Control of tissue development and cell diversity by cell cycle-dependent transcriptional filtering.

Cell cycle duration changes dramatically during development, starting out fast to generate cells quickly and slowing down over time as the organism matures. The cell cycle can also act as a transcriptional filter to control the expression of long gene transcripts, which are partially transcribed in short cycles. Using mathematical simulations of cell proliferation, we identify an emergent property that this filter can act as a tuning knob to control gene transcript expression, cell diversity, and the number and proportion of different cell types in a tissue. Our predictions are supported by comparison to single-cell RNA-seq data captured over embryonic development. Additionally, evolutionary genome analysis shows that fast-developing organisms have a narrow genomic distribution of gene lengths while slower developers have an expanded number of long genes. Our results support the idea that cell cycle dynamics may be important across multicellular animals for controlling gene transcript expression and cell fate.

Immediate action is the best strategy when facing uncertain climate change.

Mitigating the detrimental effects of climate change is a collective problem that requires global cooperation. However, achieving cooperation is difficult since benefits are obtained in the future. The so-called collective-risk game, devised to capture dangerous climate change, showed that catastrophic economic losses promote cooperation when individuals know the timing of a single climatic event. In reality, the impact and timing of climate change is not certain; moreover, recurrent events are possible. Thus, we devise a game where the risk of a collective loss can recur across multiple rounds. We find that wait and see behavior is successful only if players know when they need to contribute to avoid danger and if contributions can eliminate the risks. In all other cases, act quickly is more successful, especially under uncertainty and the possibility of repeated losses. Furthermore, we incorporate influential factors such as wealth inequality and heterogeneity in risks. Even under inequality individuals should contribute early, as long as contributions have the potential to decrease risk. Most importantly, we find that catastrophic scenarios are not necessary to induce such immediate collective action.

Evaluating the maintenance of disease-associated variation at the blood group-related gene B4galnt2 in house mice.

BACKGROUND: B4galnt2 is a blood group-related glycosyltransferase that displays cis-regulatory variation for its tissue-specific expression patterns in house mice. The wild type allele, found e.g. in the C57BL/6 J strain, directs intestinal expression of B4galnt2, which is the pattern observed among vertebrates, including humans. An alternative allele class found in the RIIIS/J strain and other mice instead drives expression in blood vessels, which leads to a phenotype similar to type 1 von Willebrand disease (VWD), a common human bleeding disorder. We previously showed that alternative B4galnt2 alleles are subject to long-term balancing selection in mice and that variation in B4galnt2 expression influences host-microbe interactions in the intestine. This suggests that the costs of prolonged bleeding in RIIIS/J allele-bearing mice might be outweighed by benefits associated with resistance against gastrointestinal pathogens. However, the conditions under which such trade-offs could lead to the long-term maintenance of disease-associated variation at B4galnt2 are unclear. RESULTS: To explore the persistence of B4galnt2 alleles in wild populations of house mice, we combined B4galnt2 haplotype frequency data together with a mathematical model based on an evolutionary game framework with a modified Wright-Fisher process. In particular, given the potential for a heterozygote advantage as a possible explanation for balancing selection, we focused on heterozygous mice, which express B4galnt2 in both blood vessels and the gastrointestinal tract. We show that B4galnt2 displays an interesting spatial allelic distribution in Western Europe, likely due to the recent action of natural selection. Moreover, we found that the genotype frequencies observed in nature can be produced by pathogen-driven selection when both heterozygotes and RIIIS/J homozygotes are protected against infection and the fitness cost of bleeding is roughly half that of infection. CONCLUSION: By comparing the results of our models to the patterns of polymorphism at B4galnt2 in natural populations, we are able to recognize the long-term maintenance of the RIIIS/J allele through host-pathogen interactions as a viable hypothesis. Further, our models identify that a putative dominant-, yet unknown protective function of the RIIIS/J allele appears to be more likely than a protective loss of intestinal B4galnt2 expression in RIIIS/J homozygotes.

Coevolutionary interactions between farmers and mafia induce host acceptance of avian brood parasites.

