Cheating does not explain selective differences at high and low relatedness in a social amoeba

<p>Abstract</p> <p>Background</p> <p>Altruism can be favored by high relatedness among interactants. We tested the effect of relatedness in experimental populations of the social amoeba <it>Dictyostelium discoideum</it>, where altruism occurs in a starvation-induced social stage when some amoebae die to form a stalk that lifts the fertile spores above the soil facilitating dispersal. The single cells that aggregate during the social stage can be genetically diverse, which can lead to conflict over spore and stalk allocation. We mixed eight genetically distinct wild isolates and maintained twelve replicated populations at a high and a low relatedness treatment. After one and ten social generations we assessed the strain composition of the populations. We expected that some strains would be out-competed in both treatments. In addition, we expected that low relatedness might allow the persistence of social cheaters as it provides opportunity to exploit other strains.</p> <p>Results</p> <p>We found that at high relatedness a single clone prevailed in all twelve populations. At low relatedness three clones predominated in all twelve populations. Interestingly, exploitation of some clones by others in the social stage did not explain the results. When we mixed each winner against the pool of five losers, the winner did not prevail in the spores because all contributed fairly to the stalk and spores. Furthermore, the dominant clone at high-relatedness was not cheated by the other two that persisted at low relatedness. A combination of high spore production and short unicellular stage most successfully explained the three successful clones at low relatedness, but not why one of them fared better at high relatedness. Differences in density did not account for the results, as the clones did not differ in vegetative growth rates nor did they change the growth rates over relevant densities.</p> <p>Conclusions</p> <p>These results suggest that social competition and something beyond solitary growth differences occurs during the vegetative stage when amoebae eat bacteria and divide by binary fission. The high degree of repeatability of our results indicates that these effects are strong and points to the importance of new approaches to studying interactions in <it>D. discoideum</it>.</p

Polymorphic members of the lag gene family mediate kin discrimination in Dictyostelium

Self and kin discrimination are observed in most kingdoms of life and are mediated by highly polymorphic plasma membrane proteins. Sequence polymorphism, which is essential for effective recognition, is maintained by balancing selection. Dictyostelium discoideum are social amoebas that propagate as unicellular organisms but aggregate upon starvation and form fruiting bodies with viable spores and dead stalk cells. Aggregative development exposes Dictyostelium to the perils of chimerism, including cheating, which raises questions about how the victims survive in nature and how social cooperation persists. Dictyostelids can minimize the cost of chimerism by preferential cooperation with kin, but the mechanisms of kin discrimination are largely unknown. Dictyostelium lag genes encode transmembrane proteins with multiple immunoglobulin (Ig) repeats that participate in cell adhesion and signaling. Here, we describe their role in kin discrimination. We show that lagB1 and lagC1 are highly polymorphic in natural populations and that their sequence dissimilarity correlates well with wild-strain segregation. Deleting lagB1 and lagC1 results in strain segregation in chimeras with wild-type cells, whereas elimination of the nearly invariant homolog lagD1 has no such consequences. These findings reveal an early evolutionary origin of kin discrimination and provide insight into the mechanism of social recognition and immunity

A new social gene in Dictyostelium discoideum, chtB

Background: Competitive social interactions are ubiquitous in nature, but their genetic basis is difficult to determine. Much can be learned from single gene knockouts in a eukaryote microbe. The mutants can be competed with the parent to discern the social impact of that specific gene. Dictyostelium discoideum is a social amoeba that exhibits cooperative behavior in the construction of a multicellular fruiting body. It is a good model organism to study the genetic basis of cooperation since it has a sequenced genome and it is amenable to genetic manipulation. When two strains of D. discoideum are mixed, a cheater strain can exploit its social partner by differentiating more spore than its fair share relative to stalk cells. Cheater strains can be generated in the lab or found in the wild and genetic analyses have shown that cheating behavior can be achieved through many pathways. Results: We have characterized the knockout mutant chtB, which was isolated from a screen for cheater mutants that were also able to form normal fruiting bodies on their own. When mixed in equal proportions with parental strain cells, chtB mutants contributed almost 60% of the total number of spores. To do so, chtB cells inhibit wild type cells from becoming spores, as indicated by counts and by the wild type cells’ reduced expression of the prespore gene, cotB. We found no obvious fitness costs (morphology, doubling time in liquid medium, spore production, and germination efficiency) associated with the cheating ability of the chtB knockout. Conclusions: In this study we describe a new gene in D. discoideum, chtB, which when knocked out inhibits the parental strain from producing spores. Moreover, under lab conditions, we did not detect any fitness costs associated with this behavior

