Marek Konarzewski

Cell size as a link between noncoding DNA and metabolic rate scaling

Accumulation of noncoding DNA and therefore genome size (C-value) may be under strong selection toward increase of body size accompanied by low metabolic costs. C-value directly affects cell size and specific metabolic rate indirectly. Body size can enlarge through increase of cell size and/or cell number, with small cells having higher metabolic rates. We argue that scaling exponents of interspecific allometries of metabolic rates are by-products of evolutionary diversification of C-values within narrow taxonomic groups, which underlines the participation of cell size and cell number in body size optimization. This optimization leads to an inverse relation between slopes of interspecific allometries of metabolic rates and C-value. To test this prediction we extracted literature data on basal metabolic rate (BMR), body mass, and C-value of mammals and birds representing six and eight orders, respectively. Analysis of covariance revealed significant heterogeneity of the allometric slopes of BMR and C-value in both mammals and birds. As we predicted, the correlation between allometric exponents of BMR and C-value was negative and statistically significant among mammalian and avian orders.

EVOLUTION OF BASAL METABOLIC RATE AND ORGAN MASSES IN LABORATORY MICE

Animal species of similar body mass vary widely in basal metabolic rate (BMR). A central problem of evolutionary physiology concerns the anatomical/physiological origin and functional significance of that variation. It has been hypothesized that such interspecific differences in wild animals evolved adaptively from differences in relative sizes of metabolically active organs. In order to minimize confounding phenotypic effects and maximize relevant genetic variation, we tested for intraspecific correlations between body‐mass‐corrected BMR and masses of four organs (heart, kidney, liver, and small intestine) among six inbred strains of mice. We found significant differences between strains in BMR and in masses of all four organs. Strains with exceptionally high (or low) BMR tended to have disproportionately large (or small) organs. The mass of each organ was correlated with the masses of each of the other three organs. Variation in organ masses accounted for 52% of the observed variation in BMR, of which 42% represented between‐strain variation, and 10% represented within‐strain variation. This conclusion is supported by published measurements of metabolic rates of tissue slices from the four organs. The correlation between BMR and intestine or heart mass arose exclusively from differences between strains, while the correlation between BMR and liver or kidney mass also appeared in comparing individual mice within the same strain. Thus, even though the masses of the four examined organs account for no more than 17% of total body mass, their high metabolic activities or correlated factors account for much of the variation in BMR among mice. We suggest that large masses of metabolically active organs are subject to natural selection through evolutionary trade‐offs. On the one hand, they make possible high‐energy budgets (advantageous under some conditions), but on the other hand they are energetically expensive to maintain.

Macrophysiology: A Conceptual Reunification

Widespread recognition of the importance of biological studies at large spatial and temporal scales, particularly in the face of many of the most pressing issues facing humanity, has fueled the argument that there is a need to reinvigorate such studies in physiological ecology through the establishment of a macrophysiology. Following a period when the fields of ecology and physiological ecology had been regarded as largely synonymous, studies of this kind were relatively commonplace in the first half of the twentieth century. However, such large‐scale work subsequently became rather scarce as physiological studies concentrated on the biochemical and molecular mechanisms underlying the capacities and tolerances of species. In some sense, macrophysiology is thus an attempt at a conceptual reunification. In this article, we provide a conceptual framework for the continued development of macrophysiology. We subdivide this framework into three major components: the establishment of macrophysiological patterns, determining the form of those patterns (the very general ways in which they are shaped), and understanding the mechanisms that give rise to them. We suggest ways in which each of these components could be developed usefully.

Metabolic Ceilings under a Combination of Peak Energy Demands

Is energy expenditure limited by shared metabolic machinery for energy assimilation or by bottlenecks specific to each mode of energy expenditure? We tested this question in mice by imposing peak energy burdens of lactation and of cold stress simultaneously. We measured food intake, body and organ masses, and small intestinal brush-border hydrolase and transporter capacities in virgin female mice and in mothers nursing approximately 5, 8, or 14 pups, at either 5° C or 23° C We had already observed that mothers of 14 pups are at a limit of lactational performance at 23° C, while virgin mice at 5° C are near their limit of food intake in response to cold stress. Nevertheless, the increments in food intake due to these two energy stresses applied simultaneously proved to be additive: food intake in lactating mice at 5° C was even higher than the peak intake in lactating mice at 23° C or in virgins at 5° C. Thus, neither during peak lactation nor during peak cold stress alone was energy expenditure limited by shared machinery for energy assimilation; assimilation could be pushed even higher by adding another energy stress. Masses of the small intestine, liver, and kidney increased with food intake even more than expected from increases in body mass. These increased organ masses are adaptive and permit energy-stressed mice to process ingested nutrients at rates exceeding the capacities of unstressed mice. Safety factors (load/capacity ratios) of three intestinal brush-border hydrolases and transporters for nutrients declined toward 1 with increasing food intake. The capacity of the brush-border enzyme sucrase to produce glucose remained matched to the capacity of the brush-border glucose transporter to absorb the resulting glucose, as both varied with food intake.

