Experimental elimination of parasites in nature leads to the evolution of increased resistance in hosts

A reduction in the strength of selection is expected to cause the evolution of reduced trait expression. Elimination of a parasite should thus cause the evolution of reduced resistance to that parasite. To test this prediction in nature, we studied the fourth- and eighth-generation descendants of guppies (Poecilia reticulata) introduced into four natural streams following experimental elimination of a common and deleterious parasite (Gyrodactylus spp.). After two generations of laboratory rearing to control for plasticity and maternal effects, we infected individual fish to assess their resistance to the parasite. Contrary to theoretical expectations, the introduced guppy populations had rapidly and repeatably evolved increased resistance to the now-absent parasite. This evolution was not owing to a resistance-tolerance trade-off, nor to differences in productivity among the sites. Instead, a leading candidate hypothesis is that the rapid life-history evolution typical in such introductions pleiotropically increases parasite resistance. Our study adds a new dimension to the growing evidence for contemporary evolution in the wild, and also points to the need for a re-consideration of simple expectations from host–parasite theory. In particular, our results highlight the need for increased consideration of multiple sources of selection and pleiotropy when studying evolution in natural contexts.     

Dynamics of mitochondrial inheritance in the evolution of binary mating types and two sexes

The uniparental inheritance (UPI) of mitochondria is thought to explain the evolution of two mating types or even true sexes with anisogametes. However, the exact role of UPI is not clearly understood. Here, we develop a new model, which considers the spread of UPI mutants within a biparental inheritance (BPI) population. Our model explicitly considers mitochondrial mutation and selection in parallel with the spread of UPI mutants and self-incompatible mating types. In line with earlier work, we find that UPI improves fitness under mitochondrial mutation accumulation, selfish conflict and mitonuclear coadaptation. However, we find that as UPI increases in the population its relative fitness advantage diminishes in a frequency-dependent manner. The fitness benefits of UPI ‘leak’ into the biparentally reproducing part of the population through successive matings, limiting the spread of UPI. Critically, while this process favours some degree of UPI, it neither leads to the establishment of linked mating types nor the collapse of multiple mating types to two. Only when two mating types exist beforehand can associated UPI mutants spread to fixation under the pressure of high mitochondrial mutation rate, large mitochondrial population size and selfish mutants. Variation in these parameters could account for the range of UPI actually observed in nature, from strict UPI in some Chlamydomonas species to BPI in yeast. We conclude that UPI of mitochondria alone is unlikely to have driven the evolution of two mating types in unicellular eukaryotes.     

The relationship between visual working memory and attention: retention of precise colour information in the absence of effects on perceptual selection

We examined the conditions under which a feature value in visual working memory (VWM) recruits visual attention to matching stimuli. Previous work has suggested that VWM supports two qualitatively different states of representation: an active state that interacts with perceptual selection and a passive (or accessory) state that does not. An alternative hypothesis is that VWM supports a single form of representation, with the precision of feature memory controlling whether or not the representation interacts with perceptual selection. The results of three experiments supported the dual-state hypothesis. We established conditions under which participants retained a relatively precise representation of a parcticular colour. If the colour was immediately task relevant, it reliably recruited attention to matching stimuli. However, if the colour was not immediately task relevant, it failed to interact with perceptual selection. Feature maintenance in VWM is not necessarily equivalent with feature-based attentional selection.     

Effect of ring-constrained phenylpropyloxyethylamines on sigma receptors

A series of ring-constrained phenylpropyloxyethylamines, partial opioid structure analogs and derivatives of a previously studied sigma (σ) receptor ligand, was synthesized and evaluated at σ and opioid receptors for receptor selectivity. The results of this study identified several compounds with nanomolar affinity at both σ receptor subtypes. Compounds 6 and 9 had the highest selectivity for both σ receptor subtypes, compared to μ opioid receptors. In addition, compounds 6 and 9 significantly reduced the convulsive effects of cocaine in mice, which would be consistent with antagonism of σ receptors.     

