Aneta Książek

Anatomic and energetic correlates of divergent selection for basal metabolic rate in laboratory mice.

The aerobic capacity model postulates that high basal metabolic rates (BMR) associated with endothermy evolved as a correlated response to the selection on maximum, peak metabolic rate V̇o2,max. Furthermore, the model assumes that BMR and V̇o2,max are causally linked, and therefore, evolutionary changes in their levels cannot occur independently. To test this, we compared metabolic and anatomical correlates of selection for high and low body mass–corrected BMR in males of laboratory mice of F18 and F19 selected generations. Divergent selection resulted in between‐line difference in BMR equivalent to 2.3 phenotypic standard deviation units. V̇o2,max elicited by forced swimming in 20°C water was higher in the low BMR than high BMR line and did not differ between the lines when elicited by exposure to heliox at −2.5°C. Moreover, the magnitude of swim‐ and heliox‐induced hypothermia was significantly smaller in low BMR mice, whereas their interscapular brown adipose tissue was larger than in high BMR mice. Our results are therefore at variance with the predictions of aerobic capacity model. The selection also resulted in correlated response in food consumption (C) and masses of metabolically active internal organs: kidneys, liver, small intestine, and heart, which fuel maximum, sustained metabolic rate (SusMR) rather than V̇o2,max. These correlated responses were strong enough to claim the existence of positive, genetic correlations between BMR and the mass of viscera as well as C. Thus, our findings support the suggestion that BMR evolved as a correlated response to selection for SusMR, not V̇o2,max. In functional terms BMR should therefore be interpreted as a measure of energetic costs of maintenance of metabolic machinery necessary to sustain high levels of energy assimilation rate.

Artificial Selection on Metabolic Rates and Related Traits in Rodents

Abstract Artificial selection experiments are potentially powerful, yet under-utilized tool of evolutionary and physiological ecology. Here we analyze and review three important aspects of such experiments. First, we consider the effects of instrumental measurement errors and random fluctuations of body mass on the total phenotypic variation. We illustrate this with the analysis of measurements of oxygen consumption in an open-flow respirometry set-ups. We conclude that measurement errors and fluctuations of body mass are likely to reduce the repeatability of oxygen consumption by about one third. Using published estimates of repeatability of metabolic rates we also showed that it does not tend to decline with increasing time between measurements. Second, we review data on narrow sense heritability (h2) of metabolic rates in mammals. The results are equivocal: many studies report very low (∼0.1) h2, whereas some recent studies (including our own estimates of h2 in laboratory mice, obtained by means of parent-offspring regression) report significant h2 ≥ 0.4. Finally, we discuss consequences of the lack of replicated lines in artificial selection experiments. We focus on the confounding effect of genetic drift on statistical inferences related to primary (selected) and secondary (correlated) traits, in the absence of replications. We review literature data and analyze them following the guidelines formulated by Henderson (1989, 1997). We conclude that most results obtained in unreplicated experiments are probably robust enough to ascribe them to the effect of selection, rather than genetic drift. However, Hendersons guidelines by no means should be treated as a legitimate substitute of the analysis of variance, based on replicated lines.

Determinants of intra-specific variation in basal metabolic rate.

Basal metabolic rate (BMR) provides a widely accepted benchmark of metabolic expenditure for endotherms under laboratory and natural conditions. While most studies examining BMR have concentrated on inter-specific variation, relatively less attention has been paid to the determinants of within-species variation. Even fewer studies have analysed the determinants of within-species BMR variation corrected for the strong influence of body mass by appropriate means (e.g. ANCOVA). Here, we review recent advancements in studies on the quantitative genetics of BMR and organ mass variation, along with their molecular genetics. Next, we decompose BMR variation at the organ, tissue and molecular level. We conclude that within-species variation in BMR and its components have a clear genetic signature, and are functionally linked to key metabolic process at all levels of biological organization. We highlight the need to integrate molecular genetics with conventional metabolic field studies to reveal the adaptive significance of metabolic variation. Since comparing gene expressions inter-specifically is problematic, within-species studies are more likely to inform us about the genetic underpinnings of BMR. We also urge for better integration of animal and medical research on BMR; the latter is quickly advancing thanks to the application of imaging technologies and ‘omics’ studies. We also suggest that much insight on the biochemical and molecular underpinnings of BMR variation can be gained from integrating studies on the mammalian target of rapamycin (mTOR), which appears to be the major regulatory pathway influencing the key molecular components of BMR.

