Vertebrate defense against parasites: Interactions between avoidance,resistance, and tolerance

Hosts can utilize different types of defense against the effects of parasitism, including avoidance, resistance, and tolerance. Typically, there is tremendous heterogeneity among hosts in these defense mechanisms that may be rooted in the costs associated with defense and lead to trade‐offs with other life‐history traits. Trade‐offs may also exist between the defense mechanisms, but the relationships between avoidance, resistance, and tolerance have rarely been studied. Here, we assessed these three defense traits under common garden conditions in a natural host–parasite system, the trematode eye‐fluke Diplostomum pseudospathaceum and its second intermediate fish host. We looked at host individuals originating from four genetically distinct populations of two closely related salmonid species (Atlantic salmon, Salmo salar and sea trout, Salmo trutta trutta) to estimate the magnitude of variation in these defense traits and the relationships among them. We show species‐specific variation in resistance and tolerance and population‐specific variation in resistance. Further, we demonstrate evidence for a trade‐off between resistance and tolerance. Our results suggest that the variation in host defense can at least partly result from a compromise between different interacting defense traits, the relative importance of which is likely to be shaped by environmental components. Overall, this study emphasizes the importance of considering different components of the host defense system when making predictions on the outcome of host–parasite interactions.     

Optimized self-excising Cre-expression cassette for mammalian cells

We observed that overexpression of Cre recombinase in 293 T cells has toxic effects and that the chicken beta-actin promoter is active in Escherichia coli, causing expression of Cre in bacteria. This led to significant problems in the cloning of Cre/loxP constructs. Leaky Cre-expression in E. coli, and toxicity of the Cre overexpression in mammalian cells, were solved by constructing a novel silent self-inactivating Cre (SSi-Cre) expression cassette. The SSi-Cre is based on modified loxP sites flanking the Cre/Int/DsRed fusion gene containing a Cre coding sequence interrupted by an intron, which prevents leaky expression of Cre in E. coli. Additionally, this system contains a reporter gene to visualize Cre activity by fluorescent microscopy. The SSi-Cre cassette provides a universal strategy for the generation of Cre/loxP constructs, as well as a solution to the toxicity caused by the overexpression of Cre in target cells. SSi-Cre should thus provide a useful tool for various applications based on the Cre/loxP system.     

Local Adaptation and Genetics of Acid-Stress Tolerance in the Moor Frog, Rana arvalis

As potential to adapt to environmental stress can be essential for population persistence, knowledge on the genetic architecture of local adaptation is important for conservation genetics. We investigated the relative importance of additive genetic, dominance and maternal effects contributions to acid stress tolerance in two moor frog (Rana arvalis) populations originating from low and neutral pH habitats. Experiments with crosses obtained from artificial matings revealed that embryos from the acid origin population were more tolerant to low pH than embryos from the neutral origin population in embryonic survival rates, but not in terms of developmental stability, developmental and growth rates. Strong maternal effect and small additive genetic contributions to variation were detected in all traits in both populations. In general, dominance contributions to variance in different traits were of similar magnitude to the additive genetic effects, but dominance effects outweighed the additive genetic and maternal effects contributions to early growth in both populations. Furthermore, the expression of additive genetic variance was independent of pH treatment, suggesting little additive genetic variation in acid stress tolerance. The results suggest that although local genetic adaptation to acid stress has taken place, the current variation in acid stress tolerance in acidified populations may owe largely to non-genetic effects. However, low but significant heritabilities (h ² 0.07–0.22) in all traits – including viability itself – under a wide range of pH conditions suggests that environmental stress created by low pH is unlikely to lower moor frog populations' ability to respond to selection in the traits studied. Nevertheless, acid conditions could lower populations' ability to respond to selection in the long run through reduction in effective population size.     

