Mice divergently selected for high and low basal metabolic rates evolved different cell size and organ mass.

Evolution of metabolic rates of multicellular organisms is hypothesized to reflect the evolution of their cell architecture. This is likely to stem from a tight link between the sizes of cells and nuclei, which are expected to be inversely related to cell metabolism. Here, we analysed basal metabolic rate (BMR), internal organ masses and the cell/nucleus size in different tissues of laboratory mice divergently selected for high/low mass-corrected BMR and four random-bred mouse lines. Random-bred lines had intermediate levels of BMR as compared to low- and high-BMR lines. Yet, this pattern was only partly consistent with the between-line differences in cell/nucleus sizes. Erythrocytes and skin epithelium cells were smaller in the high-BMR line than in other lines, but the cells of low-BMR and random-bred mice were similar in size. On the other hand, the size of hepatocytes, kidney proximal tubule cells and duodenum enterocytes were larger in high-BMR mice than other lines. All cell and nucleus sizes were positively correlated, which supports the role of the nucleus in cell size regulation. Our results suggest that the evolution of high BMR involves a reduction in cell size in specialized tissues, whose functions are primarily dictated by surface-to-volume ratios, such as erythrocytes. High BMR may, however, also incur an increase in cell size in tissues with an intense transcription and translation, such as hepatocytes.

Acclimation of thermal physiology in natural populations of Drosophila melanogaster : a test of an optimality model.

Many organisms modify their physiological functions by acclimating to changes in their environment. Recent studies of thermal physiology have been influenced by verbal models that fail to consider the selective advantage of acclimation and thus make no predictions about variation in acclimation capacity. We used a quantitative model of optimal plasticity to generate predictions about the capacity of Drosophila melanogaster to acclimate to developmental temperature. This model predicts that the ability to acclimate thermal sensitivity should evolve when temperature varies greatly among generations. Based on the model, we expected that flies from the highly seasonal environment of New Jersey would acclimate thermal sensitivity more than would flies from the less seasonal environment of Florida. When raised at constant and fluctuating temperatures, flies from these populations failed to adjust their thermal optima in the way predicted by the model, suggesting that current assumptions about functional and genetic constraints should be reconsidered.

Cell size is positively correlated between different tissues in passerine birds and amphibians, but not necessarily in mammals.

We examined cell size correlations between tissues, and cell size to body mass relationships in passerine birds, amphibians and mammals. The size correlated highly between all cell types in birds and amphibians; mammalian tissues clustered by size correlation in three tissue groups. Erythrocyte size correlated well with the volume of other cell types in birds and amphibians, but poorly in mammals. In birds, body mass correlated positively with the size of all cell types including erythrocytes, and in mammals only with the sizes of some cell types. Size of mammalian erythrocytes correlated with body mass only within the most taxonomically uniform group of species (rodents and lagomorphs). Cell volume increased with body mass of birds and mammals to less than 0.3 power, indicating that body size evolved mostly by changes in cell number. Our evidence suggests that epigenetic mechanisms determining cell size relationships in tissues are conservative in birds and amphibians, but less stringent in mammals. The patterns of cell size to body mass relationships we obtained challenge some key assumptions of fractal and cellular models used by allometric theory to explain mass-scaling of metabolism. We suggest that the assumptions in both models are not universal, and that such models need reformulation.

Altered allocation to roots and shoots in the endophyte-infected seedlings of Puccinellia distans (Poaceae).

Endophytes play an important role in ecological and evolutionary processes in plants and have marked economic value. Seed-transmitted fungal endophytes are conventionally regarded as mutualistic symbionts, but their fitness consequences for the offspring of the host are not clear. Puccinellia distans infected with the fungus Epichloë typhina (E+) produces seeds that are several times smaller than normal (E-). This observation suggests that the E+ seedlings face a developmental disadvantage. Our growth chamber experiments compared the germination rates of the small E+ and large E- seeds of P. distans and examined the biomass allocation of seedlings to roots and shoots. The E+ seedlings germinated more slowly and maintained shorter shoots and a smaller root biomass for 30-50 days after sowing. Despite this disadvantage, the E+ plants more quickly increased their total size, attaining a larger shoot and whole-plant biomass. The shoot:root biomass ratio increased more rapidly through time in the E+ seedlings, attaining a value nine times higher in the E+ than the E- group 50 days after sowing. Such differences between the E+ and E- seedlings were not explained by the growth allometry between shoots and roots. The seedlings of P. distans infected with the Epichloë endophyte were initially handicapped by their postponed emergence, but this disadvantage was quickly overcome by their superior growth capacity. The decrease in the relative allocation to roots may indicate that endophytes increase the performance of roots as resource-acquiring organs and/or reduce the role of roots in protection against herbivores.

Can optimal resource allocation models explain why ectotherms grow larger in cold?

Basically all organisms can be classified as determinate growers if their growth stops or almost stops at maturation, or indeterminate growers if growth is still intense after maturation. Adult size for determinate growers is relatively well defined, whereas in indeterminate growers usually two measures are used: size at maturation and asymptotic size. The latter term is in fact not a direct measure but a parameter of a specific growth equation, most often Bertalanffy's growth curve. At a given food level, the growth rate in determinate growers depends under given food level on physiological constraints as well as on investments in repair and other mechanisms that improve future survival. The growth rate in indeterminate growers consists of two phases: juvenile and adult. The mechanisms determining the juvenile growth rate are similar to those in determinate growers, whereas allocation to reproduction (dependent on external mortality rate) seems to be the main factor limiting adult growth. Optimal resource allocation models can explain the temperature-size rule (stating that usually ectotherms grow slower in cold but attain larger size) if the exponents of functions describing the size-dependence of the resource acquisition and metabolic rates change with temperature or mortality increases with temperature. Emerging data support both assumptions. The results obtained with the aid of optimization models represent just a rule and not a law: it is possible to find the ranges of production parameters and mortality rates for which the temperature-size rule does not hold.