicolor first burst onto Earth’s scene 4.13 million years ago and has been interacting with the forest floor in complex ways ever since. Besides being able acquire sugars and other nutrients through a mycorrhizal and saprophytic ecology, Laccaria bicolor also has carnivorous abilities! In 2001, John N. Klironomos and Miranda M. Hart exposed soil dwelling arthropods known as springtails (Folsomia candida specifically) to Laccaria bicolor and other test fungi. Klironomos and Hart found that similarly to other predatory fungi, Laccaria bicolor exudes toxins to immobilize its potential prey, prior to hyphal invasion. To make sure this fungus was acquiring nutrients from these springtails, these researchers radiolabeled the arthropods, and found that the tissues of Laccaria bicolor had significantly elevated animal derived nitrogen compared to the other test fungi.
Laccaria bicolor (a) with enriched animal nitrogen compared to other non-carnivorus species Klironomos and Hart 2001.
The Laccaria genus easily spread throughout our planet’s forest ecosystems with the evolution of a mycorrhizal ecology. What has aided in this species success is not just this mycorrhizal lifestyle, but the conservation of genes that encode for enzymes that breakdown environmental polysaccharides; a remnant of its ancestor’s saprophytic existence. Additionally, its carnivorous abilities further broaden its niche width. Compared to other fungi, the relatively large genome of Laccaria bicolor may be required to have these multiple ecological functions. Some species living from the forest floor have a specific ecology, and stick to their game plan. Other species like Laccaria bicolor are adaptive and have a repertoire of genes to utilize in order to carry out their life cycle in a wide array of environments.