James S. Ruff

Experimental viral evolution to specific host MHC genotypes reveals fitness and virulence trade-offs in alternative MHC types

The unprecedented genetic diversity found at vertebrate MHC (major histocompatibility complex) loci influences susceptibility to most infectious and autoimmune diseases. The evolutionary explanation for how these polymorphisms are maintained has been controversial. One leading explanation, antagonistic coevolution (also known as the Red Queen), postulates a never-ending molecular arms race where pathogens evolve to evade immune recognition by common MHC alleles, which in turn provides a selective advantage to hosts carrying rare MHC alleles. This cyclical process leads to negative frequency-dependent selection and promotes MHC diversity if two conditions are met: (i) pathogen adaptation must produce trade-offs that result in pathogen fitness being higher in familiar (i.e., host MHC genotype adapted to) vs. unfamiliar host MHC genotypes; and (ii) this adaptation must produce correlated patterns of virulence (i.e., disease severity). Here we test these fundamental assumptions using an experimental evolutionary approach (serial passage). We demonstrate rapid adaptation and virulence evolution of a mouse-specific retrovirus to its mammalian host across multiple MHC genotypes. Critically, this adaptive response results in trade-offs (i.e., antagonistic pleiotropy) between host MHC genotypes; both viral fitness and virulence is substantially higher in familiar versus unfamiliar MHC genotypes. These data are unique in experimentally confirming the requisite conditions of the antagonistic coevolution model of MHC evolution and providing quantification of fitness effects for pathogen and host. These data help explain the unprecedented diversity of MHC genes, including how disease-causing alleles are maintained.

MHC SIGNALING DURING SOCIAL COMMUNICATION

The major histocompatibility complex (MHC) has been known to play a critical role in immune recognition since the 1950s. It was a surprise, then, in the 1970s when the first report appeared indicating MHC might also function in social signaling. Since this seminal discovery, MHC signaling has been found throughout vertebrates and its known functions have expanded beyond mate choice to include a suite of behaviors from kin-biased cooperation, parent-progeny recognition to pregnancy block. The widespread occurrence of MHC in social signaling has revealed conserved behavioral-genetic mechanisms that span vertebrates and includes humans. The identity of the signals chemical constituents and the receptors responsible for the perception of the signal have remained elusive, but recent advances have enabled the identification of the key components of the behavioral circuit. In this chapter we organize recent findings from the literature and discuss them in relation to four nonmutually exclusive models wherein MHC functions as a signal of (i) individuality, (ii) relatedness, (iii) genetic compatibility and (iv) quality. We also synthesize current mechanistic studies, showing how knowledge about the molecular basis of MHC signaling can lead to elegant and informative experimental manipulations. Finally, we discuss current evidence relating to the primordial functions of the MHC, including the possibility that its role in social signaling may be ancestral to its central role in adaptive immunity.

Human-relevant Levels of Added Sugar Consumption Increase Female Mortality and Lower Male Fitness in Mice

Consumption of added sugar has increased over recent decades and is correlated with numerous diseases. Rodent models have elucidated mechanisms of toxicity, but only at concentrations beyond typical human exposure. Here we show that comparatively low levels of added sugar consumption have substantial negative effects on mouse survival, competitive ability, and reproduction. Using Organismal Performance Assays (OPAs) – in which mice fed human-relevant concentrations of added sugar (25% Kcal from a mixture of fructose and glucose [F/G]) and control mice compete in seminatural enclosures for territories, resources and mates – we demonstrate that F/G-fed females experience a two-fold increase in mortality while F/G-fed males control 26% fewer territories and produce 25% less offspring. These findings represent the lowest level of sugar consumption shown to adversely affect mammalian health. Clinical defects of F/G-fed mice were decreased glucose clearance and increased fasting cholesterol. Our data highlight that physiological adversity can exist when clinical disruptions are minor, and suggest that OPAs represent a promising technique for unmasking negative effects of toxicants.

