London, February 14 : Scientists at Rice University and Baylor College of Medicine (BCM) have found that even the simplest social creatures - the amoebae Dictyostelium discoideum - have over 100 genes that regulate their cooperative behaviour.
This finding is a result of the first ever genome-wide search for genes that govern social behaviour. The researchers describe their research as a large-scale attempt to combine evolutionary biology with genomics in a systematic search for genes tied to social behaviour.
"This pool of genes is going to allow us to understand the genetic architecture of social behaviour," Nature magazine quoted Joan Strassmann, Rice's Harry C. and Olga K. Wiess Professor of Ecology and Evolutionary Biology, as saying.
It took five years for the researchers to conduct the genome-wide investigation, and they use the detailed study of some 10,000 randomly mutated strains of D. discoideum for the purpose.
"The basic idea was to knock out genes at random and put each mutant through 10 rounds germination, growth and development to identify mutations that led to cheating," Strassmann said.
While social cooperation among microbes is little understood, it causes major medical and industrial problems. Medically, cooperation underlies conditions as mundane as tooth decay to more serious conditions like chronic infections associated with medical implants. Industrially, slimy colonies of bacteria also foul filters at water treatment plants and other facilities, causing millions of dollars of damage each year.
Strassmann has revealed that during the study, cheating mutations were found in more than 100 genes. As there are advantages to be gained form cheating, the researcher said that the real mystery from an evolutionary point of view was how species like D. discoideum managed to keep cheaters from out-performing and eliminating cooperation altogether.
"Cheating is to be expected. Cooperation is the real story. Since cheaters can thrive without these 100-plus genes, there has to be some other reason that they're still in the genome," Strassmann added,
Though loners in times of plenty, D. discoideum form colonies when food is scarce, and work collectively to ensure their survival. About one-fifth of the colony's individuals form a tall, thin stalk, and the rest climb the stalk and clump together into a small bulb that can be carried away to better environs by the wind or on the legs of passing insects.
The simple social system poses an evolutionary conundrum for biologists; the individuals in the stalk give themselves up altruistically to support the colony, so what's to keep more selfish strains of D. discoideum from cheating the system, avoiding the stalk and out-reproducing their altruistic neighbours?
Strassmann's team had identified a handful of cheater mutations for D. discoideum in prior studies. With a view to finding out exactly how they cheated, the researchers subjected cheaters to additional tests. They also examined the cheaters' genetic code to locate the precise site of the cheater mutations.
The tests showed that cheaters would exploit virtually any advantage to increase their share of spores in the next colony.
While strategies for cheating varied at the proteomic level, the study found some cheaters use a common genetic strategy: they piggyback onto other essential functions.
In a previous study, the researchers had identified a cheater that used the same strategy. , Strassmann said the broader study indicated that the "dual-function strategy" might be shared by other successful cheaters.
"The evolutionary opportunities for moves and countermoves appear to create a kind of genetic arms race in which cheating mutations are met with counter-mutations. In this arena, cheating is often going to be piggybacked onto essential functions, making it hard to get rid of and hard to control," Strassmann said.