Another one of those largely overlooked species of fungus, the milk-white toothed polypore (Irpex lacteus) is commonly found growing on branches of hard woods throughout temperate regions of the world. If you’re not into mycology, you might notice that something is growing on the branch and most likely, the inquires will stop there. Though, if you are immersed into the mycological discipline, you might notice its advantageous resupinate morphology; an evolutionary adaptation that enhances the species overall efficiency. 

“Crust fungi” represent a resupinate morphology. The reason for the quotations is because crust fungi are not all genetically similar. When I first got into mycology, I would find a resupinate fungus and exclaim, “Ah, a crust fungus.” I wasn’t wrong, but I also wasn’t right. This type of morphology has evolved in separate lineages several times, so exclaiming that I found a crust fungus really wasn’t saying much. But hey, I was enthusiastic. 

In a previous Fungi Friday, I wrote about how Flammulina velutipes senses gravity. It can quickly adjust its stipe so its gills are positioned perpendicular to the ground. This improves the species spore dispersal, as the potential offspring fall straight down, unimpeded by neighboring gill tissue. Similar to F. velutipes, species that form resupinate fruit bodies also sense gravity. Hyphae of Irpex lacteus, growing on woody debris elevated from the forest floor congregate and produce spore forming pore structures on the bottom of these surfaces. Essentially, this maximizes the species spore forming surface area, as fewer energy and nutrient reserves are allocated to pileus tissues. The pileus or cap surface protect the vulnerable sexual structures, but since resupinate fungi grow beneath woody surface, the substrate they’re actually feeding on act as a protective pileus. 

If the fungus extends its growth from directly beneath the branch, it will form a cap surface. Underneath the cap or crust, this polypore produces elongated pores. Though, depending on the growth conditions, the fungus can also grow a hydnoid surface also known as teeth. This is why the fungus is sometimes mistaken for a true tooth fungus. When it produces pores, the length of its pores varies with age. Overall, it has quite a variable appearance. This, along with its massive distribution has confused mycologists for years, leading to it being separately described over and over again. Because of this, there are a ridiculous number of synonyms for the species. If you search this species on Wikipedia, you will see a total of 34 scientific names given to the saprotroph! Sequencing its genome have since then clarified the taxonomic discrepancy. 

The next time you find a resupinate fungus on your next hike, think about why this morphology has evolved several times throughout Earth’s long history. Note how efficient it is for a decomposer to utilize its own substrate to protect its spore bearing surface. Realize that while most fungal species must produce a stipe and pileus, the resupinate Irpex lacteus maximizes the number of spores it can produce, ultimately enhancing its fitness. This species made its way on to Earth’s scene along with the diversification with the angiosperm trees. As long as hard woods remain growing from the forest floor, you can bet Irpex lacteus will be there too, breaking down their tissue when they fall.

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