Evolutionary adaptation to temperature. IX. Preadaptation to novel stressful environments of Escherichia coli adapted to high temperature.

Stressful environments may be considered as those that reduce fitness, sometimes due in part to the increased metabolic expenditure required to sustain life. Direct adaptation to a stressor is expected to increase fitness and reduce maintenance metabolism, with the latter leading to increased biomass production. In this study, we test the general hypothesis that such adaptation to one stressor can preadapt organisms to novel stressful environments. Six lines of Escherichia coli propagated for 2,000 generations at 41-42 degrees C (42 group), a stressful temperature, were compared to six control lines propagated for 2,000 generations at 37degrees C (37 group) and to the common ancestor of both groups. We assayed biovolume yield (a measure of growth efficiency) and competitive fitness in the 42 group's selective high temperature environment as well as five novel stressful environments-acid, alkali, ethanol, high osmolarity and peroxide. As previously reported, at high temperature the 42 group had both higher yield and fitness than the 37 group and ancestor. In the novel environments, the 42 group generally produced yields higher than the 37 group (and marginally higher than the ancestor), but we found no differences in competitive fitness among the 37 and 42 groups and the ancestor. We also found that the performance of lines within groups was not correlated across stressful environments for either yield or relative fitness. Because previous adaptation to one stressor did not improve our measure of Darwinian fitness in novel stressful environments, we conclude that the 42 group shows no useful pre-adaptation, or cross-tolerance, to these types of environments.

An experimental test of the thermoregulatory hypothesis for the evolution of endothermy.

The thermoregulatory hypothesis proposes that endothermy in mammals and birds evolved as a thermoregulatory mechanism per se and that natural selection operated directly to increase body temperature and thermal stability through increments in resting metabolic rate. We experimentally tested this hypothesis by measuring the thermoregulatory consequences of increased metabolic rate in resting lizards (Varanus exanthematicus). A large metabolic increment was induced by feeding the animals and consequent changes in metabolic rate and body temperature were monitored. Although metabolic rate tripled at 32 degrees C and quadrupled at 35 degrees C, body temperature rose only about 0.5 degrees C. The rate of decline of body temperature in a colder environment did not decrease as metabolic rate increased. Thus, increasing the visceral metabolic rate of this ectothermic lizard established neither consequential endothermy nor homeothermy. These results are inconsistent with a thermoregulatory explanation for the evolution of endothermy.

Sexual dimorphism in physiological performance of whiptail lizards (genus Cnemidophorus).

Numerous studies have examined sexual dimorphism in the morphology and behavior of vertebrates; very few, however, have explicitly investigated the possibility of gender differences in physiological performance, despite the observations of such differences in humans. In this study, I investigated physiological sexual dimorphism in the lizard genus Cnemidophorus by measuring five whole-animal traits, all of which are likely to influence fitness in these species: burst speed, endurance, maximal exertion capacity, standard metabolic rate, and evaporative water loss rate. Because at least some of these traits are known to be strongly influenced by body size, I tested for dimorphism using both absolute and size-corrected trait values. An examination of six Cnemidophorus species and subspecies revealed a strong trend toward higher absolute trait values in males for all variables except endurance. Most of the dimorphism in standard metabolic rate and evaporative water loss rate could be explained by differences in body mass between males and females; for the locomotor traits, however, body size explained only a small fraction of the overall sexual dimorphism. The portion of trait differences not explained by body size was likely due to gender differences in physiology, such as differences in relative muscularity and fat content.

Comparisons of physiological performance in sexual and asexual whiptail lizards (genus Cnemidophorus): implications for the role of heterozygosity.

Many asexual animal species are of hybrid origin, with consequent high levels of heterozygosity. Data from some studies suggest that increased heterozygosity may be functionally correlated with superior performance in a variety of fitness-related traits. Thus, hybrid asexual species could be expected to exhibit some degree of heterosis. This spontaneous heterosis hypothesis is tested in a comparative study of asexual and sexual species of the lizard genus Cnemidophorus. Asexual species of the genus are parthenogenetically reproducing hybrids of the sexual species and as a result have high levels of heterozygosity that have persisted since their origins. Five whole-organism physiological traits (burst speed, endurance, maximal exertion, standard metabolic rate, and evaporative water loss rate) were examined in five asexual species and the sexual species that gave rise to them. Trait values for sexual and asexual species were compared using a nonphylogenetic approach and two phylogenetically controlled approaches capable of dealing with reticulate phylogenies. In contrast to the predictions of the heterosis hypothesis, performance for four of the traits in asexual Cnemidophorus was not statistically different than that in their sexual parental species, and asexuals had significantly worse endurance. On the whole, the overall trend appeared to be toward worse performance in asexuals. An obvious interpretation of these results is that heterozygosity and "vigor" need not be functionally related. However, other factors may be counterbalancing possible beneficial effects of heterozygosity, including detrimental epistatic effects resulting from the karyotypically mixed genome of these hybrids, and the accumulation of deleterious mutations in the asexual lineages via Muller's ratchet.