EARTH DID NOT ALWAYS LOOK like it does today. The land we stand on feels solid, but the continents float on molten magma like dumplings on a simmering stew. For the first billion years or so, most action on the planet was geological and chemical. The ancient, lifeless supercontinents drifted apart and bashed back together, most recently only a quarter billion years ago as a single landmass called Pangaea.
Life appeared about four billion years ago during the geological period we call the Precambrian Era. For the first two billion years, sometimes disparaged as the Boring Billions, all life-forms had just one cell. The atmosphere was a blend of nitrogen and carbon dioxide gases. Life is sweet: it’s all about sugar. A few bacteria discovered how to use sunlight to make sugar from the air by stitching carbon dioxide and water molecules together. This biochemical reaction, called photosynthesis, provides the food for all life. Oxygen was a by-product. As the millennia passed, the concentration of oxygen in the atmosphere climbed to 21 percent. Other microbes scrounged the waste secretions of these photosynthetic cells, survived on their tiny carcasses, or attacked living cells directly—life surviving off death by the processes of saprobic or parasitic nutrition.1 Between 1 and 1.5 billion years ago, microbes with nuclei and multiple X-shaped chromosomes (known as eukaryotes) split into various kingdoms of life—including animals, fungi, plants, and several groups of protists.2 The bacteria (often called prokaryotes), which have no nuclei and usually just one circular chromosome, went off on their own trajectory. Based on sheer numbers of cells and species, bacteria still dominate the modern world—but that’s a different story.
We think that the last common ancestors of animals and fungi were single cells that swam in the ocean with a whip-like flagellum (pl. flagella): microscopic tadpoles called zoo-spores. The ancestors of fungi were so tiny and fragile that only a few fossils exist that provide clues to what they looked like. Today, the surviving offshoots of this ancient evolution belong to the phylum Chytridiomycota, known as chytrids. The majority of the thousand or so species live a bucolic life in freshwater. Their zoospores wag their tails and flutter from place to place. They ram their way into pollen grains or the epidermis of floating seeds, like tiny goats butting a balloon. Then they swell up into one or a few cells that fill up with zoo-spores again. Some make finger-like cellular roots called rhizoids that grab on to or penetrate the host tissue. Sometimes zoospores smash their heads together and mate. You have probably never heard of chytrids (the name comes from the Greek for “little pot” and describes the mother cells filled with zoospores), but their affection for moisture and their habit of setting up zoospore factories in plants and animals lead to some serious diseases. The well-known amphibian apocalypse (see chapter 9) is one example.
Zygomycetes (“zygos” for short) were the next fungal group to split off, when modern multicellular life began to diversify and move onto land. Most modern zygos (now reclassified into several phyla; see appendix) are fast, weedy moulds like the compost mould Rhizopus. They search out moist, sugary nooks, pop up quickly, and saturate their surroundings with asexual spores. Quite a few are involved with insects. The familiar Entomophthora muscae glues houseflies to your windows each autumn and spatters a halo of white asexual spores around the corpse. The sexual process in zygo-mycetes is more like ours than most other fungi because there is only one child per mating. Pregnancy happens outside the body, though—the sexual zygospores swell up between the tips of a mating pair (see figure on p. 223). These dark, thick-walled balls are often covered with warts or elaborate branched projections that under a microscope give them the look of steel wool or the working end of a medieval flail. There is no mechanism for sending these spores elsewhere; they just drop into the dust and hibernate until favorable conditions return. Their dramatic ornamentation protects the spores from being devoured by hungry insect larvae and nematodes.
Root-associated zygos called arbuscular mycorrhizal fungi (see chapter 4) are among the fungi that helped plants adapt to life on land. Those cartoons of a fish squiggling out of the sea, growing legs, wobbling along, and transforming into a lizard, then a dinosaur, then some kind of ape, and finally, after millions of years, Homer Simpson—it didn’t happen like that. Although the details are lost to time, most land plants probably evolved from multicelled algae that migrated into freshwater, away from the salty violence of the sea. In that first few hundred million years, algae and fungi bobbed together in shallow ponds or dried together into crusts. The intimate cooperation and competition that still exists between the fungal and plant kingdoms bloomed.
