Small Things Considered

Is a Good Offense the Best Defense?

by Merry

The budding yeast, S. cerevisiae. © Eye of Science/
Photo Researchers, Inc. Used with permission. Source.

Most eukaryotes possess an RNA interference system (RNAi) that they use to regulate gene expression and to defend against viruses and other mobile elements. However, some budding yeasts, such as Saccharomyces cerevisiae, appear to get along just fine without it even though RNAi has benefited other yeasts by silencing transposons in particular. How do these yeasts that lack functional RNAi systems compete with closely related species that do? And how come they don’t have RNAi when RNAi arose in an early eukaryote ancestor and is conserved throughout most of the fungi?

Bacteria Activate Fungal Gene Clusters

by S. Marvin Friedman

Legend: Orsellinic acid. Source.

Fungi are notorious for their ability to produce a wide variety of secondary metabolites, including antibiotics, statins, immunosuppressants, mycotoxins, and others. A veritable pharmacopeia. Interestingly, many of the gene clusters involved in the biosynthesis of these compounds are silent under normal laboratory conditions. In some cases, the synthesis of these compounds is known to depend on a symbiotic relationship with bacteria. Previous research has shown that intimate physical contact between the model fungus Aspergillus nidulans and the soil bacterium Streptomyces rapamycinicus activates the fungal ors gene cluster that encodes one of the fungal secondary metabolites, orsellinic acid and its derivatives. The effect is impressive: gene expression is 5 orders of magnitude greater when the fungus and bacterium are co-cultivated. Orsellinic acid is a polyketide, a large group of organic compounds that include antibiotics such as erythromycin, tetracyclines, and amphotericins. Orsellinic acid has pharmacological activities, including radical scavenging.

In filamentous fungi, the regulation of secondary metabolism involves the post-translational modification of histones. Nutzmann and coworkers have now analyzed the effect of several such epigenetic modifiers on the synthesis of orsellinic acid by A. nidulans co-incubated with S. rapamycinicus. Adding an inhibitor of histone acetyltransferase (HAT), lecanoic acid,  blocked transcription of the orsA gene. On the other hand, a histone deacetylase inhibitor, suberoylanilide hydroxamic acid, activated the orsA gene without the need for co-incubation with S. rapamycinicus.

How to Escape a Deadly Embrace

This is the last of three installments celebrating the Week of the Fungi on STC.

by Elio

A network of fungal hyphae modified for nematode-
trapping. Source.

The space around a single soil particle can be one hell of a battlefield. Here, nematode worms consume bacteria, bacteria fight back and kill them, fungi suck the worms dry, and so on. Not a moment of peace. These battles are as intense as they are intricate, yet we know little about all this drama. Honest, if it weren’t for C. elegans, nematodes would hardly be in my consciousness despite their enormous abundance and ecological importance in soils and waters. They are found nearly everywhere. They are major players in the McMurdo Dry Valleys of Antarctica, and have recently been found one kilometer down in a gold mine, where they feed on bacterial biofilms growing on the walls. Nematodes have their bard: a delicious 1914 article by N.A. Cobb graciously extols their glory. But this being Fungus Week at STC, I choose to relate a nematode story of life in the soil that involves fungi.

The King of the Fungi

This is the second of three installments celebrating the Week of the Fungi on STC.

by Elio

A basidiocarp (fruiting body) of Fomitiporia ellipsoidea.
(Do not try to flip it over. It is in the correct orientation.)
Source.

Small things are not being considered here. Au contraire, I'm reporting on the largest fungus known, if by 'fungus' you mean the fruiting body of the organism. The reason for making this distinction is that a fungal mycelium can reach humongous proportions. The hyphae of a kind of oyster mushroom, Armillaria ostoyae, growing in northeastern Oregon covered some 10 km² and were nearly 9000 years old.