Plant–herbivore interactions: silicon concentration in tussock sedges and population dynamics of root voles

Summary It has been hypothesized that the induction of silicon (Si)-based plant defence in response to herbivore damage may engender rodent population cycles. Many studies have also considered accumulation of Si as a process controlled by geo-hydrological factors. To test these ideas, we investigated the relationship between concentration of Si in fibrous tussock sedge (Carex appropinquata) and the population density of a major sedge consumer, the root vole (Microtus oeconomus), in field enclosures in natural habitat under a variety of natural water regimes and weather conditions. We found that a high density of voles at the end of summer resulted in the immediate accumulation of Si by rhizomes, followed by accumulation of Si in leaves with a 1-year lag time. The level of river flooding in the same year had an additional impact on Si concentration in rhizomes but did not affect silicification of leaves. Overwinter changes in concentration of Si in sedges were influenced by fluctuations in ambient temperature and the depth of snow cover (multiple freeze–thaw cycles), thus affecting the quality of winter food available for voles. Smaller voles had lower mortality during early winter than large voles, which seemed to be connected with changes in the quality of the autumn rather than the winter food base. Winter survival of voles was not associated with Si concentration in their faeces, however. Our results suggest that changes in Si concentration in fibrous tussock sedge can be induced by changes in vole population density and are also additionally affected by the amount of flooding and weather conditions.

High basal metabolic rate does not elevate oxidative stress during reproduction in laboratory mice

Increased oxidative stress (OS) has been suggested as a physiological cost of reproduction. However, previous studies reported ambiguous results, with some even showing a reduction of oxidative damage during reproduction. We tested whether the link between reproduction and OS is mediated by basal metabolic rate (BMR), which has been hypothesized to affect both the rate of radical oxygen species production and antioxidative capacity. We studied the effect of reproduction on OS in females of laboratory mice divergently selected for high (H-BMR) and low (L-BMR) BMR, previously shown to differ with respect to parental investment. Non-reproducing L-BMR females showed higher oxidative damage to lipids (quantified as the level of malondialdehyde in internal organ tissues) and DNA (quantified as the level of 8-oxodG in blood serum) than H-BMR females. Reproduction did not affect oxidative damage to lipids in either line; however, it reduced damage to DNA in L-BMR females. Reproduction increased catalase activity in liver (significantly stronger in L-BMR females) and decreased it in kidneys. We conclude that the effect of reproduction on OS depends on the initial variation in BMR and varies between studied internal organs and markers of OS.

Effect of Dietary Restriction on Immune Response of Laboratory Mice Divergently Selected for Basal Metabolic Rate

To study whether dietary restriction (DR; 70% of ad lib. feeding)–elicited immunosuppression results from the trade-off between the costs of mounting an immune response and the metabolic costs of maintenance, we subjected mice from two divergent lines selected for high basal metabolic rate (H-BMR) and low BMR (L-BMR) to 4 wk of DR and then challenged them with keyhole limpet hemocyanin (KLH) antigen. Those line types differ genetically with respect to BMR and to the mass of metabolically expensive internal organs, which are larger in H-BMR mice. In mice of both line types, DR resulted in a significant reduction of body mass, an immune response, and the downsizing of spleen, lymph nodes, thymus, heart, and kidneys but not small intestines. DR resulted in a greater reduction of the spleen and lymph nodes in mice of the H-BMR line type, whereas the thymus was more affected in L-BMR line type. In contrast, immunization resulted in an increase of liver mass in DR mice of both line types. A comparison of the results of current and earlier studies on the same mouse line types suggests that metabolic trade-offs involving the costs of an immune response are more apparent when animals are forced to increase energy demands (e.g., by cold exposure) compared to when energy demands are decreased through DR. Our findings also suggest that divelrgent selection on BMR resulted in between-line-type differences in T-cell- and B-cell-mediated types of an immune response. More generally, our results indicate that production of a wide repertoire of antibodies is not correlated with high BMR.

Relationships between dominance, testosterone level and scent marking of males in a free-living root vole (Microtus oeconomus) population.