Plants wouldn’t be as successful if they weren’t involved with fungi. Many cooperative relationships between fungi and plants developed and continued through the eons, eras, periods, epochs, and ages into the modern world. Hyphae—the thread-like cells that make up the bodies of fungi—grew inside plant leaves, stems, or roots. A few plant fossils have been found that give clues to these ancient relationships. Another strategy was for single-celled algae (or photosynthetic bacteria) to live inside colonies of a fungus, an arrangement we now call lichens.
The proliferation of fungi and land plants was dramatic and seemed to happen in sync. About 400 million years ago in the Devonian Period, the two largest fungal phyla of the modern world, Ascomycota (the ascomycetes) and Basidio-mycota (the basidiomycetes), appeared. Both interact intimately with living and dead plants, but in different ways.
Ascomycetes, or “ascos” for short, are the largest fungal phylum. You may know some of the conspicuous ones. Lumpy, coal-like, pricey truffles (Tuber species) nestle among the roots of oak trees. If dogs, wild pigs, or squirrels don’t find them first, you can dig them up and shave off some slivers to enhance your finest meals. Elusive wrinkled, egg-shaped morels (Morchella species, sometimes called Gucchi mushrooms) pop up in meadows and under trees for a few weeks each spring. They are prized by mushroom foragers, who often exhibit strong hoarding behaviors and keep their precious locations a secret.
Many of the roughly 87,000 known ascos are micro-scopic. You see them only as little dots or blobs on plants or animals. Or they hide away in decaying debris nibbling at the cellulose or starch, then bloom as an asexual, spore-spewing mould. Their sexual states are flask- or cup-shaped bodies that emerge from a nest of intermingled parental hyphae. Sexual spores form inside a larger sac-like cell called an ascus (pl. asci) that bulges out from the cell where the nuclei of the two parents merge. If the nucleus inside each ascus divides once, the fungus ends up with two ascospores per union; if twice, then 4, then 8, 16, 32, sometimes hundreds. The typical ascus, though, has eight neatly arranged ascospores and looks something like a transparent pod or sac full of beans. In most species, the asci are squirt guns: they rupture and a gush of cellular fluid propels the spores out into the air. Hundreds or thousands of asci may ripen at once, each the result of a unique union of nuclei from the original parent hyphae. It’s quite the orgy.
The ability to break down both cellulose and lignin in plants is a distinctive talent of many species of the other large fungal phylum, the basidiomycetes (“basidios”). About half of the fifty thousand species form what we generally consider mushrooms. Some have caps with gills, like the typical brown grocery store mushroom, Agaricus bisporus. Others have a spongy underside covered with tiny pores—boletes like the cep or porcini (Boletus edulis) adored by European mycophiles. Polypores (like the reishi mushroom, Ganoderma lucidum) are tough, woody bracket fungi that decay trees and lumber and have thousands of speck-like pores on the underside of their shelf-like structures. Puffballs are round fungal marshmallows about the size of a golf ball that soften with age and dispatch puffs of grayish spores into the air. (The giant puffball, Calvatia gigantea, swells to the size of a soccer ball and often gets treated like one.) Jelly fungi, like the yellow witch’s butter, Tremella mesenterica, often found on tree branches, swell into brain-like gelatinous masses when wet but shrink to a hard scab when dry. Some have yeasts as asexual states. Many microscopic basidiomycetes lurk in soil or plants too, notably the pathogenic rusts and smuts that can be so devastating to agriculture.
In many basidios, the hyphae arrange themselves into a miniature, concentrated mushroom in the soil and wait. When the showers come, the cells of this primordium inflate with liquid and a complete mushroom bursts out of the ground like a compressed sponge dropped into water. The force of this swelling can even break concrete. The gill or pore surfaces of the mature structure are covered by a layer of thousands of club-shaped microscopic cells called basidia where the nuclei originating with each parent merge. Each basidium is crowned with four tapering pins known as sterigmata. Each sterigma supports a developing basidiospore balanced off-kilter on its perch, and these spores inflate in synchrony to their final round or oval shape. Using a mechanism that involves the explosion of a tiny droplet of water, the spores pop off into the space between the gills and fall into the air current streaming around the cap.