Human exposure by mobile phones in enclosed areas

Human exposure by mobile phones in enclosed areas such as train carriages, elevators, and cars is considered. Equivalent power density and whole body specific absorption rate (SAR) are estimated by applying multimode resonant cavity theory and a straight forward worst case approach. The results show that exceeding the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines due to simultaneous use of several mobile phones in enclosed areas is highly improbable.     

Environmental and population dependency of genetic variability-fitness correlations in Rana temporaria

Abstract Considerable effort has been invested in studying the relationship between fitness and genetic variability. While evidence exists both for and against positive genetic variability-fitness correlations (GFC), the possible environment and population-dependency of GFCs has seldom been tested. We investigated GFCs in common frog (Rana temporaria) tadpoles reared under different temperatures and feeding regimes in four replicate populations. Genetic variability in eight microsatellite loci in 238 parents was used to estimate heterozygosity (H) and mean expected d2 in 158-sibships (4515 offspring). Generalized linear mixed model analyses of offspring fitness traits (survival to metamorphosis, developmental and growth rate) revealed that offspring survival probability was positively correlated with H, and that relationships were similar in all four populations tested. However, significant interaction between other genetic variability measures (d2, relatedness) and treatment conditions indicated that GFCs were detectable in some, but not in all environments. Interestingly, GFCs between survival and both heterozygosity and relatedness were most pronounced in stressful environments (i.e. limited food). Developmental and growth rates were significantly associated with d2 but less with H and relatedness. Furthermore, many of these GFCs were population-specific. These results suggest--in line with the contention that expression of inbreeding depression can be environment dependent--that GFCs can also be highly sensitive to the environmental conditions under which they are measured. The results further suggest that the observed positive correlation between H and survival probability is likely to be explainable by the 'general', rather than by the 'local' or 'direct' effect hypotheses.     

Membrane-bound pyrophosphatase of Thermotoga maritima requires sodium for activity

Membrane-bound pyrophosphatase of the hyperthermophilic bacterium Thermotoga maritima(Tm-PPase), a homologue of H(+)-translocating pyrophosphatase, was expressed in Escherichia coli and isolated as inner membrane vesicles. In contrast to all previously studied H(+)-PPases, both native and recombinant Tm-PPases exhibited an absolute requirement for Na(+) but displayed the highest activity in the presence of millimolar levels of both Na(+) and K(+). Detergent-solubilized recombinant Tm-PPase was thermostable and retained the monovalent cation requirements of the membrane-embedded enzyme. Steady-state kinetic analysis of pyrophosphate hydrolysis by the wild-type enzyme suggested that two Na(+) binding sites and one K(+) binding site are involved in enzyme activation. The affinity of the site that binds Na(+) first is increased with increasing K(+) concentration. In contrast, only one Na(+) binding site (K(+)-dependent) and one K(+) binding site were involved in activation of the Asp(703) --> Asn variant. Thus, Asp(703) may form part of the K(+)-independent Na(+) binding site. Unlike all other membrane and soluble PPases, Tm-PPase did not catalyze oxygen exchange between phosphate and water. However, solubilized Tm-PPase exhibited low but measurable PP(i)-synthesizing activity, which also required Na(+) but was inhibited by K(+). These results demonstrate that T. maritima PPase belongs to a previously unknown subfamily of Na(+)-dependent H(+)-PPase homologues and may be an analogue of Na(+),K(+)-ATPase.     

Do pheromones reveal male immunocompetence?

Pheromones function not only as mate attractors, but they may also relay important information to prospective mates. It has been shown that vertebrates can distinguish, via olfactory mechanisms, major histocompatibility complex types in their prospective mates. However, whether pheromones can transmit information about immunocompetence is unknown. Here, we show that female mealworm beetles (Tenebrio molitor) prefer pheromones from males with better immunocompetence, indicated by a faster encapsulation rate against a novel antigen, and higher levels of phenoloxidase in haemolymph. Thus, the present study indicates that pheromones could transmit information about males' parasite resistance ability and may work as a reliable sexual ornament for female choice.     