Concurrent coevolution of intra-organismal cheaters and resisters

The evolution of multicellularity is a major transition that is not yet fully understood. Specifically, we do not know whether there are any mechanisms by which multicellularity can be maintained without a single-cell bottleneck or other relatedness-enhancing mechanisms. Under low relatedness, cheaters can evolve that benefit from the altruistic behaviour of others without themselves sacrificing. If these are obligate cheaters, incapable of cooperating, their spread can lead to the demise of multicellularity. One possibility, however, is that cooperators can evolve resistance to cheaters. We tested this idea in a facultatively multicellular social amoeba, Dictyostelium discoideum. This amoeba usually exists as a single cell but, when stressed, thousands of cells aggregate to form a multicellular organism in which some of the cells sacrifice for the good of others. We used lineages that had undergone experimental evolution at very low relatedness, during which time obligate cheaters evolved. Unlike earlier experiments, which found resistance to cheaters that were prevented from evolving, we competed cheaters and noncheaters that evolved together, and cheaters with their ancestors. We found that noncheaters can evolve resistance to cheating before cheating sweeps through the population and multicellularity is lost. Our results provide insight into cheater-resister coevolutionary dynamics, in turn providing experimental evidence for the maintenance of at least a simple form of multicellularity by means other than high relatedness

In the social amoeba, Dictyostelium discoideum , density, not farming status, determines predatory success on unpalatable Escherichia coli

Background The social amoeba Dictyostelium discoideum interacts with bacteria in a variety of ways. It is a predator of bacteria, can be infected or harmed by bacteria, and can form symbiotic associations with bacteria. Some clones of D. discoideum function as primitive farmers because they carry bacteria through the normally sterile D. discoideum social stage, then release them after dispersal so the bacteria can proliferate and be harvested. Some farmer-associated bacteria produce small molecules that promote host farmer growth but inhibit the growth of non-farmer competitors. To test whether the farmers’ tolerance is specific or extends to other growth inhibitory bacteria, we tested whether farmer and non-farmer amoebae are differentially affected by E. coli strains of varying pathogenicity. Because the numbers of each organism may influence the outcome of amoeba-bacteria interactions, we also examined the influence of amoeba and bacteria density on the ability of D. discoideum to grow and develop on distinct bacterial strains. Results A subset of E. coli strains did not support amoeba proliferation on rich medium, independent of whether the amoebae were farmers or non-farmers. However, amoebae could proliferate on these strains if amoebae numbers are high relative to bacteria numbers, but again there was no difference in this ability between farmer and non-farmer clones of D. discoideum. Conclusions Our results show that farmer and non-farmers did not differ in their abilities to consume novel strains of E. coli, suggesting that farmer resistance to their own carried bacteria does not extend to foreign bacteria. We see that increasing the numbers of bacteria or amoebae increases their respective likelihood of competitive victory over the other, thus showing Allee effects. We hypothesize that higher bacteria numbers may result in higher concentrations of a toxic product or in a reduction of resources critical for amoeba survival, producing an environment inhospitable to amoeba predators. Greater amoeba numbers may counter this growth inhibition, possibly through reducing bacterial numbers via increased predation rates, or by producing something that neutralizes a potentially toxic bacterial product