What I have in mind here is the finding on southern China’s Hainan Island of a gigantic fruiting body of a bracket fungus or polypore, Fomitiporia ellipsoidea. This gargantua was a tad over 10 meters long, almost 90 cm wide, and about 5 cm thick (picture half the length of a bowling alley). Its weight was estimated to be between 400 and 500 kg. It was found in a virgin forest where it adorned the underside of the large fallen trunk of an oak tree (Cyclobalanopsis patelliformis). In the words of a fellow blogger who writes Real Monstrosities: ….that's a big shelf. Maybe even a bed. A long bed upon which about 5 people could sleep in single file. If they wanted to….. It's estimated to be around 20 years old, contain about 450 million spores and weigh about half a tonne. So definitely sleep on it and not under it.

Fungi That Spit Like Baseball Players

This is the fourth annual Week of the Fungi on STC, a sporadic undertaking. It is our way to hail the start of the fall mushroom collecting season in parts of our home territory (the northern hemisphere).

by Gemma Reguera

Although I was a tomboy as a child, I never had the stomach for spitting competitions. But that changed recently when I read an article by Nik Money and collaborators about spitting competitions among fungi. Yes, you heard me right. Some fungi spit, and they do so with amazing precision and speed. It has been long known that some fungi in the Ascomycota and Zygomycota use squirt gun mechanisms to launch their spores. But the speeds and distances that some of them can achieve have only recently been measured with precision, thanks to high speed video capture microscopy.

One group of fungi, the so-called coprophilous (which literally means 'dung-loving'), have mastered this technique and can launch spores at speeds of 4-21 meters per second! The gold medal in acceleration goes to Ascobolus immersus, which sets the record at 1,800,000 m s^–2. Such acceleration, the authors note, is the fastest recorded flight in nature. To put this number in context, a bullet leaving the muzzle of a rifle barrel approximately 0.9 m long at a speed of 600 m s^–1 will reach an acceleration of 200,000 m s^–2. The record holder for distance is Pilobolus kleinii. This fungus makes some sexy-looking fruiting bodies to propagate its spores and spits like a baseball player. Not surprisingly, it already caught the attention of Elio in two earlier postings (click here and here). Their sporangia (aka individual spores wrapped in a protective enclosure) can land as far as 2.5 meters away from the tip of the sporangiophore (aka the fungal squirt gun). Now, keep in mind that the Pilobolus fruiting body is not even a twentieth of an inch tall!

Candida's Unstable Chromosomes & Unorthodox Sex

by Dean Dawson

Candida albicans. Source.

Who hasn't heard of Candida? It’s one of the most common fungal pathogens of humans. It is also a commensal organism, living mainly in people’s gastrointestinal tract. The diseases it causes range from a fairly mild vaginitis to deadly opportunistic systemic infections. In fact, Candida species are a major cause of nosocomial bloodstream infections. Candidas are close relatives of the baker’s yeast, Saccharomyces cerevisiae, but live by very different rules.

Candida has been the focus of intense research at the molecular level for only the past fifteen years or so, and it has not been an easy road. These organisms, it turns out, have a slightly different genetic code than most others, rendering the standard molecular tools less useful. Additionally, Candida does not exhibit the mating and spore formation behaviors of its cousins Saccharomyces cerevisiae and Aspergillus, which made them popular organisms for fast-moving molecular genetic studies. But the development of new tools and a growing community of researchers have revealed in the past few years that Candida has evolved its own unorthodox mechanisms and lifestyles that probably greatly enhance its ability to adapt to changes in its host environments.

The Great Epidemic

by Merry

The "Redwood of the East." Source.

When you read the title—The Great Epidemic—what came to mind? The Black Death (Yersinia pestis) that in two years killed 20 million people in Europe—approximately 30-60% of the population? The 1918 flu pandemic with its tally of 50 million dead in three years? AIDS, with a death toll projected to reach 200 million by 2025? Or perhaps that 20^th century epidemic that struck down over three and a half billion in North America in the space of a few decades—the American chestnut blight? These chestnut trees, Castanea dentata to be precise, were stately giants often 100 feet or more in height with crowns that spanned 100 feet. Their straight trunks provided billions of dollars worth of beautiful, rot-resistant wood, and the bountiful nuts provided far more than the traditional stuffing for Thanksgiving turkeys. Combined they had made up a quarter of the forest canopy from Maine to Mississippi.