Contemporary mycologists count about twenty phyla in the fungal kingdom.3 About sixteen of these groups contain only a few species and have little impact on humans. But the A, B, C, Zs—the ascomycetes, basidiomycetes, chytrids, and zygomycetes that we just met—together encompass millions of species. They display a vast range of beneficial and harmful behaviors that influence both nature and human civilization. To see them, you need only learn how to look.
Learning to See: Bringing Fungi Into the Light
How might you describe yourself on the telephone to the stranger who volunteers to meet you at the airport? I would be one of many slightly chubby, blue-eyed, shortish, middle-aged men with straight brown hair. You might need some special vocabulary to describe the shape of my nose, the mole above my lip, and the way I walk. It would be easier if we all wore name tags or had some kind of barcode.
For me, the process of learning to see the hidden world of fungi and distinguish its members started the summer after my second year at the University of Waterloo. In our survey course on nonvascular plants, I’d discovered an unexpected attraction to mycology and wanted to test-fly my new “expertise” on home turf. So at the end of the summer before third year, I stepped into the scraggly woodlot behind our house in Sudbury. I picked a few mushrooms but quickly realized I didn’t know much about them. The local bookstore had a single copy of just one guidebook, The Mushroom Hunter’s Field Guide, by the doyen of American mycology, Alexander H. Smith (1904-1986).4 To identify mushrooms, I discovered in that book, you make a spore print by resting a cap gill-side down on a piece of white or black paper and cover it overnight with a jar. Or better, half on white paper and half on black, so you can easily distinguish prints of white or black spores. The spores fall off the cap, and in the morning you can see what color they are.
After hours of navigating backwards and forwards in identification keys, and comparing my specimens with the bewildering jargon in the descriptions, I was 70 percent certain that what I had was the honey mushroom, Armillaria mellea. As the common and Latin names indicate, the caps are vaguely honey colored and have scales that look something like the crystals that form in old honey. According to the guide, an important distinguishing feature of honeys is the rhizomorph, a black shoestring-like strand found around the base. I tromped back into the ravine and scratched through the dirt with an intensity that gratified the dog, who was obviously pleased that I had learned something from her after all. Now that I knew what to look for, the rhizomorphs were everywhere, beneath the loose bark of rotting trees and meandering through the soil and leaf litter like long strips of black licorice. This made me more confident that I had the identity right. Honey mushrooms are also what Smith called “edible and choice,” a more enticing invitation than “boring but won’t kill you.” My goal was to show up my botanical sisters by bringing a wild mushroom to the table.
One rule of wild mushroom consumption is that the first time you eat any new species, you eat just a little. So I told my mother my plan and left most of the mushrooms untouched and the guide open at the appropriate page in case the poison control center needed them later. I heated up the skillet with a pat of butter and fried up one or two mushrooms. How bad could they be? They tasted okay, a bit like regular button mushrooms but with a slight metallic tang. I sat back to wait. Nothing unfortunate happened. Three weeks later, I was in the clear.
Mushroom guides are not shy about describing the dangers of mushroom poisoning.5 Although most mushrooms are not lethal to humans, the poisonous ones are surprisingly common. Different mushroom species make different toxins, and they affect the body in different ways. As soon as two hours after dining on a mushroom that causes gastric upset, but often as long as seven or eight hours, you vomit or scramble off to the toilet. Others take even longer to make you sick. The fatal effects of the kidney toxin orellanin, produced by Cortinarius orellanus and related species, are sometimes delayed by two or three weeks.
The beautiful, juicy-looking Amanita mushrooms seem to invite sampling, but species of this genus cause the most fatalities. Their common names convey the correct message. The white North American species Amanita bisporigera and the European A. virosa are known as destroying angels. The greenish to brownish A. phalloides, a European species spreading in North America since the 1930s, is called the death cap. So all mushroom pickers are taught to recognize Amanita first, before they learn the edible mushrooms in other genera. The Amanita toxins, circular peptides like amatoxin or phallotoxin, survive cooking and pass from the stomach into the bloodstream. Twelve to twenty-four hours later the unpleasantness begins, with severe dizziness and headaches, nausea, diarrhea, hyper-peeing, coughing and shortness of breath, and back pain. There may be a brief respite after this first act, which offers a false sense of recovery as the toxins painlessly concentrate in the liver and kidneys. Then protein synthesis stops and cells burst, leading to an unpleasant death a few days later. Although urban myths suggest treating Amanita poisoning with vitamin C or penicillin, these remedies don’t help much. The only hope is enough intravenous fluids—and short-term support of vital functions—to buy enough time for a transplant from a compatible liver donor.
