Selection For Stress Resistance Research Articles

Limited phenotypic plasticity in range‐edge populations: a comparison of co‐occurring populations of two Agrimonia species with different geographical distributions

Increased importance of genetic drift and selection for stress resistance have been predicted to lead to a reduction in the degree of phenotypic plasticity in populations at margins of a species' geographical range, relative to those in the centre. We examined the effect of population positioning within the species range on degree of active morphological plasticity to vegetation shade. Importantly, we discriminated between active, size-independent morphological adjustments in response to shade and passive changes in morphology caused by the dependence of morphological traits on plant size, as only the former are considered to be adaptive. Two closely related and ecologically similar Agrimonia species were examined in the same geographical location, where one species reaches the edge of its distribution (Agrimonia pilosa) and the other does not (A. eupatoria). Plasticity to light availability is likely to be advantageous for both species as they occupy habitats with variable light conditions. However, we hypothesised that high levels of environmental stress should lead to reduced active plasticity in marginal compared with more central populations. Agrimonia eupatoria exhibited active adjustments in leaf morphology in response to tree shade, and in elongation of stems and inflorescences in response to herbaceous shade. In contrast, A. pilosa exhibited very limited active plasticity. High active plasticity allowed A. eupatoria to retain constant shoot growth in a wide range of light conditions, while the lack of active plasticity in A. pilosa resulted in a strong dependence of shoot growth on light availability. We propose that high levels of environmental stress in marginal areas of a species' range may lead to a significant reduction in the degree of active plasticity. Our results clearly indicate that discrimination between active and passive plasticity is crucial for reaching valid conclusions about differences in adaptive plasticity between marginal and non-marginal populations.

Stress specific correlated responses in fat content, Hsp70 and dopamine levels in Drosophila melanogaster selected for resistance to environmental stress

Studies of adaptation to stressful environments have frequently encountered cross resistance. This has prompted the hypothesis that certain adaptations confer resistance to multiple stressors. Some of the genes and mechanisms conferring stress resistance have been identified, however, the generality and basis of stress adaptation and cross resistance is still unclear. We investigated several physiological traits that have been previously linked to increased stress resistance: Hsp70 expression, fat content and dopamine levels. Additionally, we studied a behavioural trait, locomotor activity, as a proxy for the physiological state of the organisms. Physiology is the mechanistic link between resistance phenotype and underlying genetic background, and provides insights into the background and generality of cross resistance and correlated responses to selection for stress resistance. We assessed the relationship between the measured traits and stress resistance in a set of lines selected for increased resistance to several environmental stressors. We found that, although all physiological traits displayed significant differentiation among selection regimes, none were consistently associated with increased general stress resistance. This demonstrates that directional changes in Hsp70 expression level, dopamine level and fat content occur in response to the specific requirements of the different stress regimes, rather than as a general response to stress.

Variation in Stress Responses within a Bacterial Species and the Indirect Costs of Stress Resistance

Bacteria can exhibit high levels of resistance to one or more environmental stresses such as temperature, osmolarity, radiation, pH, starvation, as well as resistance to noxious chemicals and antibiotics. Yet evolution has not optimized stress resistance in all bacteria to all stresses. Even within a species like Escherichia coli, stress resistance is not constant between strains, suggesting that selection for stress resistance is under counterselection in some environments. The tradeoffs associated with stress resistance in E. coli are due to more than the direct cost of resistance mechanisms. A significant indirect cost is that high stress resistance is associated with a reduced ability to compete for poor growth substrates like acetate or even good substrates like glucose at suboptimal concentrations. High stress resistance also decreases the ability to use inorganic nutrients like phosphate. This tradeoff between self-preservation and nutritional competence, called the SPANC balance, is likely to be the major selective influence in natural populations. Another cost of high stress resistance in E. coli is an elevated mutation rate and the increased generation of deleterious mutations. Directional adaptations in SPANC balance and mutation rate are environment-dependent. The most common variations in SPANC are due to polymorphisms in the levels of global regulators RpoS and ppGpp between different strains. High levels favor stress resistance, and low levels allow better nutrition. The intimate association of RpoS/ppGpp with stress resistance and SPANC balancing influences numerous cellular processes and bacterial properties, including virulence.

From the stress theory of aging to energetic and evolutionary expectations for longevity.

Stress targets energy carriers. Genes for stress resistance are selected that convey high metabolic efficiency enabling adaptation to the energetically restrictive and hence stressful environments of natural populations. Data from experimental organisms and from humans are consistent with a primary role for stress resistance underlying life span, which provides a hitherto neglected procedure for assaying longevity in natural populations. Taking into account the metabolic consequences of stressful environments, the free-radical theory of aging becomes a general stress theory of aging. A recent derivative, the deprivation-syndrome theory of aging, highlights resource and hence energy shortages. Energy balances under the stress theory of aging are primary for an understanding of the evolutionary limits of longevity of organisms in their habitats. In contrast, well-nourished humans of the modern era, and laboratory, domesticated and island populations are exposed to more benign conditions which appear to provide the background for other evolutionary theories of aging, especially the mutation accumulation and antagonistic pleiotropy theories. In modern human populations where selection for stress resistance is relaxed compared with earlier harsher conditions, substantial future evolutionary extensions to maximum life span may be difficult to attain because of the mutation accumulation process. However there is an urgent need for comparative empirical studies of life-history traits including longevity under benign and harsh environments.