Protein pheromone expression levels predict and respond to the formation of social dominance networks

Communication signals are key regulators of social networks and are thought to be under selective pressure to honestly reflect social status, including dominance status. The odours of dominants and nondominants differentially influence behaviour, and identification of the specific pheromones associated with, and predictive of, dominance status is essential for understanding the mechanisms of network formation and maintenance. In mice, major urinary proteins (MUPs) are excreted in extraordinary large quantities and expression level has been hypothesized to provide an honest signal of dominance status. Here, we evaluate whether MUPs are associated with dominance in wild‐derived mice by analysing expression levels before, during and after competition for reproductive resources over 3 days. During competition, dominant males have 24% greater urinary MUP expression than nondominants. The MUP darcin, a pheromone that stimulates female attraction, is predictive of dominance status: dominant males have higher darcin expression before competition. Dominants also have a higher ratio of darcin to other MUPs before and during competition. These differences appear transient, because there are no differences in MUPs or darcin after competition. We also find MUP expression is affected by sire dominance status: socially naive sons of dominant males have lower MUP expression, but this apparent repression is released during competition. A requisite condition for the evolution of communication signals is honesty, and we provide novel insight into pheromones and social networks by showing that MUP and darcin expression is a reliable signal of dominance status, a primary determinant of male fitness in many species.

Experimental viral evolution reveals major histocompatibility complex polymorphisms as the primary host factors controlling pathogen adaptation and virulence.

Using an experimental evolution approach, we recently demonstrated that the mouse-specific pathogen Friend virus (FV) complex adapted to specific major histocompatibility complex (MHC) genotypes, which resulted in fitness tradeoffs when viruses were exposed to hosts possessing novel MHC polymorphisms. Here we report the analysis of patterns of pathogen adaptation and virulence evolution from viruses adapting to one of three hosts that differ across the entire genome (A/WySn, DBA/2J and BALB/c). We found that serial passage of FV complex through these mouse genotypes resulted in significant increases in pathogen fitness (156-fold) and virulence (11-fold). Adaptive responses by post-passage viruses also resulted in host–genotype-specific patterns of adaptation. To evaluate the relative importance of MHC versus non-MHC polymorphisms as factors influencing pathogen adaptation and virulence, we compared the magnitude of fitness tradeoffs incurred by post-passage viruses when infecting hosts possessing either novel MHC polymorphisms alone or hosts possessing novel MHC and non-MHC polymorphisms. MHC polymorphisms alone accounted for 71% and 83% of the total observed reductions in viral fitness and virulence in unfamiliar host genotypes, respectively. Strikingly, these data suggest that genetic polymorphisms within the MHC, a gene region representing only ∼0.1% of the genome, are major host factors influencing pathogen adaptation and virulence evolution.

Low-dose paroxetine exposure causes lifetime declines in male mouse body weight, reproduction and competitive ability as measured by the novel organismal performance assay

Paroxetine is a selective serotonin reuptake inhibitor (SSRI) that is currently available on the market and is suspected of causing congenital malformations in babies born to mothers who take the drug during the first trimester of pregnancy. We utilized organismal performance assays (OPAs), a novel toxicity assessment method, to assess the safety of paroxetine during pregnancy in a rodent model. OPAs utilize genetically diverse wild mice (Mus musculus) to evaluate competitive performance between experimental and control animals as they compete among each other for limited resources in semi-natural enclosures. Performance measures included reproductive success, male competitive ability and survivorship. Paroxetine-exposed males weighed 13% less, had 44% fewer offspring, dominated 53% fewer territories and experienced a 2.5-fold increased trend in mortality, when compared with controls. Paroxetine-exposed females had 65% fewer offspring early in the study, but rebounded at later time points, presumably, because they were no longer exposed to paroxetine. In cages, paroxetine-exposed breeders took 2.3 times longer to produce their first litter and pups of both sexes experienced reduced weight when compared with controls. Low-dose paroxetine-induced health declines detected in this study that were undetected in preclinical trials with doses 2.5-8 times higher than human therapeutic doses. These data indicate that OPAs detect phenotypic adversity and provide unique information that could be useful towards safety testing during pharmaceutical development.