You can appreciate that identifying mushrooms precisely is of practical importance, especially if you want to eat them. Misidentifications of fungi cause problems in all kinds of situations, from research labs to farms and hospitals. But how do you identify something you can hardly see? You can only get so far with a jeweler’s loupe. For moulds, yeasts, and other inconspicuous fungi, this is a technical process that begins with a microscope. Some citizen scientists do learn these skills. But the need for expensive lab equipment and the perceived need for a university degree often draws a boundary between the hobbyist and the academic. Nonetheless, a few of these lab procedures are important to understand.
If we want to do experiments with a fungus, we need a living culture. Capturing a fungus and getting it to grow in the lab is sometimes easy. You pick up a spore or piece of the colony and place it onto an agar medium in a Petri dish. With a bit of luck, you have the right nutrients and the right temperature, and after a few days the hyphae start to spread. The colonies can be fuzzy or slimy, waxy or powdery, furrowed or smooth, with rays or rings of color, and sometimes with hazy pigments leaching ahead of the growth. Some are fruity, many are musty, and others have no smell at all. You can store purified cultures in refrigerators for months and in freezers for decades, then start them growing again and put them to work. Many fungi don’t culture easily, though, because they need a living host or some nutrient is missing or they consider agar a questionable imitation of their homes in nature. Some fungi, maybe the majority, may not grow in culture at all. If you want to study bases, an uncontaminated culture is best, although people with appropriate microscopes and steady hands can start with a single spore.
In modern biology, the most convincing evidence is DNA left at the scene of the crime. Much of the identification of microfungi has graduated away from the microscope into DNA sequencing. This is a practical development because there are so many more people with skills in molecular biology. Anytime you see a picture of a molecular biologist, they are holding a pipettor. This is a narrow plastic cone with a plunger and a stack of ring-like dials at the top, which looks like it should be attached to a video game. Its purpose is to suck up and spit out precise but minuscule volumes of liquid-enzymes, salt solutions, or DNA extracts. Molecular labs overflow with racks full of snap-cap plastic tubes, disposable plastic tips, tubes with solutions, and wonderful little vibrating vortex mixers.
The polymerase chain reaction (PCR) is used to duplicate snippets of genes until their concentration is high enough for chemical analysis, DNA sequences of PCR-amplified genes—the bases adenine (A), cytosine (C), thymine (T), and guanine (G), or ACTG, mixed up in all sorts of ways—are determined with a very accurate method called Sanger sequencing. Technicians used to plop tabloid-sized plasticized sheets called sequencing gels onto light boxes, or hold them up against the window. Each base has its own lane filled with a ladder of black bands. To read the sequence, you follow the bands upwards, jumping from ladder to ladder one rung at a time, each step representing one base. Nowadays, Sanger reactions flow through narrow gel-filled tubes past lasers that detect the DNA. The output comes as a graph with different-colored peaks to represent each base, but most of the sequence is determined by software. In about 2010, a suite of new but slightly sloppy methods was introduced, called next-generation sequencing (next-gen).6 Because it is faster, cheaper, and has much higher throughput, next-gen is used more and more. Some methods don’t need PCR-amplified DNA and can read the sequence from strands of single DNA molecules. Next-gen methods are now used for DNA-based surveys of oceans, forests, farms, foods, plants, animals, buildings, and ourselves, to see what microbes live there.
Whatever the DNA sequencing method, the precise DNA sequences of standard genes are used as genetic fingerprints, which are like DNA barcodes for specific species. Detect the barcode and you can identify the species, DNA sequences are also used to generate the evolutionary genealogies called phylogenetic trees, which assist with the classification of species into phyla, genera, and other taxonomic categories. Differences between sequences can be used to calculate dates in evolutionary time when groups separated, a technique called the “molecular clock.” While DNA sequencing helps us decide what fungi we are looking at and how they are related to one another, a more interesting biological question is “What do they do?” To answer that, we need to step out of the lab and into nature.