Brood parasites exploit their host in order to increase their own fitness. Typically, this results in an arms race between parasite trickery and host defence. Thus, it is puzzling to observe hosts that accept parasitism without any resistance. The 'mafia' hypothesis suggests that these hosts accept parasitism to avoid retaliation. Retaliation has been shown to evolve when the hosts condition their response to mafia parasites, who use depredation as a targeted response to rejection. However, it is unclear if acceptance would also emerge when 'farming' parasites are present in the population. Farming parasites use depredation to synchronize the timing with the host, destroying mature clutches to force the host to re-nest. Herein, we develop an evolutionary model to analyse the interaction between depredatory parasites and their hosts. We show that coevolutionary cycles between farmers and mafia can still induce host acceptance of brood parasites. However, this equilibrium is unstable and in the long-run the dynamics of this host-parasite interaction exhibits strong oscillations: when farmers are the majority, accepters conditional to mafia (the host will reject first and only accept after retaliation by the parasite) have a higher fitness than unconditional accepters (the host always accepts parasitism). This leads to an increase in mafia parasites' fitness and in turn induce an optimal environment for accepter hosts.

Eco-evolutionary feedback promotes Red Queen dynamics and selects for sex in predator populations.

Although numerous hypotheses exist to explain the overwhelming presence of sexual reproduction across the tree of life, we still cannot explain its prevalence when considering all inherent costs involved. The Red Queen hypothesis states that sex is maintained because it can create novel genotypes with a selective advantage. This occurs when the interactions between species induce frequent environmental change. Here, we investigate whether coevolution and eco-evolutionary feedback dynamics in a predator-prey system allows for indirect selection and maintenance of sexual reproduction in the predator. Combining models and chemostat experiments of a rotifer-algae system we show a continuous feedback between population and trait change along with recurrent shifts from selection by predation and competition for a limited resource. We found that a high propensity for sex was indirectly selected and was maintained in rotifer populations within environments containing these eco-evolutionary dynamics; whereas within environments under constant conditions, predators evolved rapidly to lower levels of sex. Thus, our results indicate that the influence of eco-evolutionary feedback dynamics on the overall evolutionary change has been underestimated.

Which risk scenarios can drive the emergence of costly cooperation?

In collective risk dilemmas, cooperation prevents collective loss only when players contribute sufficiently. In these more complex variants of a social dilemma, the form of the risk curve is crucial and can strongly affect the feasibility of a cooperative outcome. The risk typically depends on the sum of all individual contributions. Here, we introduce a general approach to analyze the stabilization of cooperation under any decreasing risk curve and discuss how different risk curves affect cooperative outcomes. We show that the corresponding solutions can be reached by social learning or evolutionary dynamics. Furthermore, we extend our analysis to cases where individuals do not only care about their expected payoff, but also about the associated distribution of payoffs. This approach is an essential step to understand the effects of risk decay on cooperation.

Plastic behaviors in hosts promote the emergence of retaliatory parasites.

Mafia like behavior, where individuals cooperate under the threat of punishment, occurs not only in humans, but is also observed in several animal species. Observations suggest that avian hosts tend to accept a certain degree of parasitism in order to avoid retaliating punishment from the brood parasite. To understand under which conditions it will be beneficial for a host to cooperate, we model the interaction between hosts and parasites as an evolutionary game. In our model, the host's behavior is plastic, and thus, its response depends on the previous interactions with the parasite. We find that such learned behavior in turn is crucial for the evolution of retaliating parasites. The abundance of this kind of mafia behavior oscillates in time and does not settle to an equilibrium. Our results suggest that retaliation is a mechanism for the parasite to evade specialization and to induce acceptance by the host.

No evidence for size-assortative mating in the wild despite mutual mate choice in sex-role-reversed pipefishes.

Size-assortative mating is a nonrandom association of body size between members of mating pairs and is expected to be common in species with mutual preferences for body size. In this study, we investigated whether there is direct evidence for size-assortative mating in two species of pipefishes, Syngnathus floridae and S. typhle, that share the characteristics of male pregnancy, sex-role reversal, and a polygynandrous mating system. We take advantage of microsatellite-based "genetic-capture" techniques to match wild-caught females with female genotypes reconstructed from broods of pregnant males and use these data to explore patterns of size-assortative mating in these species. We also develop a simulation model to explore how positive, negative, and antagonistic preferences of each sex for body size affect size-assortative mating. Contrary to expectations, we were unable to find any evidence of size-assortative mating in either species at different geographic locations or at different sampling times. Furthermore, two traits that potentially confer a fitness advantage in terms of reproductive success, female mating order and number of eggs transferred per female, do not affect pairing patterns in the wild. Results from model simulations demonstrate that strong mating preferences are unlikely to explain the observed patterns of mating in the studied populations. Our study shows that individual mating preferences, as ascertained by laboratory-based mating trials, can be decoupled from realized patterns of mating in the wild, and therefore, field studies are also necessary to determine actual patterns of mate choice in nature. We conclude that this disconnect between preferences and assortative mating is likely due to ecological constraints and multiple mating that may limit mate choice in natural populations.