Competitive Ability in Male House Mice (Mus musculus): Genetic Influences

Conspecifics of many animal species physically compete to gain reproductive resources and thus fitness. Despite the importance of competitive ability across the animal kingdom, specific traits that influence or underpin competitive ability are poorly characterized. Here, we investigate whether there are genetic influences on competitive ability within male house mice. Additionally, we examined if litter demographics (litter size and litter sex ratio) influence competitive ability. We phenotyped two generations for a male’s ability to possess a reproductive resource––a prime nesting site––using semi-natural enclosures with mixed sex groupings. We used the “Animal Model” coupled with an extensive pedigree to estimate several genetic parameters. Competitive ability was found to be highly heritable, but only displayed a moderate genetic correlation to body mass. Interestingly, litter sex ratio had a weak negative influence on competitive ability. Litter size had no significant influence on competitive ability. Our study also highlights how much remains unknown about the proximal causes of competitive ability.

Compared to Sucrose, Previous Consumption of Fructose and Glucose Monosaccharides Reduces Survival and Fitness of Female Mice

BACKGROUND Intake of added sugar has been shown to correlate with many human metabolic diseases, and rodent models have characterized numerous aspects of the resulting disease phenotypes. However, there is a controversy about whether differential health effects occur because of the consumption of either of the two common types of added sugar-high-fructose corn syrup (fructose and glucose monosaccharides; F/G) or table sugar (sucrose, a fructose and glucose disaccharide). OBJECTIVES We tested the equivalence of sucrose- vs. F/G-containing diets on mouse (Mus musculus) longevity, reproductive success, and social dominance. METHODS We fed wild-derived mice, outbred mice descended from wild-caught ancestors, a diet in which 25% of the calories came from either an equal ratio of F/G or an isocaloric amount of sucrose (both diets had 63% of total calories as carbohydrates). Exposure lasted 40 wk, starting at weaning (21 d of age), and then mice (104 females and 56 males) were released into organismal performances assays-seminatural enclosures where mice competed for territories, resources, and mates for 32 wk. Within enclosures all mice consumed the F/G diet. RESULTS Females initially fed the F/G diet experienced a mortality rate 1.9 times the rate (P = 0.012) and produced 26.4% fewer offspring than females initially fed sucrose (P = 0.001). This reproductive deficiency was present before mortality differences, suggesting the F/G diet was causing physiologic performance deficits prior to mortality. No differential patterns in survival, reproduction, or social dominance were observed in males, indicating a sex-specific outcome of exposure. CONCLUSION This study provides experimental evidence that the consumption of human-relevant levels of F/G is more deleterious than an isocaloric amount of sucrose for key organism-level health measures in female mice.

Quantification of cerivastatin toxicity supports organismal performance assays as an effective tool during pharmaceutical safety assessment

A major problem in pharmaceutical development is that adverse effects remain undetected during preclinical and clinical trials, but are later revealed after market release when prescribed to many patients. We have developed a fitness assay known as the organismal performance assay (OPA), which evaluates individual performance by utilizing outbred wild mice (Mus musculus) that are assigned to an exposed or control group, which compete against each other for resources within semi‐natural enclosures. Performance measurements included reproductive success, survival, and male competitive ability. Our aim was to utilize cerivastatin (Baycol®, Bayer), a pharmaceutical with known adverse effects, as a positive control to assess OPAs as a potential tool for evaluating the safety of compounds during preclinical trials. Mice were exposed to cerivastatin (~4.5 mg/kg/day) into early adulthood. Exposure ceased and animals were released into semi‐natural enclosures. Within enclosures, cerivastatin‐exposed females had 25% fewer offspring and cerivastatin‐exposed males had 10% less body mass, occupied 63% fewer territories, sired 41% fewer offspring, and experienced a threefold increase in mortality when compared to controls. OPAs detected several cerivastatin‐induced adverse effects indicating that fitness assays, commonly used in ecology and evolutionary biology, could be useful as an additional tool in safety testing during pharmaceutical development.

Does allopreening control avian ectoparasites

For birds, the first line of defence against ectoparasites is preening. The effectiveness of self-preening for ectoparasite control is well known. By contrast, the ectoparasite control function of allopreening—in which one birds preens another—has not been rigorously tested. We infested captive pigeons with identical numbers of parasitic lice, and then compared rates of allopreening to the abundance of lice on the birds over time. We documented a negative relationship between rates of allopreening and the number of lice on birds. Moreover, we found that allopreening was a better predictor of louse abundance than self-preening. Our data suggest that allopreening may be a more important means of ectoparasite defence than self-preening when birds live in groups. Our results have important implications for the evolution of social behaviour.