Stable eusociality via maternal manipulation when resistance is costless

The evolution of morphological castes under decoupled control

Eusociality, where units that previously reproduced independently function as one entity, is of major interest in evolutionary biology. Obligate eusociality is characterized by morphologically differentiated castes and reduced conflict. We explore conditions under which morphological castes may arise in the Hymenoptera and factors constraining their evolution. Control over offspring morphology and behaviour seems likely to be decoupled. Provisioners (queens and workers) can influence offspring morphology directly through the nutrition they provide, while adult offspring control their own behaviour. Provisioners may, however, influence worker behaviour indirectly if offspring modify their behaviour in response to their morphology. If manipulation underlies helping, we should not see helping evolve before specialized worker morphology, yet empirical observations suggest that behavioural castes precede morphological castes. We use evolutionary invasion analyses to show how the evolution of a morphologically differentiated worker caste depends on the prior presence of a behavioural caste: specialist worker morphology will be mismatched with behaviour unless some offspring already choose to work. A mother's certainty about her offspring's behaviour is also critical-less certainty results in greater mismatch. We show how baseline worker productivity can affect the likelihood of a morphological trait being favoured by natural selection. We then show how under a decoupled control scenario, morphologically differentiated castes should be less and less likely to be lost as they become more specialized. We also suggest that for eusociality to be evolutionarily irreversible, workers must be unable to functionally replace reproductives and reproductives must be unable to reproduce without help from workers.

The evolution of manipulative cheating

A social cheat is typically assumed to be an individual that does not perform a cooperative behaviour, or performs less of it, but can still exploit the cooperative behaviour of others. However, empirical data suggests that cheating can be more subtle, involving evolutionary arms races over the ability to both exploit and resist exploitation. These complications have not been captured by evolutionary theory, which lags behind empirical studies in this area. We bridge this gap with a mixture of game-theoretical models and individual-based simulations, examining what conditions favour more elaborate patterns of cheating. We found that as well as adjusting their own behaviour, individuals can be selected to manipulate the behaviour of others, which we term 'manipulative cheating'. Further, we found that manipulative cheating can lead to dynamic oscillations (arms races), between selfishness, manipulation, and suppression of manipulation. Our results can help explain both variation in the level of cheating, and genetic variation in the extent to which individuals can be exploited by cheats.

Hybridization enables the fixation of selfish queen genotypes in eusocial colonies

A eusocial colony typically consists of two main castes: queens that reproduce and sterile workers that help them. This division of labor, however, is vulnerable to genetic elements that favor the development of their carriers into queens. Several factors, such as intracolonial relatedness, can modulate the spread of such caste‐biasing genotypes. Here we investigate the effects of a notable yet understudied ecological setting: where larvae produced by hybridization develop into sterile workers. Using mathematical modeling, we show that the coevolution of hybridization with caste determination readily triggers an evolutionary arms race between nonhybrid larvae that increasingly develop into queens, and queens that increasingly hybridize to produce workers. Even where hybridization reduces worker function and colony fitness, this race can lead to the loss of developmental plasticity and to genetically hard‐wired caste determination. Overall, our results may help understand the repeated evolution toward remarkable reproductive systems (e.g., social hybridogenesis) observed in several ant species.

Eusociality through conflict dissolution

Eusociality, where largely unreproductive offspring help their mothers reproduce, is a major form of social organization. An increasingly documented feature of eusociality is that mothers induce their offspring to help by means of hormones, pheromones or behavioural displays, with evidence often indicating that offspring help voluntarily. The co-occurrence of maternal influence and offspring voluntary help may be explained by what we call the converted helping hypothesis, whereby maternally manipulated helping subsequently becomes voluntary. Such hypothesis requires that parent-offspring conflict is eventually dissolved-for instance, if the benefit of helping increases sufficiently over evolutionary time. We show that help provided by maternally manipulated offspring can enable the mother to sufficiently increase her fertility to transform parent-offspring conflict into parent-offspring agreement. This conflict-dissolution mechanism requires that helpers alleviate maternal life-history trade-offs, and results in reproductive division of labour, high queen fertility and honest queen signalling suppressing worker reproduction-thus exceptionally recovering diverse features of eusociality. As such trade-off alleviation seemingly holds widely across eusocial taxa, this mechanism offers a potentially general explanation for the origin of eusociality, the prevalence of maternal influence, and the offspring's willingness to help. Overall, our results explain how a major evolutionary transition can happen from ancestral conflict.

Feedback loops in the major evolutionary transition to eusociality: the status and potential of theoretical approaches

In this review, we adopt a step-wise framework for the evolution a major evolutionary transition in light of eusocial insects. By focusing on the sequence of (1) group formation, (2) alignment of genetic interests, and finally (3) group integration to higher-level functioning, we highlight that these steps occasionally interact with each other through feedback. We summarize models that capture such feedback and identify cases where there is room for the development of between-step relationships. We suggest that life history traits may serve as a conduit for analyzing feedback between suites of correlated traits. Our review reveals that there are many relationships both within and between the above steps that await formal modeling.

Maternal effects and parent-offspring conflict

Maternal effects can provide offspring with reliable information about the environment they are likely to experience, but also offer scope for maternal manipulation of young when interests diverge between parents and offspring. To predict the impact of parent-offspring conflict, we model the evolution of maternal effects on local adaptation of young. We find that parent-offspring conflict strongly influences the stability of maternal effects; moreover, the nature of the disagreement between parents and young predicts how conflict is resolved: when mothers favor less extreme mixtures of phenotypes relative to offspring (i.e., when mothers stand to gain by hedging their bets), mothers win the conflict by providing offspring with limited amounts of information. When offspring favor overproduction of one and the same phenotype across all environments compared to mothers (e.g., when offspring favor a larger body size), neither side wins the conflict and signaling breaks down. Only when offspring favor less extreme mixtures relative to their mothers (something no current model predicts), offspring win the conflict and obtain full information about the environment. We conclude that a partial or complete breakdown of informative maternal effects will be the norm rather than the exception in the presence of parent-offspring conflict.

The evolution of queen control over worker reproduction in the social Hymenoptera

A trademark of eusocial insect species is reproductive division of labor, in which workers forego their own reproduction while the queen produces almost all offspring. The presence of the queen is key for maintaining social harmony, but the specific role of the queen in the evolution of eusociality remains unclear. A long‐discussed scenario is that a queen either behaviorally or chemically sterilizes her workers. However, the demographic and ecological conditions that enable such manipulation are still debated. We study a simple model of evolutionary dynamics based on haplodiploid genetics. Our model is set in the commonly observed case where workers have lost the ability to lay female (diploid) eggs by mating, but retain the ability to lay male (haploid) eggs. We consider a mutation that acts in a queen, causing her to control the reproductive behavior of her workers. Our mathematical analysis yields precise conditions for the evolutionary emergence and stability of queen‐induced worker sterility. These conditions do not depend on the queen's mating frequency. We find that queen control is always established if it increases colony reproductive efficiency, but can evolve even if it decreases colony efficiency. We further derive the conditions under which queen control is evolutionarily stable against invasion by mutant workers who have recovered the ability to lay male eggs.