Small Things Considered

An Evolutionary Tale of Zombie Ants and Fungal Villains & Knights

by Gemma Reguera

Fig. 1: An ant infected by O. unilateralis bites the underside of a
leaf. The fungal stroma emerges from the back of the ant’s head and
develops a fruiting body with a capsule full of spores. Once matured,
the capsule is released and the spores disperse on the forest floor.
Courtesy of David P. Hughes. Source.

In a recent post I shared with you some amazing things I had learnt about coprophilous (‘dung-loving’) fungi that spit their spores like pros. What I did not tell you then is that my six-year-old son also fell in love with the spitting fungi (dung + spit = child’s interest!) and wanted to learn more. So we spent hours watching online videos until we stumbled upon a BBC’s Planet Earth video narrated by the great David Attenborough about ant parasitic fungi in the genus Cordyceps. The video shows a carpenter ant (genus Camponotus) that has been infected by spores of the fungus Ophiocordyceps unilateralis. The spores germinate inside the ant’s respiratory track and the mycelia grow towards the brain while feeding on soft tissues. Once the fungus reaches the brain, it induces behavioral changes such that the ant climbs up vegetation and bites the underside of the leaves. There the ant awaits its death while the fungus continues to grow within. The stroma stalk of the fungus eventually protrudes from the back of the ant’s head and a fruiting body bearing a capsule filled with spores forms near its tip (Fig. 1). Once the spores are sexually mature, the capsule is released, and then explodes, either in the air or upon hitting the ground. This delivers the spores into the path of healthy ants, there to start a new cycle of infection.

It’s Raining Viruses!

by Merry

Caterpillar cadavers containing viral particles (OBs) are
eaten by predatory invertebrates such as this spider.
Since the alkaline pH of the insect midgut is required to
release the virions from the OBs, the OBs pass through
the predator’s digestive tract intact and are deposited
in their feces. In this way the virus gets a free ride across
relatively large distances, there to await ingestion
by a hapless caterpillar. Source.

It’s true! Each year it rains viruses, more than a trillion of them per acre over thousands of forested acres in the USA. This is the work of the airborne arm of the USDA Forest Service, part of their efforts to reduce the devastation to hardwood forests caused by the imported gypsy moth, Lymantria dispar. Each year they spray with a registered “general use pesticide” called GYPCHEK, the active ingredient of which is Lymantria dispar multinucleocapsid nuclear polyhedrosis virus (LdMNPV), an ideal ‘green’ pesticide. The virus is host-specific, self-replicating, biodegradable, and deadly. And I would add, it makes a most interesting story, provided you’re not a caterpillar.

LdMNPV is a member of the genus Alphabaculovirus, family Baculoviridae. These viruses infect arthropods, most commonly lepidopteran larvae—what you and I would call caterpillars. Because one virus can efficently convert one caterpillar into more than 10⁹ viruses, with each of those progeny poised to repeat the process, they have received attention as potential biological control agents to help us in our perpetual war against the insects.

SEM of baculovirus OBs (polyhedra). Arrows
indicate three polyhedra of different sizes.
Bar = 2 μm. Source.

Viruses are supposed to be small, something you look at with an electron microscope. But these viruses form unusually large polyhedral particles that range in size from 0.15 to 15 μm, thus are readily visible at 400× or 100× magnification. The particles, called occlusion bodies or OBs, are how the virus journeys from one caterpillar to the next. An OB is made up of many virions embedded within a stable, paracrystalline protein matrix. Nested inside each virion are several rod-shaped nucleocapsids, a nucleocapsid being a protein shell enclosing a copy of the viral genome. The whole conglomeration is assembled in the nucleus of the host cell. Thus the multinucleocapsid and the nuclear in the virus’s name. What about the polyhedrosis? This is a general term for any disease of caterpillars that is caused by polyhedral virus particles and that leads to the gruesome liquefaction of the host and the accumulation of OBs in the fluid.

The Leopard and the Mouse: A Microbiologist's Take

by Fred Neidhardt

Photo credit: Casey Gutteridge/Solentnews.biz.

