Predatory slime mould freezes prey in large groups

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Predatory slime mould freezes prey in large groups

Category: Predators and prey

Posted on: December 18, 2008 7:30 AM, by Ed Yong

http://scienceblogs.com/notrocketscience/2008/12/predatory_slime_moul

d_freezes_prey_in_large_groups.php

This post is part of a celebration of the 2-year anniversary of open-

access journal PLoS ONE.

Gathering in large numbers is usually a good way of protection

yourself against predators, and it's no surprise that mass defence

is a common strategy in the natural world. But it doesn't always

work. There is one hunter that has found a way to use group defence

to its advantage. It allows its prey to gather in large numbers and

then freezes them in place with a chemical weapon, providing it with

a bountiful banquet to eat at its leisure. It's called Dictyostelium

caveatum.

D.caveatum is a member of the dictyostelids, a group that also goes

by the names of " slime moulds " or " social amoebae " . It consists of a

single cell and its rather unassuming amoeba-like appearance hides

the fact that it is a predator par excellence. Lacking fangs, claws

or any of the weapons of multi-celled creatures, it nonetheless has

highly effective ways of killing its prey - other very closely

related social amoebae.

The majority of dictyostelids, such as D.discoideum, are some of

nature's most vivid examples of cooperation. They live most of their

lives as single cells that eat bacteria, but a lack of food drives

them to seek out company. Like shattered pieces of a T-1000, single

cells move towards each other and stick together to form clumps

(left image). These clusters elongate to become multi-celled " slugs "

(middle), which in turn reach for the sky and transform

into " fruiting bodies " (right) - a long stalk topped by a ball of

spores. When food is available again, the spores are released and

become new amoebae, starting the cycle all over again. The cells

that make up the stalk are left to die, sacrificing themselves for

the future of their peers.

It's a lovely story, but add D.caveatum into the mix and you get a

very different ending. If a group of 10,000 D.discoideum cells is

invaded by even a single D.caveatum one, they are doomed. The lone

invader eventually eats the other species, using their cells as fuel

to produce its own fruiting bodies. After 48 hours, only D.caveatum

remains. This extraordinary behaviour was discovered about two

decades ago by one Waddell but only last year, Clement Nizak

from Rockefeller University managed to observe it for the first time

under the microscope and work out how it happens.

(Watch D.caveatum (the red arrows) devour D.discoideum (the green

arrows).)

Nizak loaded cells of different social amoebae with fluorescent

chemicals, so that he could track their movements under a

microscope. His images revealed that D.caveatum, sporting a green

glow, are surprisingly active and mobile. When they come across

amoebae of D.discoideum (looking fetching in fluorescent red), they

send out extensions called pseudopods that surround the prey within

seconds. Shortly after, the engulfed victim is broken down. Its only

hope of survival is in being large enough that the predator can't

completely surround it.

D.caveatum can also eat other social amoebae after they've started

to gather into groups as Nizak showed by cutting one of these in

half and using a microscope to identify the predatory cells within.

But that doesn't explain how a single D.caveatum can consume a group

of tens of thousands of other amoebae, before it can form a fruiting

body. It simply shouldn't have enough time.

Nizak found that D.caveatum gets around its limited time frame by

freezing the development of its prey species. When amoebae clump

together, they form a distinct mound that tips over to form the

front end of the slug. But even small concentrations of D.caveatum

cells can prevent the mound from forming and without it, the prey

are stuck in the cluster stage and slowly overwhelmed.

D.caveatum manipulates other social amoebae using a chemical that

the it constantly secretes. In a wonderfully elegant experiment,

Nizak separated D.caveatum and D.discoideum with a permeable filter

that prevented the two species from interacting but would allow

molecules to pass from one to the other. On its own, D.discoideum

wasn't affected by the filter but in the presence of D.caveatum

(either alone or in a mixture) its development was completely

frozen, even without any direct cell-to-cell contact.

Nizak used dialysis to isolate the secretions from D.caveatum and

found that these alone could halt D.discoideum's life cycle,

although for a shorter time than when actual predatory cells were

around. The precise identify of the chemical in question is still a

mystery, but one thing's for certain - it doesn't kill the prey

outright and its effects are reversible. If it's removed, the

majority of D.discoideum clumps soon resume their life cycle and

produce fruiting bodies. But in natural situations, they wouldn't

get the chance with D.caveatum cells still around to pump out more

inhibitors.

This ability to freeze the development of other social amoebae is

all the more remarkable for the sheer number of species that are

affected. Of the hundred or so species of social amoeba so far

discovered, D.caveatum is the only one that has evolved to exploit

and prey upon its own kind and indeed, it develops more quickly when

it's fed with other social amoebae than on the group's traditional

diet of bacteria. Nor is it fussy about its choice of prey. Nizak

found that they will gorge themselves on at least six different

types of distantly related social amoebae, and even one species that

isn't a dictyostelid. Whether it uses different chemicals for each

species is a mystery for now.

It only draws the line at cannibalism - if it bumps into cells of

its own kind, they go their separate ways. Indeed, D.caveatum is

immune to its own chemical weapon, although how it gets away with it

is unclear. For the moment, Nizak suggests that D.caveatum may have

evolved the ability to prey upon other dictyostelids by losing or

modifying the genetic pathways that normally prevent social amoebae

from eating each other. In the future, it may be possible to test

this idea by studying genetic changes in a mutant cannibal strain of

D.caveatum (also discovered by Waddell), which doesn't avoid

eating other cells of the same species.

(D.caveatum cells meet but don't eat one another)

For the moment, what we know of D.caveatum shows it to be a

surprisingly sophisticated predator that uses finely tailored

chemicals to alter the behaviour of its prey. There are plenty of

other similar examples in the natural world. Just last year, I

blogged about the emerald cockroach wasp which uses cockroaches as

living larders for their young. It injects them with a venom that

specifically reduces their motivation to walk, but not their general

motor skills. The upshot is that the wasp can grab a stung cockroach

by its antennae and walk it around like a dog on a leash.

Reference: Clement Nizak, J. Fitzhenry, H. Kessin

(2007). Exploitation of Other Social Amoebae by Dictyostelium

caveatum PLoS ONE, 2 (2) DOI: 10.1371/journal.pone.0000212

More on awesome predators:

The spider that crushes its prey with 140 metres of webbing

The mantis shrimp has the world's fastest punch

Prehistoric great white shark had strongest bite in history

Immune snakes outrun toxic newts in evolutionary arms races

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