measured the fitness of a cheater type (who possessed a reduced number of invertase genes) relative to a cooperator (who contained all possible invertase genes). by manipulating the level of social interaction within the community by varying the population density, they found that the cheater is less fit than the cooperator at low levels of sociality, but more fit in dense communities. therefore, they propose that selection for "cheating" causes natural variation in the amount of invertase genes an individual may possess, and that variation in invertase genes reflects constant adaptation to an ever-changing biotic environment that is a consequence of the instability of cooperative interactions. altruism the second type of cooperative interactions is altruistic, or interactions that are beneficial to the recipient but costly to the actor (-/+). justifying the evolutionary benefit of altruistic behavior is a highly debated topic. a common justification for the presence of altruistic behaviors is that they provide an indirect benefit because the behavior is directed towards other individuals who carry the cooperative gene. the simplest and most common reason for two individuals to share genes in common is for them to be genealogical relatives (kin), and so this is often termed kin selection. according to hamilton, an altruistic act is evolutionarily beneficial if the relatedness of the individual that profits from the altruistic act is higher than the cost/benefit ratio this act imposes. this rationale is referred to as hamilton's rule. natural selection normally favors a gene if it increases reproduction, because the offspring share copies of that gene. however, a gene can also be favored if it aids other relatives, who also share copies. therefore, by helping a close relative reproduce, an individual is still passing on its own genes to the next generation, albeit indirectly. hamilton pointed out that kin selection could occur via two mechanisms: (a) kin discrimination, when cooperation is preferentially directed toward relatives, and (b) limited dispersal (population viscosity), which keeps relatives in spatial proximity to one another, allowing cooperation to be directed indiscriminately toward all neighbors (who tend to be relatives). in microbial systems, these two mechanisms are equally important. for example, most microbial populations often begin from a small number of colonizers. because most microbes reproduce asexually, close genetic relatives will surround cells as the population grows. these clonal populations often result in an extremely high density, especially in terrestrial systems. therefore, the probability that a cells altruistic behavior will benefit a close relative is extremely high. while altruistic behaviors are most common between individuals with high genetic relatedness, it is not completely necessary. altruistic behaviors can also be evolutionarily beneficial if the cooperation is directed towards individuals who share the gene of interest, regardless of whether this is due to coancestry or some other mechanism. an example of this is known as the "green beard" mechanism, and requires a single gene (or a number of tightly linked genes) that both causes the cooperative behavior and can be recognized by other individuals due to a distinctive phenotypic marker, such as a green beard. the most studied slime mold from this perspective is dictyostelium discoideum, a predator of bacteria that is common in the soil. when starving, the usually solitary single-celled amoebae aggregate and form a multicellullar slug that can contain 104–106 cells. this slug migrates to the soil surface, where it transforms into a fruiting body composed of a spherical tip of spores and a stalk consisting of nonviable stalk cells that hold the spores aloft (figure 2). roughly 20% of the cells develop into the non-reproductive stalk, elevating the spores and aiding their dispersal. programmed cell death (pcd) is another suggested form of microbial altruistic behavior. although programmed cell death (also known as apoptosis or autolysis) clearly provides no direct fitness benefit, it can be evolutionary adaptive if it provides indirect benefits to individuals with high genetic relatedness (kin selection). several altruistic possibilities have been suggested for pcd, such as providing resources that could be used by other cells for growth and survival in saccharomyces cerevisiae. while using kin selection to explain the evolutionary benefits of pcd is common, the reasoning contains some inherent problems. charlesworth (1978) noted that it is extremely hard for a gene causing suicide to spread because only relatives that do not share the gene would ultimately benefit. therefore, the possible solution to this problem in microbes is that selection could favor a low probability of pcd among a large population of cells, possibly depending upon individual condition, environmental conditions, or signaling. other microbial interactions quorum sensing main article: quorum sensing figure 3: diagram of quorum sensing. (left) in low density, the concentration of the autoinducer (blue dots) is relatively low and the substance production is restricted. (right) in high density, the concentration of the autoinducer is high and the bacterial substances (red dots) are produced. the integration of cooperative and communicative interactions appear to be extremely important to microbes; for example, 6–10% of all genes in the bacterium pseudomonas aeruginosa are