Hyphae, Mycelium, and Spores: A Fungus for a Day
What if Google Earth didn’t stop a few thousand feet above your house but just kept zooming in?7 Put yourself in Alice in Wonderland’s shoes and take a bite of the magic mushroom. Let your body shrink until it is ten thousand times smaller than that awkward blob of limbs and organs you occupy every day of your life. You land softly in a landscape that stretches into the distance. What were once microscopic moulds nowsway above you like trees, their spores drifting by like balloons on currents of air. Some of the other creatures and plants around you are so tall that they seem a mile high.
How do you find your way in such a curious scene? That would depend on where you were when you fell down the rabbit hole. If you were in a forest, the matrix would extend deep below and far above, with dirt, roots, worms, and insects teeming in the metropolis around you. If you landed on the back of an animal, you’d crawl out from behind its fur and search for a safe place to hide. But for now, just imagine you’re at home and that you’ve rematerialized in the muddled detritus under the kitchen table.
Looping human hairs and animal fur twist into the sky. Flakes of dandruff stick to some of the strands or sag in loose waxy mounds on the floor. Fibers from clothing and furniture droop like tissue paper streamers. Crashed pollen grains the size of airships lie cracked open and leaking on the ground. Mineral boulders and charcoal briquettes of soot block your path and hide the crevices between the floor tiles. Nematodes several times your size wriggle through the detritus and eight-legged mites thunder around like tanks. Spear-like shards of molted insect exoskeletons jut out of piles on the ground. A few creatures and microbes in your vicinity might sniff at you, but most won’t bother—don’t take it personally. Humans are unimportant here.
To get our bearings in this miniaturized world, let’s turn the looking glass around and imagine that you are a fungus.8 Your body is composed almost entirely of hyphae, cylindrical cells that are like continuously elongating pieces of spaghetti. They are rarely as wide as a human hair and typically about fifty times narrower. The “skin” around these tubular cells is stiffer and tougher than you are used to, but still flexible. Instead of relying on keratin, the fibrous protein that gives structure to animal skin and nails, fungal cell walls are wrapped with a tape of polysaccharide fibers mixed with chitin. (Polysaccharide means “many sugars”; various sugar molecules are bound into branched or unbranched chains.) Chitin is made of long chains of the sugar N-acetyl-glucosamine, the same compound that hardens the shells of insects and shellfish.
Having filaments as your main construction material might seem limiting, but hyphae can poke their way into narrow passageways or needle their way through soft barriers. They can weave together in various patterns like threads of a fabric. Hyphae provide flexibility and strength, whether they unite into a larger tissue like a mushroom or tie other material together. And outside, there are a lot of hyphae around. Estimates vary from one kind of soil to the next and from one place to another, but nearly 2,000 miles of hyphae—roughly the distance from Paris to Cairo—wind through every tea-spoon of rich organic soil.9
Try growing a few of your hyphae alongside each other and send some branches sideways. The tips of some will bump together with neighboring hyphae and meld into an irregular lattice. This joining of your different threads into a network is a peculiar fungal routine. Every time you encounter another hypha, you need to run a genetic checklist to make sure it is part of you, or a very close relative, and not someone else. It’s risky to join forces with a stranger. Cells or discrete colonies that are genetically identical are called clones. The cloning habit of fungi allows them to link together into larger networks and is one reason they are so ecologically successful.10
Eventually you end up as a three-dimensional, tangled pot of pasta called mycelium (pl. mycelia). As a fungus, you will live most of your life in mycelial form, underground or underwater, in rotten wood or organic debris—one reason why fungi are called the hidden kingdom. Being a bit more fluid than bony animals, you can ease yourself down wherever you are and spread in a meshy network. And rather than being restricted by the need to maintain an organized structure like the body of an animal or a plant, your mycelium exists as a colony. This is a free-form pastiche of connected bits and pieces that is as close as many microfungi get to organizing their mycelium.
