Intragroup and intragenomic conflict over chemical defense against predators

Insects are often chemically defended against predators. There is considerable evidence for a group‐beneficial element to their defenses, and an associated potential for individuals to curtail their own investment in costly defense while benefitting from the investments of others, termed “automimicry.” Although females in chemically defended taxa often lay their eggs in clusters, leading to siblings living in close proximity, current models of automimicry have neglected kin‐selection effects, which may be expected to curb the evolution of such selfishness. Here, we develop a general theory of automimicry that explicitly incorporates kin selection. We investigate how female promiscuity modulates intragroup and intragenomic conflicts overinvestment into chemical defense, finding that individuals are favored to invest less than is optimal for their group, and that maternal‐origin genes favor greater investment than do paternal‐origin genes. We translate these conflicts into readily testable predictions concerning gene expression patterns and the phenotypic consequences of genomic perturbations, and discuss how our results may inform gene discovery in relation to economically important agricultural products.     

Why menopause?

Summary George Williams proposed in 1957 that menopause evolved because, late in life, women have more to gain from child care than from continued fertility. I develop here a quantitative model of this idea in order to determine whether the proposed benefit is in fact larger than the cost. To make this work possible, I introduce an age-structured theory of kin selection that allows for a time delay between an act of altruism and the benefit it provides. Using this theory, I show that in age-structured populations, conventional inclusive fitness calculations are justified for effects on fertility only when either the population is stationary or there is no delay between cost and benefit. For effects on mortality, conventional calculations also require that donor and recipient be affected at the same age. I then introduce two versions of Williams' idea. Model I assumes that menopause is maintained because it reduces the risk of mortality during childbirth, thus increasing the provision of parental care. The analysis demonstrates that this model is incapable of accounting for menopause. Model II assumes that menopause facilitates parental care by reducing the time during which a woman is partially incapacitated by the demands of pregnancy and infant care. This model could not be rejected. However, a definitive test will require parameter estimates that are not yet available.     

Evolutionary dynamics of n-player games played by relatives

One of the core concepts in social evolution theory is kin selection. Kin selection provides a perspective to understand how natural selection operates when genetically similar individuals are likely to interact. A family-structured population is an excellent example of this, where relatives are engaged in social interactions. Consequences of such social interactions are often described in game-theoretical frameworks, but there is a growing consensus that a naive inclusive fitness accounting with dyadic relatedness coefficients are of limited use when non-additive fitness effects are essential in those situations. Here, I provide a general framework to analyse multiplayer interactions among relatives. Two important results follow from my analysis. First, it is generally necessary to know the n-tuple genetic association of family members when n individuals are engaged in social interactions. However, as a second result, I found that, for a special class of games, we need only measures of lower-order genetic association to fully describe its evolutionary dynamics. I introduce the concept of degree of the game and show how this degree is related to the degree of genetic association.     

Crystal toxins and the volunteer's dilemma in bacteria

The growth and virulence of the bacteria Bacillus thuringiensis depend on the production of Cry toxins, which are used to perforate the gut of its host. Successful invasion of the host relies on producing a threshold amount of toxin, after which there is no benefit from producing more toxin. Consequently, the production of Cry toxin appears to be a different type of social problem compared with the public goods scenarios that bacteria usually encounter. We show that selection for toxin production is a volunteer's dilemma. We make specific predictions that (a) selection for toxin production depends upon an interplay between the number of bacterial cells that each host ingests and the genetic relatedness between those cells; (b) cheats that do not produce toxin gain an advantage when at low frequencies, and at high bacterial density, allowing them to be maintained in a population alongside toxin‐producing cells. More generally, our results emphasize the diversity of the social games that bacteria play.     

Hamilton's rule, imprinting and parent-offspring conflict over seed mass in partially selfing plants

When genes in the offspring control the provisioning of the seed, the optimal seed size can be calculated exactly by applying Hamilton's rule. When seed size is a compromise between mother and offspring, we predict that outcrossing plant species produce larger seeds than selfers. This trend was found in the British flora and in a number of well-studied plant families. The analysis was extended to imprinting, a conditional strategy in which a gene in the offspring takes more resources when derived from the father than from the mother. The conditions for imprinting to be selected were rather restrictive. The analysis is relevant for the current debate about the evolution of imprinting in Arabidopsis thaliana.     

Individual variation and the resolution of conflict over parental care in penduline tits

Eurasian penduline tits (Remiz pendulinus) have an unusually diverse breeding system consisting of frequent male and female polygamy, and uniparental care by the male or the female. Intriguingly, 30 to 40 per cent of all nests are deserted by both parents. To understand the evolution of this diverse breeding system and frequent clutch desertion, we use 6 years of field data to derive fitness expectations for males and females depending on whether or not they care for their offspring. The resulting payoff matrix corresponds to an asymmetric Snowdrift Game with two alternative evolutionarily stable strategies (ESSs): female-only and male-only care. This, however, does not explain the polymorphism in care strategies and frequent biparental desertion, because theory predicts that one of the two ESSs should have spread to fixation. Using a bootstrapping approach, we demonstrate that taking account of individual variation in payoffs explains the patterns of care better than a model based on the average population payoff matrix. In particular, a model incorporating differences in male attractiveness closely predicts the observed frequencies of male and female desertion. Our work highlights the need for a new generation of individual-based evolutionary game-theoretic models.