Aging: The Fitness-Stress Continuum and Genetic Variability

Assuming the stress theory of aging, longevity depends upon primary selection for stress resistance and metabolic efficiency. Predominantly based upon experimental studies in the insect Drosophila melanogaster , high genetic variability for fitness, especially mortality, occurs under extreme stress. Isofemale strains derived from the progeny of recently collected single inseminated Drosophila females from the wild should provide useful biological material for extrapolating to quantitative genetic studies in man. Furthermore, environments from the benign (hormetic) to the extreme can be incorporated. Survival to old age may depend upon genes for metabolic efficiency that respond to the environmental challenges of living as limits to adaptation are approached. Under this scenario the survival of longevity mutants in man to ages analogous to the extreme life spans found in some experimental organisms under benign or protected laboratory conditions is unlikely. More future emphasis is needed on genetic variation of longevity in natural populations of experimental organisms under an array of realistically stressful environments to act as an evolutionary model for longevity in our own species.

Life span: does the limit to survival depend upon metabolic efficiency under stress?

Survival to old age in natural populations is enhanced by high vitality and resilience which depends upon substantial homeostasis and energetic amd metabolic efficiency underlain by genes for stress resistance. Under this assumption increased longevity follows from primary selection for stress resistance where stress targets energy carriers. Furthermore old and young fitness should be correlated irrespective of age under the stressful selection regime of natural populations. In contrast, antagonistic pleiotropy is most likely under the less rigorous selection regime of well-nourished humans and laboratory populations surviving to old age. Similarly, hormesis for longevity, for example from a mild temperature stress or restricted food intake is most likely under benign environmental conditions. Assuming that aging in natural populations depends upon ecological circumstances, large evolutionary increases in life span are unlikely under the stress theory of aging since organisms are frequently close to their limits of survival where metabolic efficiency is at a premium. Exceptions can occur in island populations and for mutants under laboratory conditions since the risks from environmental hazards are reduced, and life span becomes extended as a consequence. In modern human populations, selection for stress resistance is less intense than in earlier times which should be permissive of the accumulation of stress-sensitive mutants under the mutation-accumulation theory of aging. However, this process is ultimately likely to restrict the evolution of life-span extensions in the future especially if abiotic conditions deteriorate, when survival would depend more directly on metabolic efficiency under stress.

Mast- und Schlachtleistung sowie Fleisch- und Fettqualität von Piarain-Schweinen unterschiedlichen MHS-Genoryps und Geschlechts – 1. Mitteilung: Mast- und Schlachtleistung sowie Fleischqualität

Abstract. Title of the Paper: Fattening and carcass Performance and meat- and fat quality of Pietrain pigs of different MHS-genotype and sex. I. Fattening and carcass Performance and meat quality For the purpose of the comparison of the fattening and carcass Performance and meat quality of different MHS-genotypes and sexes of the Pietrain race 60 fattening pigs with equal numbers of the three MHS-genotypes (NN, Np, pp) and both sexes (castrates, gilts) had been gone in the fattening and carcass Performance testing of a litter testing Station. The animals had been fattened from 30 to 105 kg. With decreasing presence of the stress allele p (pp > Np > NN) as well in the castrates compared with the gilts there could be shown increasing growth rates combined with decreasing feed conversion, decreasing carcass quality that is higher degree of fatness and lower lean meat content of the carcass, more advantageous meat quality and a higher intramusculare fat content. Aecording to the results a general selection for stress-resistance of the Pietrain race is recommended

Hormesis: an adaptive expectation with emphasis on ionizing radiation.

Non-linear fitness gradients with maxima between extremes are expected for any environmental variable to which free-living populations are exposed. For exceedingly toxic agents, including ionizing radiation, such deviations from linearity are close to zero exposure and are conventionally called hormesis. Accordingly, hormesis is an extreme version of the non-linear fitness gradients for general environmental stresses such as temperature fluctuations, for which maximum fitness occurs at the moderate temperature fluctuations to which free-living populations are most commonly exposed. Some metabolic reserves should occur under moderate temperature stresses because of the need for pre-adaptation enabling survival during exposure to occasional periods of more extreme stress, especially at species borders where selection for stress resistance is likely to be most intense. Because heat shock proteins are induced by all stresses, adaptation to extreme temperatures should translate into adaptation to other stresses. Consequently, metabolic reserves from adaptation to extreme temperatures in the past should translate into protection from correlated abiotic stresses, especially in human populations where modern cultural processes are now ameliorating exposure to extreme stresses. Ambient and man-made radiations of non-catastrophic dimensions should therefore lead to stress-derived radiation hormesis. Other stresses can, in principle, be incorporated into this model. Accordingly, evolutionary and ecological considerations suggest two components of hormesis in relation to ionizing radiation: background radiation hormesis based upon the background exposure to which all organisms on earth are subjected; and stress-derived radiation hormesis. Exposure under stress-derived radiation hormesis is considerably larger than under background radiation hormesis, so significant deleterious effects from non-catastrophic radiation normally may be impossible to detect. Suggestions are provided for testing such postulated deviations from the commonly assumed linear no-threshold (LNT) hypothesis for the biological consequences of exposure to radiation.

