Edyta T. Sadowska

GENETIC CORRELATIONS BETWEEN BASAL AND MAXIMUM METABOLIC RATES IN A WILD RODENT: CONSEQUENCES FOR EVOLUTION OF ENDOTHERMY

Abstract According to the aerobic capacity model, endothermy in birds and mammals evolved as a correlated response to selection for an ability of sustained locomotor activity, rather than in a response to direct selection for thermoregulatory capabilities. A key assumption of the model is that aerobic capacity is functionally linked to basal metabolic rate (BMR). The assumption has been tested in several studies at the level of phenotypic variation among individuals or species, but none has provided a clear answer whether the traits are genetically correlated. Here we present results of a genetic analysis based on measurements of the basal and the maximum swim‐ and cold‐induced oxygen consumption in about 1000 bank voles from six generations of a laboratory colony, reared from animals captured in the field. Narrow sense heritability (h2) was about 0.5 for body mass, about 0.4 for mass‐independent basal and maximum metabolic rates, and about 0.3 for factorial aerobic scopes. Dominance genetic and common environmental (5 maternal) effects were not significant. Additive genetic correlation between BMR and the swim‐induced aerobic capacity was high and positive, whereas correlation resulting from specific‐environmental effects was negative. However, BMR was not genetically correlated with the cold‐induced aerobic capacity. The results are consistent with the aerobic capacity model of the evolution of endothermy in birds and mammals.

Individual variation and repeatability of basal metabolism in the bank vole, Clethrionomys glareolus

Basal metabolic rate (BMR) is a fundamental energetic trait and has been measured in hundreds of birds and mammals. Nevertheless, little is known about the consistency of the population–average BMR or its repeatability at the level of individual variation. Here, we report that average mass–independent BMR did not differ between two generations of bank voles or between two trials separated by one month. Individual differences in BMR were highly repeatable across the one month interval: the coefficient of intraclass correlation was 0.70 for absolute log–transformed values and 0.56 for mass–independent values. Thus, BMR can be a meaningful measure of an individual physiological characteristic and can be used to test hypotheses concerning relationships between BMR and other traits. On the other hand, mass–independent BMR did not differ significantly across families, and the coefficient of intraclass correlation for full sibs did not differ from zero, which suggests that heritability of BMR in voles is not high.

Is reproduction costly? No increase of oxidative damage in breeding bank voles

SUMMARY According to life-history theory, investment in reproduction is associated with costs, which should appear as decreased survival to the next reproduction or lower future reproductive success. It has been suggested that oxidative stress may be the proximate mechanism of these trade-offs. Despite numerous studies of the defense against reactive oxygen species (ROS) during reproduction, very little is known about the damage caused by ROS to the tissues of wild breeding animals. We measured oxidative damage to lipids and proteins in breeding bank vole (Myodes glareolus) females after rearing one and two litters, and in non-breeding females. We used bank voles from lines selected for high maximum aerobic metabolic rates (which also had high resting metabolic rates and food intake) and non-selected control lines. The oxidative damage was determined in heart, kidneys and skeletal muscles by measuring the concentration of thiobarbituric acid-reactive substances, as markers of lipid peroxidation, and carbonyl groups in proteins, as markers of protein oxidation. Surprisingly, we found that the oxidative damage to lipids in kidneys and muscles was actually lower in breeding than in non-breeding voles, and it did not differ between animals from the selected and control lines. Thus, contrary to our predictions, females that bred suffered lower levels of oxidative stress than those that did not reproduce. Elevated production of antioxidant enzymes and the protective role of sex hormones may explain the results. The results of the present study do not support the hypothesis that oxidative damage to tissues is the proximate mechanism of reproduction costs.

