Investment and Allocation of nectar Production in an Animal-Pollinated Plant

A plant\u27s ability to attract animal pollinators should be influenced by two important factors: the amount of available resources that is invested to the production of rewards such as nectar, and the individual\u27s size or resource state. Using Chamerion (= Epilobium) angustifolium (Onagraceae) as a model, I investigate: (1) selection pressures that determine the evolutionarily stable investment to nectar production, and (2) allocation tactics that vary with an individual\u27s current resource state. A conceptual model is presented, in which a plant\u27s attractiveness to pollinators increases with relative investment to nectar production (\u27social competition\u27). The evolutionarily stable investment strategy occurs where the expense of further nectar production outweighs the potential disadvantage of being slightly less attractive than competitors. The model may explain why C. angustifolium inflorescences produce, on average, such large amounts of nectar. Within populations, however, individuals varied greatly in the amount of nectar offered to pollinators, mainly due to variation in floral display size (number of open flowers-a correlate of resource state). Given that large, attractive inflorescences maximize pollen export by limiting the amount of pollen removed by each visitor, I predicted a size-dependent distribution of nectar within the vertical inflorescences of C. angustifolium. As predicted, small inflorescences distributed nectar nearly evenly among (lower) female- and (upper) male-phase flowers, whereas larger inflorescences allocated extra nectar to female-phase flowers. In experimental inflorescences, I distributed the same volume of nectar to mimic the \u27large\u27 and \u27small\u27 allocation types. Nectar-foraging bumblebees visited a mean of 3.2 fewer male-phase flowers on the \u27large\u27, relative to \u27small\u27 type, as expected if the nectar distribution of large displays functions to limit pollen removal during individual visits. I propose that the nectar gradient may adaptively mediate the schedule of pollen removal by manipulating the patch departure behaviour of pollinators

Kin discrimination, negative relatedness, and how to distinguish between selfishness and spite

Spiteful behaviors occur when an actor harms its own fitness to inflict harm on the fitness of others. Several papers have predicted that spite can be favored in sufficiently small populations, even when the harming behavior is directed indiscriminately at others. However, it is not clear that truly spiteful behavior could be favored without the harm being directed at a subset of social partners with relatively low genetic similarity to the actor (kin discrimination, causing a negative relatedness between actor and harmed recipient). Using mathematical models, we show that (1) the evolution of spite requires kin discrimination; (2) previous models suggesting indiscriminate spite involve scenarios where the actor gains a direct feedback benefit from harming others, and so the harming is selfish rather than spiteful; (3) extreme selfishness can be favored in small populations (or, more generally, under local competition) because this is where the direct feedback benefit of harming is greatest

Are greenbeards intragenomic outlaws?

Greenbeard genes identify copies of themselves in other individuals and cause their bearer to behave nepotistically toward those individuals. Hence, they can be favored by kin selection, irrespective of the degree of genealogical relationship between social partners. Although greenbeards were initially developed as a thought experiment, a number of recent discoveries of greenbeard alleles in real populations have led to a resurgence of interest in their evolutionary dynamics and consequences. One issue over which there has been disagreement is whether greenbeards lead to intragenomic conflict. Here, to clarify the "outlaw" status of greenbeards, we develop population genetic models that formally examine selection of greenbeard phenotypes under the control of different loci. We find that, in many cases, greenbeards are not outlaws because selection for or against the greenbeard phenotype is the same across all loci. In contrast, when social interactions are between genealogical kin, we find that greenbeards can be outlaws because different genes can be selected in different directions. Hence, the outlaw status of greenbeard genes crucially depends upon the particular biological details. We also clarify whether greenbeards are favored due to direct or indirect fitness effects and address the relationship of the greenbeard effect to sexual antagonism and reciprocity.</p

The evolution of index signals to avoid the cost of dishonesty

Animals often convey useful information, despite a conflict of interest between the signaller and receiver. There are two major explanations for such ‘honest’ signalling, particularly when the size or intensity of signals reliably indicates the underlying quality of the signaller. Costly signalling theory (including the handicap principle) predicts that dishonest signals are too costly to fake, whereas the index hypothesis predicts that dishonest signals cannot be faked. Recent evidence of a highly conserved causal link between individual quality and signal growth appears to bolster the index hypothesis. However, it is not clear that this also diminishes costly signalling theory, as is often suggested. Here, by incorporating a mechanism of signal growth into costly signalling theory, we show that index signals can actually be favoured owing to the cost of dishonesty. We conclude that costly signalling theory provides the ultimate, adaptive rationale for honest signalling, whereas the index hypothesis describes one proximate (and potentially very general) mechanism for achieving honesty

Presence of a loner strain maintains cooperation and diversity in well-mixed bacterial communities

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.ISSN:0962-8452ISSN:1471-295

Fig3

Colony forming unit counts of a 4-strain evolution competition tracking the proportions of cooperator, defector, loner, and loner defector strains for five days

Fig1

Colony forming unit counts for strains competing in two-way competitions and corrected proportions accounting for the evolution of de novo cheats

Fig2

Colony forming unit counts for a ten day evolution experiment, where proportions of all three strain (loner, cooperator, and defector) cycle over time