Structure of the Essential Diversity-Generating Retroelement Protein bAvd and Its Functionally Important Interaction with Reverse Transcriptase

1. Download : Download high-res image (245KB)
2. Download : Download full-size image

Highlights? bAvd forms a highly positively charged pentameric barrel ? bAvd binds both DNA and RNA, but without sequence preference ? The coding sequence for bAvd serves dual purposes ? The interaction of bAvd with bRT is likely to be important for retrohoming     

Balloon Venoplasty of Subclavian Vein and Brachiocephalic Junction to Enable Left Ventricular Lead Placement for Cardiac Resynchronisation Therapy

This report describes the successful implantation of a LV lead using balloon venoplasty to overcome a very tight stenosis of the right subclavian vein / brachiocephalic junction for cardiac resynchronisation therapy (CRT-P) in a patient with a right sided CRT-P system and a failed epicardial LV lead. It is important for device implanters to be familiar with interventional equipments and techniques such as balloon venoplasty to overcome difficult venous access.     

Metabolites from intestinal microbes shape Treg

Intestinal bacterial metabolites are an important communication tool between the host immune system and the commensal microbiota to establish mutualism. In a recent paper published in Science, Wendy Garrett and her colleagues report an exciting role of the three most abundant microbial-derived short-chain fatty acids (SCFA), acetic acid, propionic acid and butyric acid, in colonic regulatory T cell (cTreg) homeostasis.A number of studies have shown that increased cTreg numbers and their immunoregulatory function are promoted by the presence of commensal intestinal microbes (either individual species such as Bacteroides fragilis¹, defined benign consortia of bacteria such as the altered Schaedler flora² or groups of Clostridia³). In a recent paper in Science, Garrett and colleagues report how these effects are generated through molecular exchanges between the host and the enormous load of microbes carried in the lower intestine⁴.Smith et al.⁴ investigated the role of SCFA, which are bacterial fermentation products produced by a wide variety of bacteria through anaerobic acidogenic pathways. SCFA released by colonic bacteria have long been known to be important as a carbon source for colonic epithelial cells⁵. From this new work we can now see that signaling effects of SCFA also regulate cTreg homeostasis.Microbiota-derived SCFA were found to increase total (thymic-derived) cTreg numbers. The homing characteristics to the colon and the regulatory functions of these cells (such as IL-10 production) were also enhanced through SCFA treatment.These effects are mediated by the G-protein-coupled free fatty acid receptor 43 (GPR43). Using mice that are genetically deficient in this receptor, Smith et al. showed that this signaling pathway is responsible for the increased cTreg numbers in vivo and that signaling by SCFA reduces the susceptibility to chronic intestinal inflammation. As they found GPR43 expression on cTreg (compared with lower GPR43 expression on Treg from other sites) this may be a direct effect, e.g. alterations in histone deacetylation. However, other cell types in the GI tract also express GPR43, including enteroendocrine cells and other leukocytes, therefore indirect effects are not yet excluded. In fact, Atarashi and colleagues have recently published their studies of how Clostridial species induce cTreg⁶. They found that bacterial-derived SCFA stimulate epithelial cells to produce TGFβ, contributing to Treg differentiation and expansion.Whereas other species-specific bacterial molecules, such as B. fragilis-derived PSA, have previously been demonstrated to have immunomodulatory functions², the report by Smith et al. is an elegant demonstration of the ubiquitous and pervasive bacterial metabolites that impact on the mucosal immune system. There is really a rather promiscuous exchange of metabolites between the microbiota and the host, with metabolic pathways that require components of both eukaryotic and prokaryotic cells. Bile acids are a great example of such a mixed pathway, where a dysbiosis caused by obesity promotes liver cancer through alterations in the microbial bile acid metabolism⁷. Although Smith et al. do not see any SCFA-mediated effects on central Treg compartments (outside the colon), other bacterial metabolites that reach systemic sites likely modulate adaptive or innate immune cell function at systemic sites. This may eventually rationalize the observed increased incidence of intestinal inflammation and systemic immune-mediated disorders such as autoimmune or allergic diseases (Figure 1), which are often linked to changes within the microbiota due to diet or antibiotic use⁸.Open in a separate windowFigure 1Bacterial metabolites that reach systemic sites likely modulate adaptive or innate immune cell function at systemic sites. This may eventually rationalize the observed correlation of microbiota composition and susceptibility to systemic immune-mediated disorders such as autoimmune or allergic diseases.A clinical situation in which the colon faces a deficiency of SCFA happens after surgery that diverts the fecal stream into a stoma bag, leaving the distal colon without its normal contents. This operation may be carried out to protect a low surgical anastomosis after removal of a tumor. The result is that the defunctioned colon frequently becomes inflamed, a condition recognized as ''diversion colitis''. In some cases, treatment with SCFA has been able to treat the condition successfully⁹. The lack of SCFA as a carbon source for colonocytes was previously considered as a key factor in the aetiopathogenesis of the condition, although this will need to be reviewed in the light of the new data on the effects of SFCA on colonic Treg numbers and function.Our colonic health depends on our intestinal microbiota and what we feed them. Changes in Western dietary patterns, e.g., due to reduced intake of plant fibers, might drastically impact the production of SCFA within the intestine. Furthermore, Smith et al. demonstrate a direct effect of antibiotic (vancomycin) treatment on SCFA levels, which in turn affects intestinal immune regulation by reducing the number of cTreg.Taken together, this draws a picture of a superorganism composed of the host (us) and our microbiota, with the metabolic interface as an important communication tool. This allows the host and the microbiota to adapt to and communicate with each other. Originally, germ-free animals were derived to challenge the notion that the existence of higher organisms was irrevocably linked to their associated microbiotas¹⁰. Although the germ-free program succeeded¹¹, it has provided us with powerful tools to show that the original notion was justified: pervasive metabolic interactions and signaling make us the sum of our prokaryotic and eukaryotic cellular components.     

