Evolutionary dynamics of altruism and cheating among social amoebas - PubMed (original) (raw)
Comparative Study
Evolutionary dynamics of altruism and cheating among social amoebas
A Brännström et al. Proc Biol Sci. 2005.
Abstract
Dictyostelium discoideum is a eukaryotic amoeba, which, when starvation is imminent, aggregates to form fruiting bodies consisting of a stalk of reproductively dead cells that supports spores. Because different clones may be involved in such aggregations, cheater strategies may emerge that allocate a smaller fraction of cells to stalk formation, thus gaining a reproductive advantage. In this paper, we model the evolutionary dynamics of allocation strategies in Dictyostelium under the realistic assumption that the number of clones involved in aggregations follows a random distribution. By determining the full course of evolutionary dynamics, we show that evolutionary branching in allocation strategies may occur, resulting in dimorphic populations that produce stalkless and stalked fruiting bodies. We also demonstrate that such dimorphisms are more likely to emerge when the variation in the number of clones involved in aggregations is large.
Figures
Figure 1
Dispersal success as a function of stalk quality according to equation 2.4, plotted for all combinations of _q_=1/2, 1, 2, 3 and _ϵ_=0.1, 0.4. Curves with the same line style correspond to identical values of q, with lower curves corresponding to _ϵ_=0.1.
Figure 2
(a) Combinations of the mean μ of inverse founder spore numbers and the corresponding standard deviation σ for which evolutionary branching is possible (black region) and impossible (white region). Combinations in the grey region are logically infeasible. The cross in the white region corresponds to the distribution in the right panel. (b) Probability distribution of founder spore numbers at sites receiving at least one spore according to field measurements by Fortunato et al. (2003_a_).
Figure 3
Pairwise invasibility plots for different founder spore distributions and dispersal success functions. (a) Founder spore distributions according to figure 2_b_, and dispersal success function according to equation 2.3 with parameters _q_=2 and _d_0=0.05. Note that the evolution of incipient altruism is precluded. (b) Geometric founder spore distribution with parameter 0.04 and dispersal success function according to equation 2.4 with parameters _q_=4 and _ϵ_=0.2. Here, the evolution of altruism can take off from _r_=0.
Figure 4
Dimorphic evolutionary dynamics after branching. (a) The region of coexistence, in which protected dimorphisms are possible, is shown in grey. Arrows indicate the selection gradient's direction for each of the two resident trait values. The directions of these selective pressures change at the shown evolutionary isoclines, defined by vanishing selection gradients. Thin isoclines are evolutionarily stable, whereas thick isoclines are not. The figure shows that evolutionary change after branching will converge on an evolutionarily stable dimorphism of a cheater and an altruist, resulting in the trait values (0.48,0) at which the evolutionarily stable isocline (thin line) touches the boundary. (b) Simulations of the polymorphic evolutionary dynamics based on 1000 fruiting bodies, illustrating all four predicted phases of evolutionary change. In both panels, dispersal success follows equation 2.4 with parameters _q_=4 and _ϵ_=0.2, and the number of founder spores is distributed geometrically with parameter 0.04, resulting in an average of 25 founder spores per fruiting body.
References
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