In many species, dominance increases a males mating success via intrasexual competition and/or female choice. The level of androgen hormones, mainly testosterone (T), the intensity of scent marking and body mass are traits that are known to be linked to mammalian male social rank. Recently, however, it has been noted that this link between male dominance and the aforementioned traits in natural free-living populations is not universal and does not exist in some species. That is why we tested the hypothesis of whether a males social rank is related to the expression of T, scent-marking and his body mass. We conducted the study on the promiscuous rodent species, root voles (Microtus oeconomus), which originated from a natural population (wild-born). These tests provided support for the following conclusions: (1) the social status of a male root vole is partly related to his level of testosterone; (2) the highest T level was observed in subdominant males; (3) T levels proved to be independent of male body mass; (4) marking frequency was not dependent on a males social status nor their body mass; and (5) the mean body mass of dominant, subdominant and subordinate individuals was similar. Our results indicate that in natural free-living populations, the link between the T levels and dominance behaviour of root vole males is ambiguous. Moreover, there is no link between the social status and the intensity of scent-marking. We therefore conclude that in this species, male marking intensity cannot be used as an indicator of social rank.

Immunocompetence and high metabolic rates enhance overwinter survival in the root vole, Microtus oeconomus.

Despite its presumed significance, the association between immune defence, energy expenditures and overwinter survival is rarely studied. We analysed individual variation in immunocompetence quantified as neutrophil-to-lymphocyte ratio (N/L), total white blood cells (WBC) and natural antibody levels, along with resting (RMR) and peak metabolic rates (PMR) and mortality during three consecutive winter seasons in a natural population of the root vole, Microtus oeconomus. In early winter, WBC count was negatively correlated with RMR, whereas N/L ratio was negatively correlated with swim-elicited PMR. We suggest that while the first correlation reflected the trade-off between energy allocation in immunocompetence and other metabolically demanding processes, the latter correlation stemmed from stress-induced immunosuppression elicited by the necessity to cope with swimming in frequently flooded habitat. In addition, the analysis carried out during the first year of study characterized by a high population density and prevalence of infestation with a blood parasite—Babesia spp., showed that its intensity was inversely correlated with the N/L ratio. In summary, our results suggest that elevated N/L ratio increases the winter survival of free-ranging rodents by increasing their ability to cope with parasitic infections.

Shaving increases daily energy expenditure in free-living root voles.

Experimental manipulation of energy expenditure has long been recognized as an effective means for identifying causative effects and avoiding confounded interpretations arising from spurious correlations. This approach has been successfully applied mainly to studies on birds, particularly on reproducing adults, whereas manipulations in mammals have proved more problematic. Here, we tested the hypothesis that shaving off 50% of the dorsal pelage should effectively increase energy expenditure in wild root voles (Microtus oeconomus) in their natural environment. We measured daily energy expenditure (DEE), using doubly labelled water in shaved and unshaved voles at the beginning of winter. The difference in DEE (corrected for body mass and year effects) between experimental and control group fluctuated from 11.5% to 17.3%. Probability of recapture over the 3 day DEE assay was strongly dependent on body mass, but did not differ between shaved and unshaved animals; however, a prevalence of larger (heavier) shaved individuals was observed. Shaved animals lost more weight between the first and second trapping. Shaving therefore appears to be an effective method of increasing the cost of total DEE in wild endotherms in their natural environment.

Heat dissipation does not suppress an immune response in laboratory mice divergently selected for basal metabolic rate (BMR)

ABSTRACT The capacity for heat dissipation is considered to be one of the most important constraints on rates of energy expenditure in mammals. To date, the significance of this constraint has been tested exclusively under peak metabolic demands, such as during lactation. Here, we used a different set of metabolic stressors, which do not induce maximum energy expenditures and yet are likely to expose the potential constraining effect of heat dissipation. We compared the physiological responses of mice divergently selected for high (H-BMR) and low basal metabolic rate (L-BMR) to simultaneous exposure to the keyhole limpet haemocyanin (KLH) antigen and high ambient temperature (Ta). At 34°C (and at 23°C, used as a control), KLH challenge resulted in a transient increase in core body temperature (Tb) in mice of both line types (by approximately 0.4°C). Warm exposure did not produce line-type-dependent differences in Tb (which was consistently higher by ca. 0.6°C in H-BMR mice across both Ta values), nor did it result in the suppression of antibody synthesis. These findings were also supported by the lack of between-line-type differences in the mass of the thymus, spleen or lymph nodes. Warm exposure induced the downsizing of heat-generating internal organs (small intestine, liver and kidneys) and an increase in intrascapular brown adipose tissue mass. However, these changes were similar in scope in both line types. Mounting a humoral immune response in selected mice was therefore not affected by ambient temperature. Thus, a combined metabolic challenge of high Ta and an immune response did not appreciably compromise the capacity to dissipate heat, even in the H-BMR mice. Summary: Heat dissipation may constrain physiological processes associated with compulsory heat generation (e.g. lactation), but is unlikely to constrain immune responses in mice.