Biomedical subjects

Peter A Parsons

Publications and source records attributed to Peter A Parsons.

11 recordsLinked to original sources

PubMed2006

Survival and longevity improvements at extreme ages: an interpretation assuming an ecological stress theory of aging.

The primary determinant of survival during aging is the energetic efficiency and metabolic stability required to counter the accumulated internal and external stresses of a lifetime. Hence, genetically stress-resistant individuals should accumulate with age; frailer, less robust, less energetically efficient and less metabolically stable individuals should succumb in parallel. This selection process implies the accumulation of energetically efficient stress-resistant individuals with age to the exclusion of all others. High additive genetic variability for survival is expected under extreme circumstances, however there is limited evidence close to the absolute extremes of life that diversity may fall. At this stage, only a few highly adaptive, oxidative-stress-resistant and presumably somewhat homozygous genotypes should remain. Therefore a fall in variability may occur in these outliers, when frailer individuals are unable to cope and are eliminated at extreme ages. This process could provide an explanation of mortality-rate declines in domesticated (laboratory) and free-living populations of the extremely old. That is, mortality-rate declines may be an expectation from a process of genetic sorting resulting from the accumulated responses to environmental stress over time. Application of an ecological stress theory of aging, which combines the external stresses to which organisms are exposed with internal stresses, appears to be the prerequisite for this conclusion.

Aging↗

PubMed2006

Energetic efficiency under stress underlies positive genetic correlations between longevity and other fitness traits in natural populations.

Evolutionary relationships among fitness traits are considered in terms of the near-to-universal scenario of stressful environments leading to a resource-deficient and hence energy-deficient world. Fitness approximates to energetic (and metabolic) efficiency under this environmental model. When fitness is high, stress resistance (reducible to oxidative-stress resistance) and metabolic stability are maximal, and energy expenditure is minimal. Rapid development should then be favored followed by a long lifespan and high adult survival. Positive associations among diverse fitness or life-history traits are expected, controlled by stress-resistant 'good genotypes'. Heterozygotes tend to show higher energetic efficiency and hence higher fitness than do corresponding homozygotes under extreme environments, and to give parallel associations among life-history traits. Energy budgets under abiotic environments are pivotal for integrative evolutionary studies of life histories in natural populations.

Animals↗

PubMed2006

The natural history of Drosophila ehrmanae.

Drosophila ehrmanae [Parsons P. A., and Bock I. R. (1977). Aust. J. Zool. 25: 249-268] is a desiccation-sensitive species of undisturbed southern Australian temperate-zone rainforests. In this habitat category, nine Drosophila species have been collected, all belonging to the dominant Australian subgenus, Scaptodrosophila. Adults of these species are collected by sweeping foliage in damp habitats so providing temperature/desiccation information on adaptive behavioral selection within microhabitats. Biochemical and metabolic analyses of two common species of this subgenus indicate that D. ehrmanae should be alcohol dehydrogenase null with low ADH activity, should not utilize ethanol and its derivatives as a major resource, and therefore should not be attracted to these metabolites. Ultimately, such species may offer possibilities for behavioral/genetic analyses.

Animals↗

PubMed2005

Environments and evolution: interactions between stress, resource inadequacy and energetic efficiency.

Evolutionary change is interpreted in terms of the near-universal ecological scenario of stressful environments. Consequently, there is a premium on the energetically efficient exploitation of resources in a resource-inadequate world. Under this environmental model, fitness can be approximated to energetic efficiency especially towards the limits of survival. Furthermore, fitness at one stage of the life-cycle should correlate with fitness at other stages, especially for development time, survival and longevity; 'good genotypes' under stress should therefore be at a premium. Conservation in the wild depends primarily on adaptation to abiotically changing habitats since towards the limits of survival, genomic variation is rarely restrictive. The balance between energetic costs under variable environments and energy from resources provides a model for interpreting evolutionary stasis, punctuational and gradual change, and specialist diversification. Ultimately, a species should be in an equilibrium between the physiology of an organism and its adaptation to the environment. The primary key to understanding evolutionary change should therefore be ecological, highlighting energy availability in a stressed world; this approach is predictive for various patterns of evolutionary change in the living and fossil biota.