Dienstanweisung für das Betriebspersonal der Desinfections-Anstalt I zu Berlin

DIENSTANWEISUNG FÜR DAS BETRIEBSPERSONAL DER DESINFECTIONS-ANSTALT I ZU BERLIN Dienstanweisung für das Betriebspersonal der Desinfections-Anstalt I zu Berlin / Kirschner, K. A. Martin (Public Domain) ( - ) Title page ( - ) Reichsarbeitsministerium Bücherei ( - ) Contents ( - ) I. Der Maschinist ( - ) II. Der Heizer (13) III. Die Aufseher (15) IV. Die Desinfectoren und Arbeiter (18) [Eingeklebter Zettel] (26a) V. Die Ausführung der Effectendesinfection (34) VI. Die Ausführung der Wohnungsdesinfection (37) Tabelle zum Gebrauch bei Wohnungsdesinfectionen mit Formaldehyd (51) ColorChart ( -

Nationwide indoor smoking ban and impact on smoking behaviour and lung function: a two-population natural experiment

INTRODUCTION Many countries have implemented indoor smoking bans over the past two decades. Although smoking bans have been shown to reduce cardiovascular outcomes, little is known about their impact on respiratory health. This study investigated the impact of a nationwide indoor smoking ban on smoking behaviour and lung function. METHODS We used repeated cross-sectional data from two large cohorts of the general population comprising 31 807 Swiss and 62 093 Danish adults. We compared associations between smoking ban and smoking prevalence and prebronchodilator lung function trends in Denmark (indoor smoking ban introduced in 2007) and Switzerland (indoor smoking ban introduced in 2010) from 2005 to 2010 using a quasi-experimental study design. We performed difference-in-difference analyses with linear regression models adjusted for age, sex, weight and height. RESULTS Denmark had a stronger decrease in active smokers compared with Switzerland. Also, forced expiratory volume in the first second was higher in Danish adults than in Swiss adults: 26 mL (95% CI 2.4 to 49) 1 year, 88 mL (65 to 112) 2 years, and 74 mL (51 to 98) 3 years after smoking ban implementation. Correspondingly, forced vital capacity was higher in Danish adults compared with Swiss adults (80 mL (50 to 109) after 1 year and 126 mL (97 to 155) after two and 3 years). Improvements were observed in both never-smokers and ever-smokers, most pronounced in ever-smokers. CONCLUSIONS Nationwide indoor smoking ban is associated with less smoking and improved lung function in the general population. Implementing an indoor smoking ban can improve lung function by influencing smoking behaviour and reducing secondhand smoke

NO2 and PM2.5 Exposures and Lung Function in Swiss Adults: Estimated Effects of Short-Term Exposures and Long-Term Exposures with and without Adjustment for Short-Term Deviations

Background: The impact of nitrogen dioxide (NO2) and particulate matter with an aerodynamic diameter of less than or equal to 2.5. microns (PM2.5) exposures on lung function has been investigated mainly in children and less in adults. Furthermore, it is unclear whether short-term deviations of air pollutant concentration need to be considered in long-term exposure models. Objectives: The aims of this study were to investigate the association between short-term air pollution exposure and lung function and to assess whether short-term deviations of air pollutant concentration should be integrated into long-term exposure models. Methods: Short-term (daily averages 0–7 d prior) and long-term (1- and 4-y means) NO2 and PM2.5 concentrations were modeled using satellite, land use, and meteorological data calibrated on ground measurements. Forced expiratory volume within the first second (FEV1) of forced exhalation and forced vital capacity (FVC) were measured during a LuftiBus assessment (2003–2012) and linked to exposure information from the Swiss National Cohort for 36,085 adults (ages 18–95 y). We used multiple linear regression to estimate adjusted associations, and additionally adjusted models of long-term exposures for short-term deviations in air pollutant concentrations. Results: A 10μg/m3 increase in NO2 and PM2.5 on the day of the pulmonary function test was associated with lower FEV1 and FVC (NO2: FEV1 −8.0 ml [95% confidence interval: −13.4, −2.7], FVC −16.7 ml [−23.4, −10.0]; PM2.5: FEV1 −15.3 ml [−21.9, −8.7], FVC −18.5 ml [−26.5, −10.5]). A 10μg/m3 increase in 1-y mean NO2 was also associated with lower FEV1 (−7.7 ml; −15.9, 0.5) and FVC (−21.6 ml; −31.9, −11.4), as was a 10μg/m3 increase in 1-y mean PM2.5 (FEV1: −42.2 ml; −56.9, −27.5; FVC: −82.0 ml; −100.1, −63.9). These associations were robust to adjustment for short-term deviations in the concentration of each air pollutant. Conclusions: Short- and long-term air pollution exposures were negatively associated with lung function, in particular long-term PM2.5 exposure with FVC. Our findings contribute substantially to the evidence of adverse associations between air pollution and lung function in adults. https://doi.org/10.1289/EHP752