DOES SELECTION FOR STRESS RESISTANCE LOWER METABOLIC RATE?

Environmental stress reduces the fitness of organisms, and in this manner may shape species distribution and evolutionary change. Ecologists have long been interested in physiological mechanisms that reduce the effects of stress and result in improved fitness. It has previously been suggested (1) that selection for stress resistance will bring about the evolution of lower standard metabolic rates and (2) that organisms with increased stress resistance will have a lower metabolic rate during stressful conditions, compared to stress-sensitive organisms. We have tested these hypotheses using replicate populations of Drosophila melanogaster. Desiccation and starvation-selected populations and their controls, as well as populations selected for postponed senescence and their short-generation controls were examined. The metabolic rate of each selected and control population was measured under three different environmental conditions: (1) desiccation (no food or water), (2) starvation (water present, no food), and (3) food condition (food and water present). The carbohydrate and lipid content of each population were also measured. All selected and control populations lowered their metabolic rates when measured under starving conditions compared to food conditions. The metabolic rates of flies under desiccating conditions did not differ from food conditions regardless of selection treatment. When comparing the mass-specific metabolic rates of stress-selected populations to their controls, stress-selected populations had a significantly lower mass-specific metabolic rate in all cases, regardless of the environmental conditions under which the measurements were made. However, once the mass of nonmetabolizing lipid and carbohydrate was accounted for and metabolic rate was expressed in carbohydrate–lipid free mass, selected populations did not differ significantly in metabolic rate from control populations, regardless of the environmental condition under which the measurements were made. We conclude that in the case of D. melanogaster, resistance to stress does not require the evolution of a lower metabolic rate. Instead, the accumulation of metabolic resources such as carbohydrate and lipid may be important for resistance to specific forms of stress.

Does Selection for Stress Resistance Lower Metabolic Rate?

Does Selection for Stress Resistance Lower Metabolic Rate?

The limit to human longevity: an approach through a stress theory of ageing

Survival to old age is enhanced by high vitality and resilience associated with substantial physiological and morphological homeostasis. This is underlain by genes for stress resistance, which confer high metabolic efficiency and hence adaptation to the energy costs of the stresses to which free-living populations are exposed. Under the stress theory of ageing, selection for genes for stress resistance is primary, and achieved life-span is secondary. In some human populations of the modern era, selection for stress resistance is less intense than in earlier times, because of adequate nutrition and reduced exposure to environmental stresses. Such relaxed selection should permit the accumulation of deleterious mutants that are likely to be stress sensitive. Accordingly, increased maximum life-span in future human populations would appear difficult to achieve.

Evolutionary response to drought stress: Conservation implications

Organisms in free-living populations are normally subjected to stressful environments, especially hydrological extremes followed by nutritional inadequacy. Although adaptation to increased desiccation stress can occur, reduced fitness is a cost restricting the potential for spread into extreme habitats. Selection for stress resistance should be lower in captive populations reducing survival on release into free-living conditions. Therefore in developing protocols to promote conservation I recommend monitoring stress response traits of direct ecological importance emphasizing outlier populations close to extinction. This approach gains support from evidence that genetic variability in some stressed outlier populations is not lacking but may be enhanced.

Selection for Increased Longevity in <i>Drosophila melanogaster</i><i>:</i> A Reply to Lints

An important tool in the genetic analysis of longevity and aging in Drosophila melanogaster is the use of strains selected directly for late-age reproduction and indirectly for extended longevity. Following some initial failures to select for extended longevity, there are now a number of laboratories which have successfully selected for long life, using the techniques of late-age reproduction as well as selection for stress resistance. Baret and Lints [Gerontology 1993;39:252-259] have recently cast doubt on the reality of a number of these selected strains, including our own, suggesting that the difference in longevity between the long-lived and normal-lived strains disappears when the data are examined as a function of the number of days after the beginning of the selection experiment instead of as a function of the number of generations. With regard to our selected lines, they based their analysis on a subset of the published data dealing with these strains, and which covered 21 generations, or 40 months, of selection. We now present data for over 70 generations, or 155 months, of selection and maintenance. The Baret-Lints hypothesis makes two strong predictions, namely that (1) the longevity difference between the several strains should disappear when the data are replotted according to their fashion, and (2) there should be no other significant biological difference between the strains. Our data falsifies both of these predictions. The Baret-Lints hypothesis is flawed and should be disregarded.(ABSTRACT TRUNCATED AT 250 WORDS)