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.

Peak Sustained Metabolic Rate and Its Individual Variation in Cold-Stressed Mice

Is long-term sustained metabolic rate (SusMR) subject to a ceiling value? If so, what physiological or evolutionary factors impose that ceiling, and is the ceiling value independent of the type of energy stress? We determined food intake and daily digestible energy intake (DEI) as measures of SusMR, and resting metabolic rate (RMR), in mice that were energy stressed by being maintained at cold ambient temperatures. We compared these values with those from previous studies of mice that were energy stressed by peak lactation. We also measured intestinal brush-border nutrient uptakes, body lean and fat masses, and masses of the small intestine, heart, kidneys, and liver. Even with ad lib. quantities of a high-fat diet available, mice could not survive at temperatures below -15° C. Body mass declined at low temperatures because of depletion of body fat reserves. Food intake increased 2.5-fold, RMR 1.5-fold, and masses of the small intestine, heart, and kidneys by 30%-70% with a decrease in temperature from 23° C to -15° C. Intestinal hypertrophy served to restore the reserve capacity of intestinal nutrient transporters that would otherwise have been swamped by the increased food intake. The values reached by sustained metabolic scope (SusMR/RMR), food intake, and intestinal mass were still considerably below those of lactating mice, even though mice at temperatures below -15° C died in the presence of excess food. Thus, intestinal capacity was not the ultimate reason for an inability to survive at lower temperatures. Analysis of individual variation showed that those mice with unusually high food intake, DEI, or RMR tended to have unusually large hearts, kidneys, and intestines. Those organs are essentialfor high energy budgets (reflected in high DEIs), but they also incur large maintenance costs themselves (reflected in high RMRs).

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.

Metabolic and Organ Mass Responses to Selection for High Growth Rates in the Domestic Chicken (Gallus domesticus)

Evolutionary hypotheses suggest that higher rates of postembryonic development in birds should either lower the resting metabolic rate (RMR) in a trade‐off between the costs of growth and maintenance or increase RMR because of a buildup of metabolic machinery. Furthermore, some suggest that higher rates of postembryonic development in birds should reduce peak metabolic rate (PMR) through delayed tissue maturation and/or an increased energy allocation to organ growth. We studied this by comparing metabolic rates and organ sizes of fast‐growing meat‐type chickens (broilers) with those of birds from a laying strain, which grow much slower. During the first week of life, despite growing six times faster, the RMR of the broiler chickens was lower than that of birds of the laying strain. The difference between strains in RMR disappeared thereafter, even though broilers continued to grow twice as fast as layers. The differences between strains in growth rate during the first week after hatching were not reflected in similar differences in the relative masses of the heart, liver, and small intestine. However, broilers had heavier intestines once they reached a body mass of 80 g. In contrast, broilers had relatively smaller brains than did layers. There was a positive correlation, over both strains, between RMR and the masses of leg muscles, intestine, and liver. Furthermore, despite delayed maturation of muscle tissue, broilers exhibited significantly higher PMR. We hypothesize that a balance between the larger relative muscle mass but lower muscle maturation level explains this high PMR. Another correlation, between leg muscle mass and PMR, partly explained the positive correlation between RMR and PMR.

Cell Size but Not Genome Size Affects Scaling of Metabolic Rate in Eyelid Geckos

The metabolic theory of ecology (MTE) predicts the ubiquity of the of 3/4 scaling exponent relating metabolic rate (MR) to body mass, as well as cell‐size invariance coupled with body‐size dependence of cellular MR in quickly dividing cells. An alternative prediction is that MR scales interspecifically with a coefficient that is between 2/3 and 1, depending on the cell size and cell MR, which is mostly driven by the cell surface‐to‐volume ratio. We tested (1) the contribution of cell size to interspecific differences in MR and (2) whether the cell size–MR relationship is mediated by genome size (GS), which usually correlates positively with cell size. We tested (1) and (2) using erythrocyte area as a proxy for cell size in 14 eyelid geckos, which belong to a monophyletic group exhibiting large body‐size variation. The scaling of standard MR (SMR) was significantly lower than 3/4, whereas mass‐specific SMR correlated with erythrocyte area in both phylogenetically adjusted and conventional analyses. This points to cell‐size variation as the factor governing metabolic rate scaling, which questions predictions of the MTE. However, the nonsignificance of the correlation between mass‐specific SMR and GS undermines the strength of the relation between GS and cell size, at least in these species.