Social conflicts in Dictyostelium discoideum : a matter of scales

In this manuscript, the authors describe two different conceptual perspectives for understanding the evolution of aggregation and collective behavior in the social amoeba Dictyostelium discoideum. The first perspective is the "strain-level" where empirically strain frequencies are used to determine the success of cooperative (less spore bias) or non-cooperative (more spore bias) strategies. The second perspective is the "cell-level" where individual cell fates, spore or stalk, are affected both local biotic and abiotic conditions and by stochastic forces. The paper has a lot of excellent detail about how aggregation and cooperation in Dictyostelium functions and might be evolutionarily stable. The "cell-level" perspective highlights a number of important mechanisms that contribute to spore or stalk bias including the cell-cell signaling, cell position, and cell-cycle stage. These mechanisms suggest there is important and understudied complexity in the experimental results of chimeric mixtures and suggest evolutionary models must account for these mechanistic details in order to truly describe how aggregation evolves and is maintained in Dictyostelium lineages.

Two reviewers have read the manuscript and agree that preprint is interesting and provides a valuable perspective. They provides a few important areas for improvement that I think the authors should consider. One area that I would like to highlight specifically is the reviewer's comments about the "the balance of arguments in favor of the strain- level vs. the cell-level perspective". I agree here with the reviewer that manuscript reads a bit more as an exposition on the importance of the cell-level perspective and less of a full comparison of the benefits and drawbacks of both approaches. I also agree that simply signaling this goal earlier in the paper would be a good way to address this issue.

One place that I think the manuscript needs more substantial modification is in its description of the mathematical theory in relationship to the strain vs cell-level perspective. On page 9 in section 3, the authors suggest pure strategy models are sufficient for the strain-level perspective and mixed strategy models are necessary for the cell-level perspective. In actuality, the cell-level perspective doesn't necessitate the use of mixed strategies any more than the strain-level does. A pure strategy can be deterministic or probabilistic. In fact, many models of cooperation use a continuous variable to measure the level of cooperation, which conceptually is no different than if that same variable measures a continuous probability of cooperating. Mixed strategies become relevant when one considers the possibility of a mixture of discrete pure strategies. But in many cases this is indistinguishable from a continuous of pure strategies where the strategy is a probability. The second issue on page 9 is that the paragraph suggests that simply by using a mixed strategy, certain models allow coexistence of different behaviors (stalk vs spore I assume). However, these models really show an equilibrium with both behaviors because they setup a game that is no longer a simple prisoner's dilemma (PD); rather, these games are likely snowdrift (SD) games where a mixed strategy or intermediate value is stable. In other words, its the change of the game structure in these models, not their consideration of mixed strategies per se, that leads to coexistence. This applies to n-players games too where some n-players games result in PD like games and others have nonlinearities that lead to SD like outcomes, but being an n-player game per se doesn't result in nonlinearities (e.g., "Such games naturally introduce frequency-dependent payoffs and non-linearities" on page 10). In a few other places the authors suggest that multiplayer games add additional complexity that requires new game theoretic approaches rather than traditional deterministic approaches (the paragraph on page 21 starting "In evolutionary game theory"). This also isn't true (see for example Peña et al 2014 J Theor Biol and Peña and Nöldeke 2015 J Theor Biol). In general, the authors should take more care about connecting any specific feature of the strain or cell perspective to a technical limitation of a specific game theoretic approach or model; its much more likely that specific models made specific biological assumptions such as regarding the payoff structure of the game the organisms play that resulted in the model's predictions rather than a specific technical aspect of the model analysis.

Specific comments

• Page 5: "many rounds of the game". In evolutionary game theory, this really is many generations.
• Page 5: "cost" and "benefit" should be qualified by "fitness" (i.e., fitness cost and fitness benefit).
• Page 6: where citing Fletcher and Doebeli 2009, the authors should also cite Queller 1992 Evolution.
• Page 7: "assemble locally" should be "assemble from locally".
• Page 9: "still contrasted". Not clear what this means.
• Page 12: "lead Maeda" should be "led Maeda".
• Page 13: "back of the slug" and "rear form the stalk". What is different between "back" and "rear"?
• Page 17: "Fruiting bodies with large heads may be more prone to collapse and would then reduce the potential to disperse of both strains, thus undercutting the reproductive success of the cheater itself." This kind of feature is exactly what sets up the payoffs for a SD game instead of a PD game!
• Page 18: "allows to" should be "allows us to".
• Page 18: "Variation...respectively. I don't think this is an accurate description of the indirect genetic effects approach.
• Page 19: "statistical description of the outcome of interactions does not inform on the underlying processes.". This seems to reference multilevel selection or kin selection approaches. These approaches are not purely statistical and usually do build on mechanistic biological assumptions.
• Page 21: "In evolutionary game theory...processes". I'd ditch this whole paragraph.
• Page 22: "allow nowadays" should be "allow us nowadays"

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