Increasing reducing power output (NADH) of glucose catabolism for reduction of xylose to xylitol by genetically engineered Escherichia coli AI05

Anaerobic homofermentative production of reduced products requires additional reducing power (NADH and/or NADPH) output from glucose catabolism. Previously, with an anaerobically expressed pyruvate dehydrogenase operon (aceEF-lpd), we doubled the reducing power output to four NADH per glucose (or 1.2 xylose) catabolized anaerobically, which satisfied the NADH requirement to establish a non-transgenic homoethanol pathway (1 glucose or 1.2 xylose ? 2 acetyl-CoA + 4 NADH ? 2 ethanol) in the engineered strain, Escherichia coli SZ420 (?frdBC ?ldhA ?ackA ?focA-pflB ?pdhR::pflBp6-pflBrbs-aceEF-lpd). In this study, E. coli SZ420 was further engineered for reduction of xylose to xylitol by (1) deleting the alcohol dehydrogenase gene (adhE) to divert NADH from the ethanol pathway; (2) deleting the glucose-specific PTS permease gene (ptsG) to eliminate catabolite repression and allow simultaneous uptake of glucose and xylose; (3) cloning the aldose reductase gene (xylI) of Candida boidinii to reduce xylose to xylitol. The resulting strain, E. coli AI05 (pAGI02), could in theory simultaneously uptake glucose and xylose, and utilize glucose as a source of reducing power for the reduction of xylose to xylitol, with an expected yield of four xylitol for each glucose consumed (Y_RPG = 4) under anaerobic conditions. In resting cell fermentation tests using glucose and xylose mixtures, E. coli AI05 (pAGI02) achieved an actual Y_RPG value of ~3.6, with xylitol as the major fermentation product and acetate as the by-product.