Temporal variation in predation risk: stage-dependency, graded responses and fitness costs in tadpole antipredator defences

Temporal variation in predation risk may be an important determinant of prey antipredator behaviours. According to the risk allocation hypothesis, the strongest antipredator behaviours are expected when periods of high risk are short and infrequent. We tested this prediction in a laboratory experiment where common frog Rana temporaria tadpoles were raised form early larval stages until metamorphosis. We manipulated the time a predatory Aeshna dragonfly larva was present and recorded behavioural responses (activity) of the tadpoles at three different time points during the tadpoles' development. We also investigated how tadpole shape, size and age at metamorphosis were affected by temporal variation in predation risk. We found that during the two first time points activity was always lowest in the constant high-risk situation. However, antipredator response in the two treatments with brief high-risk situation increased as tadpoles developed, and by the third time point, when the tadpoles were close to metamorphosis, activity was as low as in the constant high-risk situation. Exposure to chemical cues of a predation event tended to reduce activity during the first time period, but caused no response later on. Induced morphological changes (deeper tail and shorter relative body length) were graded the response being stronger as the time spent in the proximity of predator increased. Tadpoles in the brief risk and chemical cue treatments showed intermediate responses. Modification of life history was only found in the constant high-risk treatment in which tadpoles had longer larval period and larger metamorphic size. Our results indicate that both behavioural and morphological defences were sensitive to temporal variation in predation risk, but behaviour did not respond in the manner predicted by the risk allocation model. We discuss the roles of concentration of predator chemical cues and prey stage-dependency in determining these responses.     

Breeding habitat selection and larval performance of two anurans in freshwater rock-pools

I studied breeding habitat choice of common toad Bufo bufo and common frog Rana temporaria in 221 freshwater rock-pools on 15 small islands in the southwestern coast of Finland. I monitored tadpole growth rate in a number of rock-pools differing in size, competitors and predator regime. Furthermore, I carried out a short-term field experiment to investigate the effects of different predators on larval survival. Rana bred in pools of virtually all sizes, although the very smallest ones were usually avoided. A logistic regression analysis showed that the amount of aquatic macrophyte vegetation and pool surface area were the most important pool characters for Rana. Bufo preferred the largest pools, and pool surface area was by far the most important habitat variable. Rana avoided pools with the lowest pH. but was more common than expected at pools with pH around 6. In Bufo such an effect was not found, but this may be due to relatively low number of pools occupied by Bufo. In Rana there was a positive correlation between pool area and tadpole body length in early June, but two weeks later this relationship was not significant. In Bufo tadpole body size was not significantly affected by pool area. Presence of predators or presence of the other tadpole species did not affect growth or tadpole body size in either Rana or Bufo. A short-term experiment in wading pools indicated that both species suffered heavy mortality in the presence of a diving beetle larva. In the presence of predatory fish, mortality of Bufo did not differ from controls, whereas mortality was high among Rana tadpoles. It appears that although predators prey heavily on Rana tadpoles, they do not exclude Rana from larger rock-pools. However, it is not evident from this study why Bufo avoids smaller pools.     

MODE OF SWARMING IN RELATION TO REPRODUCTIVE ISOLATION IN MAYFLIES

Mayfly males swarm, that is they fly in a fixed pattern by a specific object, the swarm marker. Females orientate to the same markers. Leptophlebia marginata mayflies were observed to orientate to two kinds of objects in a single locality in central Finland: to trees and to horizontal pale objects on the ground; when dispersed or moved to the other type of marker, they returned to their former orientation. Tree swarming is by far the most common mode of swarming, but some horizontally orientating populations were found. Sympatric populations are genetically and morphologically distinct, whereas other populations appear to have some gene flow between the swarming types. The tree-swarming mode appears to be primitive and the horizontal mode derived; wind rather than predation is the factor favoring swarming close to the ground. Swarming constitutes an effective mechanism of premating isolation in mayflies.