A fungus walks into a singles bar

Article by Fred Bunnell

I could have titled this “Fungi – why so many and so confusing”. That would have been accurate, but not as much fun. It doesn’t take much of a walk through the forest in the late summer or fall to appreciate there are a lot of different fungi. Provided the residents are not overly zealous in mowing their lawns, you come to the same conclusion walking through suburban areas. With more effort you would reach that same conclusion in a flowing stream, any ocean or biome. It would take different techniques, but poking about would expose many fungi in ice crystals of the arctic and Antarctic or reveal 15 or more species of micro-fungi in a single conifer needle. That encourages two questions: why so many and why so confusing?

Why so many fungi?

The answer is relatively straightforward. There are two parts: fungi have been around a long time and the job description of fungi is very broad. We quibble about just how long fungi have been around (we don’t even try for slime molds). Fungi don’t make very good fossils. The most revealing fossils are compression fossils in sedimentary rock, but fungi are often too squishy to leave good compression fossils. Slime molds are even squishier. Some researchers believe fungi diverged from other life forms about 1,500 million years ago; others argue that it was about 600 million years ago, but they are arguing for more advanced forms of fungi. Disagreement occurs primarily because early fungi were more squishy than current forms and left no fossils that are clearly fungi. What seems true is that multicellular, deep water organisms with attributes of fungi were around about 1,500 million years ago. This also is a common sense argument because something having the job description of fungi had to be around to allow higher life forms to evolve. Despite that, I show the more conservative approach in the graph below.

Graph

Like all early creatures, other than lithotrophs or rock-eating bacteria, fungi first appeared in water. We have no good idea how many archaea and bacteria species there are. The estimated number of algae species seems to be settling down at around 40,000, but the estimated number of fungal species ranges from 1.3 to 1.5 million. Algae did not do as well upon reaching land as did the fungi. Birds (about 10,000 species), and mammals (less than 6,000) lag well behind. Insects have the advantage of short generation times that speed the rate of evolution and now number about 1 million species. Both algae and fungi reached land about 500 million years ago. They have had ample time to diversify, but fungi diversified as fungi, entrapped by their mode of attaining energy, while algae overreached and evolved into wholly different creatures – ferns, fern allies and vascular plants. Fungi clearly have been around long enough to diversify. Their job description encouraged diversity.

That job description apparently goes something like this: decompose or break down all organic material so that nutrients in it can be reused. As we lean back and leisurely digest our meal, we may think we’re doing that very thing. We’re not. The human body carries about 100 trillion microorganisms in its intestines, a number ten times greater than the total number of human cells in the body. The microbiota of the gut includes 100’s of species of archaea, bacteria and fungi. In return for shelter, they are kind enough to break down our food and release the nutrients to us. Some writers refer to the gut microbiota as the ‘forgotten organ’. We tend to think so highly of ourselves that few of us remember that, in aggregate, the gut microbiota inside us contains about 100 times as many genes as the human genome.
Cordyceps-militaris-copy-300x300 Without fungi, the living world would consist of organisms that produced nutrients themselves – archaea, bacteria and perhaps a few lucky algae. Rather boring. The fungus job description says decompose everything organic and they fulfil that perfectly. Nothing organic is piling up. Nutrients and minerals are freed to build other organisms. To fulfil that task, fungi have tweaked their enzymes until nothing organic can resist them. We use this prowess by extracting useful enzymes from fungi for a whole host of industrial processes – lipases, amylases, cellulases, invertases, proteases and xylanases (see Fungi – the ‘good’ and the ‘bad’ blog). One result is that as new species evolved, the fungi have kept pace, producing new fungal species that can decompose them and release their nutrients for re-use. That has led to specialization. In some instances, a fungus decomposes a specific insect species, or specific parts of a specific insect species. This is handy for us. We wouldn’t want to be neck-deep in insect legs.

In their enthusiasm, some fungal species begin early – while the insect is still alive. Caterpillar fungus is a misnomer – it usually parasitizes moth pupa. Typically the pupa is buried and you have to ease the fungus out of the soil. The photo of Cordyceps is from the Fungi of Great Britain and Ireland web page, but the fungus occurs locally.

