Figure 1. Life cycle of D. discoideum (reproduced from Strssmann and Queller, 2011). |
In response to these signals, when bacteria are scarce and amoeba density has reached a certain threshold, D. discoideum enter one of two stages: the sexual or social stage. During the social stage amoebas aggregate following a cAMP gradient produced by the cells themselves. A multicellular slug forms and cells differentiate in such way that the tip becomes the anterior extreme and organizes the movement towards light and away from ammonia. During movement some posterior cells detach from the slug allowing exploitation of bacterial rich patches (Kuzdzal-Fick et al., 2007; Strassmann and Queller, 2011).
Eventually, the anterior cells migrate down towards the center of the aggregate to initiate fruiting body formation. The aggregate differentiates in a stalk (20% of the cells) and a spore ball at the tip (80% of the cells). In this division, stalk cells die forming a cellulose wall that confers the strength for holding the spores, allowing for spore dispersion. This division then involves the sacrifice of the stalk cells for the spore cells survival, and is, therefore, of behavioral interest (Strassmann and Queller, 2011). The social stage is also of behavioral interest but is a topic for another blog entry.
The reason why altruism is such an interesting phenomenon is because it defies evolutionary understanding. The problem with altruism is to understand how can it be stably maintained in the face of cheaters (individuals that obtain the benefits without paying the cost). If a cheater appears in the population (by mutation, migration or any other means) it will have more descendants that would probably be cheaters as well. The population will collapse and altruism will not be maintained. Evolutionary biologists have been aware of this problem for a long time, and different factors have been proposed as promoters of altruism evolution (or at least helping to prevent cheating).
Kin Selection
Imagine however that cooperators only cooperate with their close relatives, enhancing the survival of other individuals with a high probability of being cooperators as well. Cheaters will not spread in the population because cooperative behavior between cooperators will enhance their fitness. If a cheater mutant arises in one of such groups, it will not be able to spread because its close relatives will also have a high probability of being cheaters. In order to be successful, cheaters need cooperators to cheat; it is not possible to copy in an exam if everybody is also copying (Actually, the success of the cheaters directly depends on how many people really studied for that test). William D. Hamilton (1964) formalized this model of kin selection as an explanation for cooperative traits. He proposed that cooperative traits could spread in the population if:
rB>C
where r is the degree of relatedness of cooperator entities (the actor and the recipient), B is the benefit for the cooperator and C the cost of the interaction. In short, direct and indirect benefits (benefits for my kin) need to exceed the costs. Nevertheless, some hypotheses and details about this theory (in particular those concerning the evolutionary outcomes) have been difficult to test due to long generation times of species, as well as the difficulty to control different variables.
D. discoideum as a model organism for the evolution of cooperation
In this sense, D. discoideum is ideal to study evolution of cooperative traits. It has a cooperative life cycle, is small and easy to manipulate in lab conditions, and has a short generation time. In addition, there are many genetic tools developed for this organism including a fully sequenced genome! As far as testing cooperation, one of the first questions to ask is, can D. discoideum recognize its kin? One could expect some sort of recognition because different strains might coexist in the soil and the cost of cooperation is huge (you either die or reproduce). In other words it would not be evolutionary convenient to associate with unrelated strains during the social stage.
Figure 2. Proportion of fluorescent spores relative to a mean value. |
D. discoideum is definitively an organism worth of a nature documentary, but further more is a valuable organism to our understanding of cooperation and its evolution. This essay is not intended to be an extensive review, but in terms of cooperation there is still much to say about this amoeba.
References
Burdine, V. and M. Clarke. 1995. Genetic and physiologic modulation of the prestarvation response in Dyctiostellium discoideum. Mol Biol Cell. 6(3):311-325
Hamilton, W. D.1964. The genetical evolution of social behaviour. I. J Theor Biol 7:1–16.
Kuzdzal-Fick, J.J., K. R. Foster, D. C. Queller and J. E. Strassmann. 2007. Exploiting new terrain: an advantage to sociality in the slime mold Dictyostelium discoideum. Behavioral Ecology: 433-437
Ostrowski E.A., M. Katoh, G. Shaulsky, D. C. Queller and J. E. Strassmann .2008. Kin discrimination increases with genetic distance in a social amoeba. PLoS Biol 6:e287.
Strassmann, J. E. and D. C. Queller. 2011. Evolution of cooperation and control of cheating in a social microbe. PNAS 108(2): 10855–10862
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