http://www.eurekalert.org/pub_releases/2011-01/nsf-lha011911.php
Contact: Cheryl Dybas
cdybas from nsf.gov
703-292-7734
National Science Foundation
Like humans, amoebae pack a lunch before they travel
Amoebae increase survival odds through rudimentary form of
agriculture; finding has implications for human diseases
Some amoebae do what many people do. Before they travel, they pack a
lunch.
In results of a study reported today in the journal Nature,
evolutionary biologists Joan Strassmann and David Queller of Rice
University show that long-studied social amoebae Dictyostellum
discoideum (commonly known as slime molds) increase their odds of
survival through a rudimentary form of agriculture.
Research by lead author Debra Brock, a graduate student at Rice, found
that some amoebae sequester their food--particular strains of
bacteria--for later use.
"We now know that primitively social slime molds have genetic
variation in their ability to farm beneficial bacteria as a food
source," says George Gilchrist, program director in the National
Science Foundation's Division of Environmental Biology, which funded
the research. "But the catch is that with the benefits of a portable
food source, comes the cost of harboring harmful bacteria."
After these "farmer" amoebae aggregate into a slug, they migrate in
search of nourishment--and form a fruiting body, or a stalk of dead
amoebae topped by a sorus, a structure containing fertile spores. Then
they release the bacteria-containing spores to the environment as
feedstock for continued growth.
The findings run counter to the presumption that all "Dicty" eat
everything in sight before they enter the social spore-forming stage.
Non-farmer amoebae do eat everything, but farmers were found to leave
food uneaten, and their slugs don't travel as far.
Perhaps because they don't have to.
The advantages of going hungry now to ensure a good food supply later
are clear, as farmers are able to thrive in environments in which non-
farmers find little food.
The researchers found that about a third of wild-collected Dicty are
farmers.
Instead of consuming all the bacteria they encounter, these amoebae
eat less and incorporate bacteria into their migratory systems.
Brock showed that carrying bacteria is a genetic trait by eliminating
all living bacteria from four farmers and four non-farmers--the
control group--by treating them with antibiotics.
All amoebae were grown on dead bacteria; tests confirmed that they
were free of live bacteria.
When the eight clones were then fed live bacteria, the farmers all
regained their abilities to seed bacteria colonies, while the non-
farmers did not.
Dicty farmers are always farmers; non-farmers never learn.
Rice graduate student Tracy Douglas co-authored the paper with Brock,
Queller and Strassmann. She confirmed that farmers and non-farmers
belong to the same species and do not form a distinct evolved group.
Still, mysteries remain.
The researchers want to know what genetic differences separate farmers
from non-farmers. They also wonder why farmer clones don't migrate as
far as their counterparts.
It might be a consequence of bacterial interference, they say, or an
evolved response, since farmers carry the seeds of their own food
supply and don't need to go as far.
Also, some seemingly useless or even harmful bacteria are not consumed
as food, but may serve an as-yet-undetermined function, Brock says.
That has implications for treating disease as it may, for instance,
provide clues to the way tuberculosis bacteria invade cells, says
Strassmann, infecting the host while resisting attempts to break them
down.
The results demonstrate the importance of working in natural
environments with wild organisms whose complex ties to their living
environment have not been broken.