controlled by cell-cell signaling systems. one way that microbes communicate and organize with each other in order to partake in more advanced cooperative interactions is through quorum sensing. quorum sensing describes the phenomenon in which the accumulation of signaling molecules in the surrounding environment enables a single cell to assess the number of individuals (cell density) so that the population as a whole can make a coordinated response. this interaction is fairly common among bacterial taxa, and involves the secretion by individual cells of 'signaling' molecules, called autoinducers or pheromones. these bacteria also have a receptor that can specifically detect the signaling molecule. when the inducer binds the receptor, it activates transcription of certain genes, including those for inducer synthesis. there is a low likelihood of a bacterium detecting its own secreted inducer. thus, in order for gene transcription to be activated, the cell must encounter signaling molecules secreted by other cells in its environment. when only a few other bacteria of the same kind are in the vicinity, diffusion reduces the concentration of the inducer in the surrounding medium to almost zero, so the bacteria produce little inducer. however, as the population grows the concentration of the inducer passes a threshold, causing more inducer to be synthesized. this forms a positive feedback loop, and the receptor becomes fully activated. activation of the receptor induces the up regulation of other specific genes, causing all of the cells to begin transcription at approximately the same time. in other words, when the local concentration of these molecules has reached a threshold, the cells respond by switching on particular genes. in this way individual cells can sense the local density of bacteria, so that the population as a whole can make a coordinated response. in many situations, the cost bacterial cells pay in order to coordinate behaviors outweighs the benefits unless there is a sufficient number of collaborators. for instance, the bioluminescent luciferase produced by vibrio fischeri would not be visible if it was produced by a single cell. by using quorum sensing to limit the production of luciferase to situations when cell populations are large, v. fischeri cells are able to avoid wasting energy on the production of useless product. in many situations bacterial activities, such as the production of the mentioned public goods, are only worthwhile as a joint activity by a sufficient number of collaborators. regulation by quorum sensing would allow the cells to express appropriate behavior only when it is effective, thus saving resources under low density conditions. therefore, quorum sensing has been interpreted as a bacterial communication system to coordinate behaviors at the population level. the opportunistic bacteria pseudomonas aeruginosa also uses quorum sensing to coordinate the formation of biofilms, swarming motility, exopolysaccharide production, and cell aggregation. these bacteria can grow within a host without harming it, until they reach a certain concentration. then they become aggressive, their numbers sufficient to overcome the host's immune system, and form a biofilm, leading to disease within the host. another form of gene regulation that allows the bacteria to rapidly adapt to surrounding changes is through environmental signaling. recent studies have discovered that anaerobiosis can significantly impact the major regulatory circuit of quorum sensing. this important link between quorum sensing and anaerobiosis has a significant impact on production of virulence factors of this organism. it is hoped that the therapeutic enzymatic degradation of the signaling molecules will prevent the formation of such biofilms and possibly weaken established biofilms. disrupting the signalling process in this way is called quorum inhibition. implications while the evolution of cooperative interactions allowed microbial taxa to increase their fitness, it is hypothesized that cooperation provided a proximate cause to other major evolutionary transitions, including the evolution of multicellularity. this idea, often referred to as the colonial theory, was first proposed by haeckel in 1874, and claims that the symbiosis of many organisms of the same species (unlike the symbiotic theory, which suggests the symbiosis of different species) led to a multicellular organism. in a few instances, multicellularity occurs by cells separating and then rejoining (e.g., cellular slime molds) whereas for the majority of multicellular types, multicellularity occurs as a consequence of cells failing to separate following division. the mechanism of this latter colony formation can be as simple as incomplete cytokinesis, though multicellularity is also typically considered to involve cellular differentiation. the advantage of the colonial theory hypothesis is that it has been seen to occur independently numerous times (in 16 different protoctistan phyla). for instance, during food shortages dictyostelium discoideum cells group together in a colony that moves as one to a new location. some of these cells then slightly differentiate from each other. other examples of colonial organisation in protozoa are volvocaceae, such as eudorina and volvox. however, it can often be hard to separate colonial protists from true multicellular organisms, as the two concepts are not distinct. this problem plagues most hypotheses of how multicellularisation could have occurred.