Interaction effects of cell diffusion, cell density and public goods properties on the evolution of cooperation in digital microbes.

Microbial cooperation typically consists in the sharing of secreted metabolites (referred to as public goods) within the community. Although public goods generally promote population growth, they are also vulnerable to exploitation by cheating mutants, which no longer contribute, but still benefit from the public goods produced by others. Although previous studies have identified a number of key factors that prevent the spreading of cheaters, little is known about how these factors interact and jointly shape the evolution of microbial cooperation. Here, we address this issue by investigating the interaction effects of cell diffusion, cell density, public good diffusion and durability (factors known to individually influence costs and benefits of public goods production) on selection for cooperation. To be able to quantify these effects across a wide parameter space, we developed an individual-based simulation platform, consisting of digital cooperator and cheater bacteria inhabiting a finite two-dimensional continuous toroidal surface. Our simulations, which closely mimic microbial microcolony growth, revealed that: (i) either reduced cell diffusion (which keeps cooperators together) or reduced public good diffusion (which keeps the public goods closer to the producer) is not only essential but also sufficient for cooperation to be promoted; (ii) the sign of selection for or against cooperation can change as a function of cell density and in interaction with diffusion parameters; and (iii) increased public goods durability has opposing effects on the evolution of cooperation depending on the level of cell and public good diffusion. Our work highlights that interactions between key parameters of public goods cooperation give rise to complex fitness landscapes, a finding that calls for multifactorial approaches when studying microbial cooperation in natural systems.

What traits are carried on mobile genetic elements, and why?

Although similar to any other organism, prokaryotes can transfer genes vertically from mother cell to daughter cell, they can also exchange certain genes horizontally. Genes can move within and between genomes at fast rates because of mobile genetic elements (MGEs). Although mobile elements are fundamentally self-interested entities, and thus replicate for their own gain, they frequently carry genes beneficial for their hosts and/or the neighbours of their hosts. Many genes that are carried by mobile elements code for traits that are expressed outside of the cell. Such traits are involved in bacterial sociality, such as the production of public goods, which benefit a cell's neighbours, or the production of bacteriocins, which harm a cell's neighbours. In this study we review the patterns that are emerging in the types of genes carried by mobile elements, and discuss the evolutionary and ecological conditions under which mobile elements evolve to carry their peculiar mix of parasitic, beneficial and cooperative genes.

Kin selection and the evolution of sexual conflict.

Males and females do not always share the same evolutionary interests. This is particularly true in the case of multiple mating, where male-male competition can often lead to adaptations that are harmful to the female, and females can evolve counter adaptations to reduce the benefits males gain from such traits. Although social evolution has made substantial progress from kin selection theory, most studies of sexual conflict have ignored the effects of genetic relatedness. Here, I use a model of male harm and female resistance to investigate how kin selection affects the evolution of sexual conflict. Building on models of social evolution, I show that relatedness inhibits sexual conflict, in terms of male harm, whereas it has no effect on the evolution female resistance. This study examines a previously neglected mechanism that can potentially help to resolve sexual conflict over mating and highlights the potential importance of considering relatedness in empirical studies of sexual conflict.

Mobile DNA can drive lineage extinction in prokaryotic populations.

Natural selection ultimately acts on genes and other DNA sequences. Adaptations that are good for the gene can have adverse effects at higher levels of organization, including the individual or the population. Mobile genetic elements illustrate this principle well, because they can self-replicate within a genome at a cost to their host. As they are costly and can be transmitted horizontally, mobile elements can be seen as genomic parasites. It has been suggested that mobile elements may cause the extinction of their host populations. In organisms with very large populations, such as most bacteria, individual selection is highly effective in purging genomes of deleterious elements, suggesting that extinction is unlikely. Here we investigate the conditions under which mobile DNA can drive bacterial lineages to extinction. We use a range of epidemiological and ecological models to show that harmful mobile DNA can invade, and drive populations to extinction, provided their transmission rate is high and that mobile element-induced mortality is not too high. Population extinction becomes more likely when there are more elements in the population. Even if elements are costly, extinction can still occur because of the combined effect of horizontal gene transfer, a mortality induced by mobile elements. Our study highlights the potential of mobile DNA to be selected at the population level, as well as at the individual level.

Species-level selection reduces selfishness through competitive exclusion.

Adaptation does not necessarily lead to traits which are optimal for the population. This is because selection is often the strongest at the individual or gene level. The evolution of selfishness can lead to a 'tragedy of the commons', where traits such as aggression or social cheating reduce population size and may lead to extinction. This suggests that species-level selection will result whenever species differ in the incentive to be selfish. We explore this idea in a simple model that combines individual-level selection with ecology in two interacting species. Our model is not influenced by kin or trait-group selection. We find that individual selection in combination with competitive exclusion greatly increases the likelihood that selfish species go extinct. A simple example of this would be a vertebrate species that invests heavily into squabbles over breeding sites, which is then excluded by a species that invests more into direct reproduction. A multispecies simulation shows that these extinctions result in communities containing species that are much less selfish. Our results suggest that species-level selection and community dynamics play an important role in regulating the intensity of conflicts in natural populations.

Resolving the tragedy of the commons: the feedback between intraspecific conflict and population density.

Competition and conflict among individuals can favour exploitative strategies that undermine the common good. Theory suggests that this can lead to a tragedy of the commons and ultimately population extinction, a phenomenon known as evolutionary suicide. Here, I present a model of the evolutionary tragedy of the commons that explicitly considers the population dynamics where individuals invest in individually costly competitive traits. In the simplest form, this supports the notion that selection for high levels of conflict can cause evolutionary suicide. However, as competition comes with survival and fecundity costs, a feedback between the investment in competition and population density can act to reduce the level of conflict and prevent the population from going extinct. This suggests that the interaction between population ecology and the evolution of competition and conflict among individuals may be an important mechanism in resolving the level of competition and conflict among individuals.

Therapy supervision: the phenomena and the need.

Nursing has lagged behind in development, utilization, and valuing of therapy supervision. Nevertheless the mental health nursing clinical specialist is expected to be supervised and to be a supervisor of other professional staff. The integral nature of supervision in mental health nursing practice is demonstrated when one defines "therapy supervision," identifies its characteristics, and studies its clinical phases. As the role of the mental health clinical nurse specialist continues to expand, the importance of supervision in developing high level therapeutic skill has become more evident. In this paper various meanings of the concept of supervision and its similarities and differences with associated concepts, such as teaching and therapy, are explored. Essential characteristics are identified, and the application of the concept in practice and research is discussed. The inclusion of supervision training in graduate nursing curriculum, continuing education and in-service programs is advocated.