Genetic Correlations in a Wild Rodent: Grass-Eaters and Fast-Growers Evolve High Basal Metabolic Rates

Basal metabolic rate (BMR), commonly used as a measure of the cost of living, is highly variable among species, and sources of the variation are subject to an enduring debate among comparative biologists. One of the hypotheses links the variation in BMR with diversity of food habits and life-history traits. We test this hypothesis by asking how BMR of a particular species, the bank vole Myodes (= Clethrionomys) glareolus, would change under selection for high growth rate (measured as a postweaning body mass change; MDPW) and the ability to cope with a low-quality herbivorous diet (measured as body mass change during a four-day test; MDLQD). We show that both of the traits are heritable in the narrow sense (MDPW: h2 = 0.30; MDLQD: h2 = 0.19), and are genetically correlated with mass-independent BMR (additive genetic correlation, rA = 0.28 for MDPW and 0.37 for MDLQD). Thus, both of the traits could change in response to a selection, and the selection would also result in a correlated evolution of the level of metabolism. The results are consistent with the hypothesis that a part of the interspecific variation in BMR evolved in response to selection for life-history and ecological traits such as food habits.

Laboratory model of adaptive radiation: a selection experiment in the bank vole.

In a laboratory colony of a wild rodent, the bank vole Myodes (=Clethrionomys) glareolus, a multiway artificial selection experiment was applied to mimic evolution toward high aerobic metabolism achieved during locomotor activity, predatory behavior, and ability to cope with herbivorous diet. Four lines for each of the selection directions and four unselected control lines have been maintained. After three generations of within‐family selection, the maximum rate of oxygen consumption achieved during swimming was 15% higher in the selected than in the control lines (least square means, adjusted for body mass: 252.0 vs. 218.6 mL O2/h, P = 0.0001). When fed a low‐quality diet made of dried grass, voles from the lines selected for ability to cope with herbivorous diet lost about 0.7 g less mass than voles from the control lines (−2.44 vs. −3.16 g/4 d, P = 0.008). In lines selected for predatory behavior toward crickets, proportion of “predatory” individuals was higher than in the control lines (43.6% vs. 24.9%; P = 0.045), but “time to capture” calculated for the successful trials did not differ between the lines. The experiment continues, and the selected lines of voles will provide a unique model for testing hypotheses concerning correlated evolution of complex traits.

Evolution of basal metabolic rate in bank voles from a multidirectional selection experiment

A major theme in evolutionary and ecological physiology of terrestrial vertebrates encompasses the factors underlying the evolution of endothermy in birds and mammals and interspecific variation of basal metabolic rate (BMR). Here, we applied the experimental evolution approach and compared BMR in lines of a wild rodent, the bank vole (Myodes glareolus), selected for 11 generations for: high swim-induced aerobic metabolism (A), ability to maintain body mass on a low-quality herbivorous diet (H) and intensity of predatory behaviour towards crickets (P). Four replicate lines were maintained for each of the selection directions and an unselected control (C). In comparison to C lines, A lines achieved a 49% higher maximum rate of oxygen consumption during swimming, H lines lost 1.3 g less mass in the test with low-quality diet and P lines attacked crickets five times more frequently. BMR was significantly higher in A lines than in C or H lines (60.8, 56.6 and 54.4 ml O2 h−1, respectively), and the values were intermediate in P lines (59.0 ml O2 h−1). Results of the selection experiment provide support for the hypothesis of a positive association between BMR and aerobic exercise performance, but not for the association of adaptation to herbivorous diet with either a high or low BMR.

Initial Molecular-Level Response to Artificial Selection for Increased Aerobic Metabolism Occurs Primarily through Changes in Gene Expression

Experimental evolution combined with genome or transcriptome resequencing (Evolve and Resequence) represents a promising approach for advancing our understanding of the genetic basis of adaptation. Here, we applied this strategy to investigate the effect of selection on a complex trait in lines derived from a natural population of a small mammal. We analyzed the liver and heart transcriptomes of bank voles (Myodes [=Clethrionomys] glareolus) that had been selected for increased aerobic metabolism. The organs were sampled from 13th generation voles; at that point, the voles from four replicate selected lines had 48% higher maximum rates of oxygen consumption than those from four control lines. At the molecular level, the response to selection was primarily observed in gene expression: Over 300 genes were found to be differentially expressed between the selected and control lines and the transcriptome-wide pattern of expression distinguished selected lines from controls. No evidence for selection-driven changes of allele frequencies at coding sites was found: No single nucleotide polymorphism (SNP) changed frequency more than expected under drift alone and frequency changes aggregated over all SNPs did not separate selected and control lines. Nevertheless, among genes which showed highest differentiation in allele frequencies between selected and control lines we identified, using information about gene functions and the biology of the selected phenotype, plausible targets of selection; these genes, together with those identified in expression analysis, have been prioritized for further studies. Because our selection lines were derived from a natural population, the amount and the spectrum of variation available for selection probably closely approximated that typically found in populations of small mammals. Therefore, our results are relevant to the understanding of the molecular basis of complex adaptations occurring in natural vertebrate populations.