Moving from place to place by extending a hypha is slow. To make a quicker jump, you push individual hyphae—or larger bundles or tangles of mycelium in structures like mushrooms—above ground to make spores. A lot of fungi make spores that are single round cells, but some stretch into any number of cells and shapes that look like stars, bananas, hats, bells, worms, or spaceships and are decorated with warts, antler-like growths, or gelatinous tails. Whatever their shape and size, spores contain complete sets of genes wound into chromosomes encased in cells, each like a message in a bottle containing instructions to make a new colony.11 The drifting spores fly off into the sky like a mass of helium balloons. Their concentration in outdoor air varies a lot but is often a hundred or more times greater than the amount of pollen. Eventually they settle, nearby or far away, falling onto the ground or, with a little dab of glue that sweats through their cell wall, sticking to a plant or insect. Spores sometimes rest for years in soil like a dormant seed, burning small sparks of energy and absorbing a few molecules of oxygen, waiting for the right temperature or amount of water, or a special signal. Or they might just go for it and hope for the best. The slumbering genes in the chromosomes crank up to produce enzymes that soften a part of the spore’s cell wall. A bump swells out into a new hypha, which extends tentatively into the new surroundings to search out food.
A sampling of fungal spore shapes
And as a fungus, you will be hungry all the time. Your hyphae can only extend at the end, so to satisfy your constant cravings, you send out more tubes in all directions to search for food. Your hyphal tip is a sensitive probe attracted to water and nutrients. It monitors temperature, gravity, and light as you grow. Typically, it takes a few days or a week to grow an inch, although speed demons like the red bread mould, Neurospora crassa, need only six hours.12 The tip is also where enzymes ooze out to break down plant and animal tissues for food and energy. Your mycelium pushes out in a fan shape, winds into cords, or expands in a doughnut-like ring. The mycelium flows over wood like water, and the resulting pattern morphs into an irregular patchwork. Time-lapse videos of fungal colonies advancing over and around wood blocks show hyphae starting and stopping and changing direction. It looks as if the mycelium pauses to think about which way to turn. The colony sends out and recalls hyphal sentries, evaluating signals—for example, whether there is better food in one direction, or an enemy or a potential mating partner nearby—and then readjusts its growth.13
If you are a typical fungus, you will be a saprobe. Unable to make your own food as plants do with photosynthesis, and unable to attack a living host filled with easily absorbed sugars, you will wrap your hyphae around some dead organic debris. Biodegradation is the breakdown of organic matter, either waste products from living cells or the dead carcasses themselves, by saprobic microorganisms. For fungi, it is the equivalent of digestion. Since you have no hands or mouth, you pump more than a hundred different enzymes into the surroundings to sop up the minerals, vitamins, and sugars that you need through your hyphal tip for further biochemical processing. Crane-like molecules on the outside of your cells use grappling hooks to grab specific molecules, especially sugars, and pull them into your cells through specialized channels and pores. Breaking down these sugars and poly-saccharides gives you energy. After you finish, the leftovers—burps and other gases, all the by-product molecules, ions, and water liberated by your cellular biochemistry—flow back into your surroundings. It may seem gross, but almost all living organisms eat and excrete this way. Animals just hide this process inside.
Day after day, month after month, as a fungus you inch your way through your environment, away from your starting point. If you are blessed with a generous hyphal tip, it dispatches some nutrients back to the older cells left behind, like a grateful son or daughter sending care packages home. But as you continue to grow outwards, the old parts left behind die and slowly dissolve. This releases the precious nitrogen locked up in your proteins and your cytoplasm for reuse by the younger part of your colony. Wherever you are, you run out of nutrients or water eventually. The only thing to do is escape. That’s when you should make spores, which often happens in the autumn.
Sex is not your first option. Odds are that you are one of the fungi that make more than one kind of spore. It’s far easier to pump out thousands of asexual spores. They will all be identical clones of you, and the softest breeze will send them swarming like shock troops to attack new territory. Asexual spores skip the genetic exchange that defines sex. If you are one of those fungi that can make more than one kind of spore, and need more flexibility than a clone can offer, then sex is the way to go.
