Some bacteriuns also betray their families and live unrestrainedly at the expense of their neighbors. But still: If the going gets really tough for everyone, some social parasites can also return penitently to the bosom of the community and to the path of virtue. Myxococcus xanthus is a social bacterium: the microbes move together, hunt together, and when food is scarce, they combine into fruiting bodies of over 100,000 cells. However, only a few bacteria survive the hard times as resistant spores. Of course, this situation provides a good starting point for opportunistic microbial scammers: some strains of Myxococcus xanthus let themselves be nourished by the loss-prone fruiting bodies, but do not contribute to their formation themselves.
But the parasite tribes have to wait for the right time for this. To do this, they eavesdrop on the cooperating bacteria as soon as they communicate biochemical signals about when they need to join together to form a fruiting body. The parasitic strains cannot release these messenger substances themselves, but with the help of signals from their cooperative fellow species, they transform into spores much more efficiently than they do. When a population of cooperating and cheating bacteria goes through repeated periods of deprivation, the cheats gradually gain the upper hand-until there are too few cooperating bacteria left calling to form a fruiting body. In many cases, the population then dies out at the next emergency. Biologists refer to this as evolutionary suicide.
But it doesn't have to come to that, as researchers led by Gregory Velicer from the Max Planck Institute for Developmental Biology in Tübingen discovered in a population of cooperating bacteria that they mixed with a fraudulent strain. Over several cycles of food scarcity and abundance, this rapidly gained the upper hand over the cooperating bacteria. Eventually, the fraudulent tribe nearly wiped out the entire population. In another cycle, however, the scientists suddenly found spores again. Apparently, a new generation of formerly cheating myxococci has evolved so that the bacteria can once again cooperate.
Now the scientists searched the genome for changes that triggered resocialization. They found that a single mutation was enough to turn the cheating bacteria back into cooperating bacteria. The mutation caused the new strain - inspired by the ancient myth scientists call it the phoenix - to produce higher levels of the enzyme acetyltransferase than its ancestors. It also sporulated even more effectively than its cooperating relatives. The enzyme triggers a previously unknown mechanism that influences social behavior. However, the researchers do not yet know exactly how this mechanism works. In any case, he also ensures that Phoenix no longer allows himself to be exploited by his fraudulent ancestors. The new strain soon dominated the entire population.
Biologists call the fraudulent behavior of some myxococcal strains obligate: like parasites or symbionts, they depend on other organisms, in this case their conspecifics. Until now, biologists did not know whether organisms can shed this dependent way of life. And how long, if any, it remains possible for an obligate way of life to revert to an independent one. According to the findings of the scientists, at least the myxococci keep this chance for several generations. They have even taken a major evolutionary step forward, via the intermediate step of obligatory fraudulent behavior. Because the bacteria of the Phoenix tribe were more successful with their social way of life than the cooperating tribes that had supplanted the scammers. With their work, the scientists provide an insight into how social systems develop. © Max Planck Society
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