Under high stakes and uncertainty the rich should lend the poor a helping hand.

In social dilemmas, there is tension between individual incentives to optimize personal gain versus social benefits. An additional cause of conflict in such social dilemmas is heterogeneity. Cultural differences or financial inequity often interfere with decision making when a diverse group of individuals interact. We address these issues in situations where individuals are either rich or poor. Often, it is unclear how rich and poor individuals should interact - should the poor invest the same as the rich, or should the rich assist the poor? Which distribution of efforts can be considered as fair? To address the effects of heterogeneity on decisions, we model a collective-risk dilemma where players collectively have to invest more than a certain threshold, with heterogeneity and multiple rounds. We aim to understand the natural behavior and to infer which strategies are particularly stable in such asymmetric collective-risk games. Large scale individual based simulations show that when the poor players have half of the wealth the rich players posses, the poor contribute only when early contributions are made by the rich players. The rich contribute on behalf of the poor only when their own external assets are worth protecting. Under a certain degree of uncertainty we observe the rich maintain cooperation by assisting the poor.

The evolution of strategic timing in collective-risk dilemmas.

In collective-risk dilemmas, a group needs to collaborate over time to avoid a catastrophic event. This gives rise to a coordination game with many equilibria, including equilibria where no one contributes, and thus no measures against the catastrophe are taken. In this game, the timing of contributions becomes a strategic variable that allows individuals to interact and influence one another. Herein, we use evolutionary game theory to study the impact of strategic timing on equilibrium selection. Depending on the risk of catastrophe, we identify three characteristic regimes. For low risks, defection is the only equilibrium, whereas high risks promote equilibria with sufficient contributions. Intermediate risks pose the biggest challenge for cooperation. In this risk regime, the option to interact over time is critical; if individuals can contribute over several rounds, then the group has a higher chance to succeed, and the expected welfare increases. This positive effect of timing is of particular importance in larger groups, where successful coordination becomes increasingly difficult.

Evolutionary dynamics of strategic behavior in a collective-risk dilemma.

A collective-risk social dilemma arises when a group must cooperate to reach a common target in order to avoid the risk of collective loss while each individual is tempted to free-ride on the contributions of others. In contrast to the prisoners' dilemma or public goods games, the collective-risk dilemma encompasses the risk that all individuals lose everything. These characteristics have potential relevance for dangerous climate change and other risky social dilemmas. Cooperation is costly to the individual and it only benefits all individuals if the common target is reached. An individual thus invests without guarantee that the investment is worthwhile for anyone. If there are several subsequent stages of investment, it is not clear when individuals should contribute. For example, they could invest early, thereby signaling their willingness to cooperate in the future, constantly invest their fair share, or wait and compensate missing contributions. To investigate the strategic behavior in such situations, we have simulated the evolutionary dynamics of such collective-risk dilemmas in a finite population. Contributions depend individually on the stage of the game and on the sum of contributions made so far. Every individual takes part in many games and successful behaviors spread in the population. It turns out that constant contributors, such as constant fair sharers, quickly lose out against those who initially do not contribute, but compensate this in later stages of the game. In particular for high risks, such late contributors are favored.

Holotestoid: a computational model for testing hypotheses about echinoid skeleton form and growth.

Regular echinoid skeletons, or tests, comprise plate patterns and overall shapes that have proven challenging to analyse solely on the basis of any one approach or process. Herein, we present a computational model, Holotestoid, that emulates four macrostructural ontogenic processes involved in test growth (plate growth, plate addition, plate interaction, and plate gapping). We devise a geometric representation for analysing tests and describe how we use analogies (bubble interactions and close-packing) to emulate the processes. In the computational model, the emulated processes are used to determine the plate size and plate shape and combined to simulate a growth zone. We simulated growth zones for Arbacia punctulata and for Strongylocentrotus franciscanus by changing the value for one parameter, the ambulacral column angle. We quantitatively compared morphological features for simulated forms to those for real specimens to test the computational model. Additionally, we simulated growth zones for A. punctulata, S. franciscanus, Eucidaris thouarsii, and Mellita quinquiesperforata by changing three parameters, ambulacral column angle, peristome radius to apical system radius ratio, and apical system radius to column length ratio. Holotestoid can be used to explain morphological disparity among echinoid tests.