Adaptation, Physiological↗

PubMed2004

From energy efficiency under stress to rapid development and a long life in natural populations.

Simplistically, high fitness depends upon high energy efficiency in the stressful habitats of organisms in the wild. Rapid development and high survival to adulthood should be followed by long-lived stress-resistant genotypes under this reductionist model. Empirical evidence is very limited because of the common use of benign laboratory environments, which remains a major difficulty in understanding relationships between life-history traits under more natural settings. Heterozygotes tend to show greater energy efficiency than do corresponding homozygotes especially in stressful environments, which leads the above connections among fitness traits. In particular rapid development and increased longevity should be correlated, and underwritten by the availability of metabolic energy. Empirical work is needed based upon severe stresses where a small environmental perturbation cases lethality; drought, heat and nutritional inadequacy are suitable candidate stresses. The importance of viewing fitness in energy terms is emphasized throughout. A byproduct is the potential to fuse functional and evolutionary biology under stressful environments.

Animals↗

PubMed2003

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↗

PubMed2003

Energy, stress and the invalid linear no-threshold premise: a generalization illustrated by ionizing radiation.

The linear no-threshold (LNT) premise for environmental agents is assessed in the context of the habitats of organisms where exposure to a multiplicity of environmental agents occurs. Adaptation towards high energy efficiency or fitness to counter the metabolic consequences of the stresses from environmental agents is expected over time. This evolutionary process leads to non-linear continua for energy efficiency across environments whereby maximum efficiency should occur at around background exposures; this therefore is a description of hormesis in energy terms. Consequently the LNT premise is invalid for all environmental agents including ionizing radiation. However, published longevity and survival data, being measures of fitness or energy efficiency, indicate that non-linearity and hence radiation hormesis extends to exposures substantially in excess of background radiation. An interpretation is suggested based upon the metabolic and energy reserves required for the simultaneous adaptation to the metabolic consequences of the various environmental agents to which organisms are exposed in their habitats, especially from stresses of climatic origin.

Biological Evolution↗

PubMed2003

Metabolic efficiency in response to environmental agents predicts hormesis and invalidates the linear no-threshold premise: ionizing radiation as a case study.

Hormesis derives from high metabolic efficiency and hence high fitness that evolve in response to single and multiple environmental agents in low to moderate stress habitats. Consequently, nonlinear fitness continua are an evolutionary expectation for all environmental agents, which invalidates the LNT premise. For ionizing radiation, hormesis is interpreted to be adaptation to background radiation exposures, combined with adaptation to higher radiation exposures dependent on metabolic protection from the array of other abiotic stresses in the environment. This model of radiation hormesis renders suggestions of therapeutic radiation supplementation redundant because of similar health effects from other environmental agents. Furthermore, the model is compatible with a return of exposure levels for radiation protection to higher doses than are presently permissible, a deduction with substantial economic benefits.

Adaptation, Physiological↗

PubMed2002

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.

Adaptation, Physiological↗

PubMed2002

Radiation hormesis: challenging LNT theory via ecological and evolutionary considerations.

Ecological and evolutionary considerations suggest that radiation hormesis is made up of two underlying components. The first (a) is background radiation hormesis based upon the background exposure to which all organisms are subjected throughout evolutionary time. The second and much larger component (b) is stress-derived radiation hormesis arising as a protective mechanism derived from metabolic adaptation to environmental stresses throughout evolutionary time especially from climate-based extremes. Since (b) > > (a), hormesis for ionizing radiation becomes an evolutionary expectation at exposures substantially exceeding background. This biological model renders linear no-threshold theory invalid. Accumulating evidence from experimental organisms ranging from protozoa to rodents, and from demographic studies on humans, is consistent with this interpretation. Although hormesis is not universally accepted, the model presented can be subjected to hypothesis-based empirical investigations in a range of organisms. At this stage, however, two consequences follow from this evolutionary model: (1) hormesis does not connote a value judgement usually expressed as a benefit; and (2) there is an emerging and increasingly convincing case for reviewing and relaxing some recommended radiation protection exposure levels in the low range.

Adaptation, Physiological↗

PubMedDate not supplied

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.

Adaptation, Physiological↗