Nineteen-year-old photography student Casey Gutteridge captured this extraordinary scene at the Santago Rare Leopard Project in Hertfordshire, UK. Casey, who was photographing the leopard Sheena for a course project, said: I have no idea where the mouse came from—he just appeared in the enclosure after the keeper had dropped in the meat for the leopard. He didn't take any notice of the leopard, just went straight over to the meat and started feeding himself. But the leopard was pretty surprised—she bent down and sniffed the mouse and flinched a bit like she was scared. In the meantime the mouse just carried on eating like nothing had happened. But even a gentle shove does not deter the little creature from getting his fill...the mouse continued to eat the leopard's lunch and show the leopard who was boss! Sheena was brought in to the Santago Rare Leopard Project from a UK zoo when she was just four months old.

So, what was going on? How can one explain this unusual behavior of (likely) predator and (likely) prey?

Acoustical Mimicry

by Elio

Myrmica schencki. Source.

We seldom stray from microbial domains, but here is a stunning (and delightful) reason for wandering further afield. After all, examples of parasitism are surely not confined to the Small Things and lessons can be learned wherever they appear. According to a group of researchers from England and Italy, parasites of some ants mimic the sounds made by the queen of a colony. The ants in question are a red European species, Myrmica schencki. Their queens make distinctive sounds by scraping a plectrum across a file, somewhat like a washboard player who makes music by scratching his instrument. Worker ants use the same mechanism to make their own refrain, but they differ enough morphologically from the queens that their instruments produce a recognizably different sound. (The ants can tell them apart. Can you? Click here to download audio clips.) Much like a Zydeco player, the royal performer makes sounds that “elicit benevolent responses from workers, reinforcing their supreme social status.” (Source) This behavior can be observed in the laboratory, using appropriate tapes and speakers.

Tales From Planet Fungus

by Ilham Naili

It was two years ago that I offered to my sister for her birthday the DVD set of Planet Earth (wonderful tales by David Attenborough that I recommend to everybody), and I started to watch some episodes with her. A particular episode was memorable for both of us, as we watched an ant being literally possessed by an infecting fungus. It was such a strange notion, that this ant could be manipulated to leave its colony, climb upwards on a stem, clamp its mandibles to securely hold the stem…and die there! The vision of this fungus slowly growing out of the ant’s head was by far the strangest and most amazing thing that I got to see on television. My little sister, who never got to close her mouth during this whole time, finally looked at me with her big eyes and said, “This is SO cool!! How does it work?? Have you ever studied this in one of your classes?”

From David Attenborough’s BBC Planet Earth. Programme 3: Jungles. 2006

“No”…but it is not true anymore.

Fungal Cuckoo

A Common Cuckoo being raised by
a Reed Warbler. Source: Wikipedia

Cuckoo birds have an unsavory reputation for laying their eggs in somebody else’s nest. There the eggs are tended with care by duped surrogate parents. When hatched, the young cuckoos unceremoniously push their nestmates over the edge and, in a case of consummate avian chutzpah, open their maw to make their "hosts" feed them as if they were their own. By the time they are fledglings, the young cuckoos may be considerably larger than their foster parents.

Nu, you ask, why this feathery preamble? The reason is that this brood parasitism, as the phenomenon is called, has its counterpart in the fungal world.

Dive! Dive! Dive!

By Eddy Mateescu

This urgent call, borrowed from movies about submarine warfare, applies to this non-microbial posting that is intended as an appetizer for the fascinating subject of parasite modification of host behavior. Elio promises to follow up on this subject.

Source: Shaoni Bhattacharya (2005)
Parasites Brainwash Grasshoppers Into
Death Dive. New Scientist 31: August.

In the most general sense, all living organisms must be able to detect specific, essential aspects of their environment and to react appropriately. Therefore, any component involved in the detection-decision process (from the uptake/sensory machinery and its output to the regulation/decision making system) is susceptible to manipulation by a parasite for its own advantage.

An interesting non-microbial example is that of the parasitic Nematomorph worms which brainwash grasshoppers and crickets to leave their forest habitat and take a fatal plunge into water so that the worms can emerge and mate (Movie).