[New Paper] Misconceptions on the application of biological market theory to the mycorrhizal symbiosis

Kiers ET, West SA, Wyatt GAK, Gardner A, Bücking H & Werner G (2016) Misconceptions on the application of biological market theory to the mycorrhizal symbiosis. Nature Plants 2, 16063.


The symbiosis between plants and arbuscular mycorrhizal fungi has been described as a biological market based on evidence that plants supply more carbohydrates to fungal partners that provide more soil nutrients, and vice versa. A recent paper by Walder and van der Heijden challenges this view. However, their challenge is based on misunderstandings of biological market theory, and evolutionary theory more generally.

[New Paper] Sex ratios, virginity & local resource enhancement in a quasisocial parasitoid

Kapranas A, Hardy ICW, Tang X, Gardner A & Li B (2016) Sex ratios, virginity and local resource enhancement in a quasisocial parasitoid. Entomologia Experimentalis et Applicata 159, 243-251.


Sclerodermus harmandi (Buysson) (Hymenoptera: Bethylidae) is an economically beneficial species of parasitoid wasp that has an unusual level of sociality: groups of female foundresses reproduce on a single host and exhibit cooperative post-ovipositional brood care. The beneficial effects females have on each other’s reproductive success provide, via the theory of local resource enhancement (LRE), an explanation for their female-biased progeny sex ratios, which is part of the same framework for understanding sex-ratio evolution as the more often invoked theory of local mate competition (LMC). Here, we show that S. harmandi sex ratios are overdispersed, with high variance largely attributable to the common occurrence (60%) of developmental mortality. Developmental mortality is also positively associated with the proportion of broods which contain only females at emergence (virgin broods). Virginity is more common when broods are produced by fewer foundresses. Virginity is expected to be disadvantageous under LRE, as it is under LMC, but theory for LRE is less extensively developed. We suggest approaches for the development of LRE theory, in particular using models of ‘population elasticity’ in which the intensity of kin competition is reduced because extra resources are available to local populations that are more cooperative. For S. harmandi, such extra resources may include large hosts that can only be successfully utilised if multiple foundresses cooperate.

[New Paper] Intragenomic conflict in polyembryonic parasitoid wasps

Rautiala P & Gardner A (in press) Intragenomic conflict over soldier allocation in polyembryonic parasitoid wasps. American Naturalist. doi: 10.1086/685082


Understanding the selection pressures that have driven the evolution of sterile insect castes has been the focus of decades of intense scientific debate. An amenable empirical test bed for theory on this topic is provided by the sterile-soldier caste of polyembryonic parasitoid wasps. The function of these soldiers has been a source of controversy, with two basic hypotheses emerging: the “brood-benefit” hypothesis that they provide an overall benefit for their siblings and the “sex-ratio-conflict” hypothesis that the soldiers mediate a conflict between brothers and sisters by killing their opposite-sex siblings. Here, we investigate the divergent sex-ratio optima of a female embryo’s maternal-origin and paternal-origin genes, to determine the potential for, and direction of, intragenomic conflict over soldiering. We then derive contrasting empirically testable predictions concerning the patterns of genomic imprinting that are expected to arise out of this intragenomic conflict, for the brood-benefit versus the sex-ratio-conflict hypothesis of soldier function.

[New Paper] Restricting mutualistic partners to enforce trade reliance

Wyatt GAK, Kiers ET, Gardner A & West SA (2016) Restricting mutualistic partners to enforce trade reliance. Nature Communications 7, 10322 doi: 10.1038/ncomms10322


Mutualisms are cooperative interactions between members of different species, often involving the trade of resources. Here, we suggest that otherwise-cooperative mutualists might be able to gain a benefit from actively restricting their partners’ ability to obtain resources directly, hampering the ability of the restricted partner to survive and/or reproduce without the help of the restricting mutualist. We show that (i) restriction can be favoured when it makes the resources of the restricting individual more valuable to their partner, and thus allows them to receive more favourable terms of trade; (ii) restriction maintains cooperation in conditions where cooperative behaviour would otherwise collapse; and (iii) restriction can lead to either an increase or decrease in a restricted individual’s fitness. We discuss the applicability of this scenario to mutualisms such as those between plants and mycorrhizal fungi. These results identify a novel conflict in mutualisms as well as several public goods dilemmas, but also demonstrate how conflict can help maintain cooperation.


[New Paper] Presence of a loner strain maintains cooperation & diversity in bacteria

Inglis RF, Biernaskie JM, Gardner A & Kümmerli R (2016) Presence of a loner strain maintains cooperation and diversity in well-mixed bacterial communities. Proceedings of the Royal Society of London Series B — Biological Sciences 283, 20152682.

Cooperation and diversity abound in nature despite cooperators risking exploitation from defectors and superior competitors displacing weaker ones. Understanding the persistence of cooperation and diversity is therefore a major problem for evolutionary ecology, especially in the context of well-mixed populations, where the potential for exploitation and displacement is greatest. Here, we demonstrate that a ‘loner effect’, described by economic game theorists, can maintain cooperation and diversity in real-world biological settings. We use mathematical models of public-good-producing bacteria to show that the presence of a loner strain, which produces an independent but relatively inefficient good, can lead to rock–paper–scissor dynamics, whereby cooperators outcompete loners, defectors outcompete cooperators and loners outcompete defectors. These model predictions are supported by our observations of evolutionary dynamics in well-mixed experimental communities of the bacterium Pseudomonas aeruginosa. We find that the coexistence of cooperators and defectors that produce and exploit, respectively, the iron-scavenging siderophore pyoverdine, is stabilized by the presence of loners with an independent iron-uptake mechanism. Our results establish the loner effect as a simple and general driver of cooperation and diversity in environments that would otherwise favour defection and the erosion of diversity.

[PhD opportunities] 30 studentships for Chinese nationals

The University of St Andrews, in conjunction with the China Scholarship Council, is offering up to 30 PhD studentships for Chinese nationals. If you’re interested in pursuing PhD research in evolutionary theory, and meet the eligibility criteria, then do get in touch. More details can be found here. Note that the deadline is 30 Nov 2015.

[New Paper] The constant philopater hypothesis

Rodrigues AMM & Gardner A (in press) The constant philopater hypothesis: a new life history invariant for dispersal evolution. Journal of Evolutionary Biology doi: 10.1111/jeb.12771

Surprising invariance relationships have emerged from the study of social interaction, whereby a cancelling-out of multiple partial effects of genetic, ecological or demographic parameters means that they have no net impact upon the evolution of a social behaviour. Such invariants play a pivotal role in the study of social adaptation: on the one hand, they provide theoretical hypotheses that can be empirically tested; and, on the other hand, they provide benchmark frameworks against which new theoretical developments can be understood. Here we derive a novel invariant for dispersal evolution: the ‘constant philopater hypothesis’ (CPH). Specifically, we find that, irrespective of variation in maternal fecundity, all mothers are favoured to produce exactly the same number of philopatric offspring, with high-fecundity mothers investing proportionally more, and low-fecundity mothers investing proportionally less, into dispersing offspring. This result holds for female and male dispersal, under haploid, diploid and haplodiploid modes of inheritance, irrespective of the sex ratio, local resource availability and whether mother or offspring controls the latter’s dispersal propensity. We explore the implications of this result for evolutionary conflict of interests – and the exchange and withholding of contextual information – both within and between families, and we show that the CPH is the fundamental invariant that underpins and explains a wider family of invariance relationships that emerge from the study of social evolution.