One Symbiont Is Good, Two Are Better: The Forever Fascinating Story of the Leaf-Cutting Ants and Their Bacteria

by Elio

Leaf- cutting ants returning to their nest. Source.

Now here’s a question you’ve been asking all along about the interaction between the leaf cutting (Attine) ants, the fungi they cultivate, and the bacteria that make antifungals against unwanted fungal species. Have these bacteria evolved along with the ants to protect their gardens from unwanted "weeds," or do the ants pick up such bacteria from their environment? New data suggest both things happen.

To remind you, leaf-cutting ants practice fungiculture, and have been doing this for about 50 million years. The bits of leaves and flowers that they bring to the nest get chewed up, fertilized, placed in suitable “gardens” within the nest, and seeded with fungal material from previous gardens. Obviously, fungal gardens can be overrun by unwanted species, with disastrous results for the colony. To keep this from happening, the ants depend on selective antifungals made by actinomycetes. We have visited this topic in the past (click here and here).

Trees─Phylogenetic and Real

This is the third post for this year's Week of the Fungi on STC, a sporadic undertaking. This annual festival is our way to hail the start of the fall mushroom collecting season in parts of our home territory (the northern hemisphere).

by Elio

Cyttaria darwinii in Patagonia. Courtesy of Janet Fraser.

Picture yourself walking in a woods where the branches of the trees are festooned with bouquets of yellow objects that on close inspection look a bit like golf balls, dimples and all. This is what you would come across in the beech forests of the southern hemisphere. The yellow balls are fruiting bodies of members of the genus Cyttaria, ascomycetes just like baker’s yeast and morel mushrooms. Cyttarias are plentiful. When mature, they fall off their branches, making a layer up to 15 cm deep on the forest floor. Hard to miss.

Beech Trees, Lake Hauroko, New Zealand. Source.

Cyttarias caught the attention of Charles Darwin during his visit to Tierra el Fuego. He noted that the natives, the Yaganes, ate these mushrooms, although oddly they bypassed fresh specimens in favor of older, wizened ones. Some years ago, I came up with a possible explanation. Uniquely among mushrooms, Cyttaria have a concentration of fermentable sugars. Indeed, in Chile some people use them for the production of an alcoholic beverage called “chicha de llau-llau.” So, could it be that the natives of Tierra del Fuego favored the older specimens undergoing fermentation? These people were surprisingly hardy; they were very scantily dressed, yet living under very harsh climatic conditions. I posited that a little alcohol from fermented cyttarias may have gone a long ways towards good cheer (Ref. 1). Nobody has come up with a better idea.

Color Me Aphid

This is the second post for this year's Week of the Fungi on STC, a sporadic undertaking. This annual festival is our way to hail the start of the fall mushroom collecting season in parts of our home territory (the northern hemisphere).

by Elio

Is it OK with you if on Fungus Week we post an item that deals with fungal genes instead of whole fungi? We want to do this because of a startling finding, that some aphids are colored because they possess fungus-derived genes for making carotenoids. This is news because until now it was thought that animals cannot synthesize such pigments—only prokaryotes, fungi and plants can (which is why vitamin A—a carotenoid—is a vitamin). But there you have it, some aphids are green and others are reddish depending on which carotenoids they make. It had been thought that these animals get their carotenoids either from their diet or from bacterial endosymbionts. However the sap they live on has no such things, and their symbionts lack the genes for carotenoid biosynthesis. In addition, curing the insects of the symbionts does not affect the color. Lastly, the symbionts are inherited maternally, whereas color follows Mendelian inheritance.

        Coloration of pea aphids. (A) typical green; (B) typical red; (C) a green mutant that arose
        from a red clone. Source.

Of course color matters. Animals use color to respond to enemies, find food, and in some cases, seek out a mate. The predators of the aphids are good at telling colors. Lady beetles find red aphids to their liking, whereas certain wasps prefer green ones. And who knows in how many other ways color makes a difference to these insects!