Resource spending between microbes Like many other organisms, bacteria rely on iron intake for its biological processes. However, iron is sometimes difficult to access in certain environments, like soil. Some bacteria have evolved
siderophores, iron-chelating particles that seek and bring back iron for the bacteria. Siderophores are not necessarily specific to its producer—sometimes another individual could take up the particles instead.
Pseudomonas fluorescens is a bacterium commonly found in the soil. Under low-iron conditions,
P. fluorescens produces siderophores, specifically
pyoverdine, to retrieve the iron necessary for survival. However, when iron is readily available, either from freely diffusing in environment or another bacterium's siderophores,
P. fluorescens ceases production, allowing the bacterium to devote its energy towards growth. One study showed that when
P. fluorescens grew in association with
Streptomyces ambofaciens, another bacterium that produces the siderophore
coelichelin, no pyoverdine was detected. This result suggested that
P. fluorescens ceased siderophore production in favor of taking up iron-bound coelichelin, an association also known as siderophore piracy.
Selection pressure in Pseudomonas aeruginosa By definition, individuals cheat to gain benefits that their non-cheating counterparts do not receive. This raises the question of how a cooperative system can exist in face of these cheaters. One answer is that the cheaters actually have a reduced fitness compared to the non-cheaters. In a study by Dandekar et al., the researchers examined the survival rates of cheating and non-cheating bacteria populations (
Pseudomonas aeruginosa) under varying environmental conditions. These microorganisms, like many species of bacteria, use a cell-cell communication system called
quorum sensing that detect their population density and prompt the transcription of various resources when needed. In this case, the resources are publicly shared proteases that break down a food source like casein, and privately used adenosine hydrolase, which breaks down another food source, adenosine. The problem arises when some individuals ("cheaters") do not respond to these quorum sensing signals and therefore do not contribute to the costly protease production yet enjoy the benefits of the broken down resources. When
P. aeruginosa populations are placed into growth conditions where cooperation (and responding to the quorum signal) is costly, the number of cheaters increases, and the public resources are depleted, which can lead to a
tragedy of the commons. However, when
P. aeruginosa populations are placed into growth conditions with a proportion of adenosine, the cheaters are suppressed because the bacteria that respond to the quorum signal now produce adenosine hydrolase that they privately use for themselves to digest adenosine food source. In wild populations where the presence of adenosine is common, this is an explanation for how individuals that cooperate could have higher fitness than those that cheat, thereby suppressing the cheaters and maintaining cooperation.
Policing/punishment in insects Cheating is also commonly found in insects. The social and seemingly altruistic communities found in insects such as ants and bees provide ample opportunities for cheaters to take advantage of the system and accrue additional benefits at the expense of the community. Sometimes, a colony of insects is called a "
superorganism" for its ability to take on properties greater than those of the sum of individuals. A colony of insects in which different individuals are specialized for specific tasks means a greater colony production and greater efficiency. Moreover, based on the
kin-selection theory, it is collectively beneficial for all the individuals in the community to have the queen lay eggs rather than the workers lay eggs. This is because if the workers lay eggs, it benefits the egg-laying worker individually, but the rest of the workers are now twice removed from this worker's offspring. Therefore, though it is beneficial for one individual to have its own offspring, it is collectively beneficial to have the queen lay the eggs. Therefore, a system of worker and queen policing exists against worker-laid eggs. One form of policing occurs by the oophagy of the worker-laid eggs, found in many ant and bee species. An example of a combination of queen and worker policing is found in ants, in the genus
Diacamma, in which worker-laid eggs are taken by other workers and fed to the "queen". In general, these signals that identify the eggs as queen-laid are likely incorruptible, since it must be an honest signal to be maintained and not be used by cheating workers. In fact, 91% of the worker-laid eggs were policed within one day. They also found that about 20% of workers laying eggs were prevented from doing so through both the queen's and workers' aggressive behavior. The workers and the queen would grab the egg-laying worker and try to sting her or push her off the cell. This usually results in the worker removing her abdomen and not depositing her eggs.
Policing/punishment in other organisms Aggression and punishment are not just found in insects. For example, in
naked mole-rats, punishments by the queen are a way she motivates the lazier, less-related workers in their groups. The queen would shove the lazier workers, with the number of shoves increasing when there are fewer active workers. Reeve found that if the queen is removed when colonies are satiated, there is a significant drop in weight of the active workers because the lazier workers are taking advantage of the system. Punishment is also a method used by cichlid
Neolamprologous pulcher in their cooperative breeding systems. It is a pay-to-stay system where helper fish are allowed to stay in certain territories in exchange for their help. Similar to the naked mole rats, the helpers that were prevented from helping, the "idle helpers", receive more aggression than control helpers in the study. Researchers theorize that this system developed because the fish are usually not closely related (so kinship benefits have little impact), and because there is a high level of predation risk when the fish is outside the group (therefore a strong motivator for the helper fish to stay in the group). These results show that aggression as punishment is a way to encourage members to work together and share food when it is found.
Interspecific countermeasures Cheating and constraints of cheating are not limited to intraspecific interactions; it can also occur in a mutualistic relationship between two species. A common example is the mutualistic relationship between cleaner fish
Labroides dimidiatus and reef fish. Bshary and Grutter found that cleaner wrasse prefers the client tissue mucus over ectoparasites. This creates a conflict between the cleaner fish and reef fish, because the reef fish only benefit when the cleaner fish eats the ectoparasites. Further studies revealed that in a lab setting, the cleaner fish undergoes behavioral change in face of deterrents against eating their preferential food. In this relationship, nitrogen fixing bacteria
rhizobium fixes atmospheric N2 from inside the roots of leguminous plants, providing this essential source of nitrogen to these plants while also receiving organic acids for themselves. However, some bacteria are more mutualistic, while others are more parasitic because they consume the plant's resources but fix little to no N2. Moreover, these plants cannot tell whether the bacteria are more or less parasitic until they are settled in the plant nodules. To prevent cheating, these plants seem to be able to punish the rhizobium bacteria. In a series of experiments, researchers forced non-cooperation between the bacteria and the plants by placing various nodules in nitrogen-free atmosphere. They saw a decrease in the rhizobium reproductive success by 50%. West et al. created a model for legume sanctioning the bacteria and hypothesizes that these behaviors exist to stabilize mutualistic interactions. Thus there is selection against the "cheaters" who try to use the yucca plant without providing the benefits of pollination. ==References==