Being around a long time and having the capability to respond to a challenging job description explains why there are so many fungi, but is only part of the explanation for why their identification can be so confusing. The rest of the explanation is fungi’s unique approach to reproduction.

A fungus walks into a singles bar

We don’t know the punch line, but we can try to creep up on it. The fungal approach to sexual reproduction is uniquely theirs. There is no such thing as gender in fungi; no males, no females. A few mate with themselves – homothallic (same body) – meiosis occurs in the fungus but the gametes look identical, merely having somewhat different allele expression. Most are heterothallic (different bodies), but the bodies aren’t male or female. In them, the isolates (call them gametes or spores if you want) must be from different ‘mating types’ to mate. While many species you see out walking have only two mating types, some fungi have four, some 10 and some 20,000 or more. They all can reproduce asexually – by fragmentation of the hyphae. Break off a piece of hyphae and you’ll get another of the same fungus, just like slime molds, but they don’t seem to think as clearly as do slime molds (see No feet, no brains, no problem – slime molds). Some species go full tilt boogie and create both sexual spores to permit gene mixing and asexual spores that produce identical clones, no gene mixing. For one form of fungal gene ‘mixing’ we had to invent a new word. In these fungi, the nuclei of cells can pair off, join up and cohabit the same cell without fusing. We call two separate nuclei in a cell a dikaryon. This variety gives fungus remarkable freedom to move genes about or not.

Source: Studyblue

The hyphae on the upper left is dikaryotic – what all the advantages of that are we haven’t imagined yet. It is clear that dikaryotic hyphae are simultaneously ‘male’ and ‘female’. That in turn encourages diversity of nuclear and mitochondrial genomes. Different hyphae may fuse with or without mixing of genetic material. The germinating spores on the right have one nucleus per cell like all other life forms. If these meet the right mating type, two haploid cells can create a diploid creature, just as sperm and ova do. Dikaryons propagate hidden diversity. We already have enough trouble identifying visible diversity in species that produce both asexual and sexual spores – like the candlesnuff fungus.

Candlesnuff fungus – both photos from Brooksdale

Linnaeus and others began naming fungi long before we knew there were genes. The two forms on the left were identified as two different species. They actually are two forms of the same species – candlesnuff fungus (Xylaria hypoxylon). The form on the left is producing asexual spores (white powder) to clone itself. The form on the right is producing sexual spores to participate in gene mixing. A lot of fungal species do this. The gene jockeys are still sorting it out. The confusion grows when we consider mating types. For the first few decades that I practiced forestry, one root rot was the bane of foresters. It attacked pretty much every commercial tree species in British Columbia. We called it Armillaria mellea.

We don’t do that anymore. Since we learned to culture fungi and probe their DNA, we have learned there are at least 9 mating types of Armillaria. Worse, by the time the taxonomists were done, the name Armillaria mellea applied only to a mating type that occurred east of the Great Lakes. While the mating types have no problem distinguishing each other, several types are indistinguishable to us until we try to mate them. We now simply call them the ‘honey mushroom group’, but at least two species in BC are relatively easily distinguishable. One of these produces the oldest and largest individuals found on the planet – far larger than a blue or sperm whale and much older than the oldest tree. The largest is 3.8 km across in the Malheur forest of Oregon and could be 8,650 years old (no one can remember). It is also quite tasty.

Honey mushroom at Brooksdale

To the right is a representative of the largest and oldest organism on the planet. Taxonomists call it Armillaria solidipes (formerly ostoyae). Most of us call all mating forms of what was once Armillaria mellea ‘honey mushrooms’. This mating type is tasty when thoroughly cooked, but don’t try any honey mushroom growing on hardwoods. Other fungi have more mating types. Different combinations of mating types may or may not produce something that looks the same. There are gradations out there that are very useful to the fungi, but confound the taxonomists trying to draw lines between species.

That diversity brings us right back to the fungus entering the singles bar. We don’t know the punch line because there may be a heap of compatible mating types in that bar or none at all, even if they all look alike. And you thought it was going to be something about “I’m a fun guy”.