First Molecular Evidence for Puumala Hantavirus in Poland

Puumala virus (PUUV) causes mild to moderate cases of haemorrhagic fever with renal syndrome (HFRS), and is responsible for the majority of hantavirus infections of humans in Fennoscandia, Central and Western Europe. Although there are relatively many PUUV sequences available from different European countries, little is known about the presence of this virus in Poland. During population studies in 2009 a total of 45 bank voles were trapped at three sites in north-eastern Poland, namely islands on Dejguny and Dobskie Lakes and in a forest near Mikołajki. S and M segment-specific RT-PCR assays detected PUUV RNA in three animals from the Mikołajki site. The obtained partial S and M segment sequences demonstrated the highest similarity to the corresponding segments of a PUUV strain from Latvia. Analysis of chest cavity fluid samples by IgG ELISA using a yeast-expressed PUUV nucleocapsid protein resulted in the detection of two seropositive samples, both being also RT-PCR positive. Interestingly, at the trapping site in Mikołajki PUUV-positive bank voles belong to the Carpathian and Eastern genetic lineages within this species. In conclusion, we herein present the first molecular evidence for PUUV in the rodent reservoir from Poland.

Experimental Evolution on a Wild Mammal Species Results in Modifications of Gut Microbial Communities

Comparative studies have shown that diet, life history, and phylogeny interact to determine microbial community structure across mammalian hosts. However, these studies are often confounded by numerous factors. Selection experiments offer unique opportunities to validate conclusions and test hypotheses generated by comparative studies. We used a replicated, 15-generation selection experiment on bank voles (Myodes glareolus) that have been selected for high swim-induced aerobic metabolism, predatory behavior toward crickets, and the ability to maintain body mass on a high-fiber, herbivorous diet. We predicted that selection on host performance, mimicking adaptive radiation, would result in distinct microbial signatures. We collected foregut and cecum samples from animals that were all fed the same nutrient-rich diet and had not been subjected to any performance tests. We conducted microbial inventories of gut contents by sequencing the V4 region of the 16S rRNA gene. We found no differences in cecal microbial community structure or diversity between control lines and the aerobic or predatory lines. However, the cecal chambers of voles selected for herbivorous capability harbored distinct microbial communities that exhibited higher diversity than control lines. The foregut communities of herbivorous-selected voles were also distinct from control lines. Overall, this experiment suggests that differences in microbial communities across herbivorous mammals may be evolved, and not solely driven by current diet or other transient factors.

Learning ability in bank voles selected for high aerobic metabolism, predatory behaviour and herbivorous capability

Although great progress has been made in understanding neurological mechanisms of cognitive processes, the questions concerning interrelation between evolution of cognitive abilities and evolution of diverse life histories and adaptive strategies remains largely open. We approached the problem using a unique experimental evolution model system: lines of bank voles selected for high swim-induced aerobic metabolism, intensity of predatory behaviour towards crickets, and ability to grow on a low-quality herbivorous diet. To test a hypothesis that selection for these traits resulted in correlated changes in spatial learning and memory, voles from generation 13 of the selected and unselected control lines were examined in Morris water maze. Most of the individuals successfully learned the position of the platform hidden under water surface, but the spatial learning scores did not differ significantly between selection directions. The results are not consistent with either the hypothesis of a functional trade-off between high cognitive abilities and capability to cope with adverse nutritional conditions, or the hypothesis of a positive link between evolution of cognitive abilities and high aerobic exercise performance.