Whole Genome Sequencing of Mutation Accumulation Lines Reveals a Low Mutation Rate in the Social Amoeba Dictyostelium discoideum

Spontaneous mutations play a central role in evolution. Despite their importance, mutation rates are some of the most elusive parameters to measure in evolutionary biology. The combination of mutation accumulation (MA) experiments and whole-genome sequencing now makes it possible to estimate mutation rates by directly observing new mutations at the molecular level across the whole genome. We performed an MA experiment with the social amoeba Dictyostelium discoideum and sequenced the genomes of three randomly chosen lines using high-throughput sequencing to estimate the spontaneous mutation rate in this model organism. The mitochondrial mutation rate of 6.76×10(-9), with a Poisson confidence interval of 4.1×10(-9) - 9.5×10(-9), per nucleotide per generation is slightly lower than estimates for other taxa. The mutation rate estimate for the nuclear DNA of 2.9×10(-11), with a Poisson confidence interval ranging from 7.4×10(-13) to 1.6×10(-10), is the lowest reported for any eukaryote. These results are consistent with low microsatellite mutation rates previously observed in D. discoideum and low levels of genetic variation observed in wild D. discoideum populations. In addition, D. discoideum has been shown to be quite resistant to DNA damage, which suggests an efficient DNA-repair mechanism that could be an adaptation to life in soil and frequent exposure to intracellular and extracellular mutagenic compounds. The social aspect of the life cycle of D. discoideum and a large portion of the genome under relaxed selection during vegetative growth could also select for a low mutation rate. This hypothesis is supported by a significantly lower mutation rate per cell division in multicellular eukaryotes compared with unicellular eukaryotes

A bacterial symbiont is converted from an inedible producer of beneficial molecules into food by a single mutation in the gacA gene

Stable multipartite mutualistic associations require that all partners benefit. We show that a single mutational step is sufficient to turn a symbiotic bacterium from an inedible but host-beneficial secondary metabolite producer into a host food source. The bacteria\u27s host is a farmer clone of the social amoeba Dictyostelium discoideum that carries and disperses bacteria during its spore stage. Associated with the farmer are two strains of Pseudomonas fluorescens, only one of which serves as a food source. The other strain produces diffusible small molecules: pyrrolnitrin, a known antifungal agent, and a chromene that potently enhances the farmer\u27s spore production and depresses a nonfarmer\u27s spore production. Genome sequence and phylogenetic analyses identify a derived point mutation in the food strain that generates a premature stop codon in a global activator (gacA), encoding the response regulator of a two-component regulatory system. Generation of a knockout mutant of this regulatory gene in the nonfood bacterial strain altered its secondary metabolite profile to match that of the food strain, and also, independently, converted it into a food source. These results suggest that a single mutation in an inedible ancestral strain that served a protective role converted it to a domesticated food source