Humans tend to focus on the emotional and physical aspects of sex, overlooking the nifty evolutionary benefits that arise when genes from two individuals can mix. The first step of your fungal quest for union is familiar—you have to find a partner of a compatible mating type. In fungi, there are “males” (seekers) and “females” (receivers), which with an atypical scientific display of gender sensitivity and characteristic lack of poetic imagination, mycologists call A and a. Let’s say you are an A and want to meet an a. There are hormones for that. Your hyphal tip will aim towards any signal that suggests a receptive partner. When you find one, the a will look pretty much the same as you. Your hyphal tips will examine each other for a moment like two curious dogs booping noses. The same genetic compatibility quiz that you used to recognize your own clone reverses to make sure the cylinder of interest is from a different clone. Your tips fuse and your nuclei dance around inside the conjoined cell, then merge. The genetic gymnastics begin, the swapping of pieces of chromosomes and re-sorting of genes. Rearranging your genes increases the chance that your offspring can cope a bit better in a new environment. With luck, they will inherit some improved enzymes.14
If all goes well, a few more cell divisions and you will have four or eight spores (or sometimes many more). Unlike more nurturing humans, your uncaring ascus or basidium will blithely toss your spore children out into the world with only the nutrients they can carry, and only a copy of the family genetic guidebook to help them achieve success. Many will land close by, but spores of some plant diseases cross oceans looking for a new host. Others hunker down in soil or husks of dead plants and wait for spring.
Some basidios also have B and b mating genes. It’s an unusual strategy to have two sets of mating genes, and to work, each pair has to match to allow mating. There are four main genders (AB, ab, aB, Ab), but different members of one species can also have different variants of their A/a or B/b genes. If you count each combination of variants as a different gender and do the math for some species, you end up with about 23,000 genders.15 Hence, the flamboyant stories on the internet about fungi being exceptionally sexy.
The frequency of clones and the tendency to pump out thousands of asexual spores makes it quite likely that when you are on the hunt for love, you might encounter an identical twin. Imagine if every time you went out, you just kept meeting yourself. Your reaction would probably be “Oh no, not me again.” Mating with your own clone is like mating with yourself; it’s pointless because almost all genes are the same. A few fungi do go it alone because they are A and a at the same time; they clap their hyphae together and the sexual process begins. A few transgender yeasts switch back and forth between A and a during their life, but they still need a compatible partner if they are after sex.
The subconscious signal that leads you to your sexual partner is a chemical called a pheromone. Air, water, and soil are lush with small signaling molecules that elicit dramatic biological responses at very low concentrations. Your biochemical reactions, pathways, and by-products—the process known as metabolism—assemble and manipulate small molecules called metabolites. A common set of biochemical pathways hums along in the cells of all organisms. The process requires oxygen and chops sugars back into carbon dioxide, storing the released energy in a compound called adenosine triphosphate (ATP) that fuels the biochemical reactions of all cells. This is primary metabolism, or respiration. It is controlled by a core set of genes present in all organisms and powered within cells by tiny engines known as mitochondria. Metabolites produced in side reactions to the core biochemical cycles are secondary metabolites.16 They are the chemical signals that you and your hyphae react to—pigments, flavors, smells, invitations, warnings. If you want to learn a second language, this is the one to study because it has the most native speakers: all life-forms, even predominantly visual animals like humans, use many of the same metabolites that fungi do. But individual species combine these chemicals in unique ways or interpret the signals differently.
As a socially aware fungus, you will take part in these noisy chemical conversations, broadcasting molecular signals to and receiving them from other fungi and microbes in the vicinity. These signals will often involve volatile metabolites that float through the air like gases or dissolve in water, often odorless or tasteless but not always. You might register common fruity or fragrant molecules called aldehydes or alcohols that often indicate the availability of sweet food. Or foul-smelling compounds that contain chlorine or sulfur, usually meant as a warning. Hormones tend to be larger targeted metabolites that transmit precise information within a colony or body or between individuals of a species. We call the hormones that affect behavior, such as an interest in mating, pheromones.
There is a purpose to all this communication, of course. You are not alone in the world. There are many of your own kind, fungal colleagues, as well as species of all the other kingdoms of life. You will be bumping hyphae with the mycelium of other species all the time. Other fungi and bacteria may want to share the same space or pass through openings in your webby colonies. You will have to get used to an intimate, colonial lifestyle. Some neighbors will help you out as long as you are not after the same food. Some will be downright nasty and aggressive. You need to figure out who is cooperative, who is competitive, and how to work with them in either case.