09 octubre, 2024

Ageing

Wear and tear theory

DEF: Ageing is the result of fundamental limitations, inevitable small random changes that accumulate with time (stochastic theories of ageing); a tendency of all matter to decay into a more random, less ordered state (entropy).

source: http://www.programmed-ageing.org

Wear and tear theories of ageing - problems

  • a great variety of life-span in living organisms even if their size, activity and metabolism are controled
  • entropy can be reversed using energy; extensive maintenance and repair capability of organisms

Non-programmed ageing

Mutation accumulation MA (Medewar 1952)

exponential decline -> half-life x reproduction -> differential reproductive effect of cohorts -> evolutionary importance of individuals decreases with age (deterious mutations accumulated with time are not sweep out by selection due to low reproductive effect of the older cohorts)

Medawar, P.B, An Unsolved Problem of Biology., 1952. H.K. Lewis & Co., London.

MA and seelction



Fabian D, Flatt T (2011) The Evolution of ageing. Nature Education Knowledge 3(10):9

Mutation accumulation MA - emipiric data

Amyloid hypothesis and Alzheimer disease

Makin S (2018) Alzhiemer’s on trial. Nature 559

Mutation accumulation MA - problems

  • a great variation of organisms life-span (even if controlled for various confounding variables); the model organism as put by Medewar does not exist

  • a number of diverse organisms (e.g. salmon, octopus, marsupial mouse) display instances of death closely following an act of sexual reproduction

More criticisms:
Makin S (2018) Alzhiemer’s on trial. Nature 559
Bowles J (2000) Shattered: Medawar’s test tubes and their enduring legacy of chaos. Med Hypotheses 54:326–339

Antagonistic pleiotropy AP (Williams 1957)

Pleiotropy: a situation in which a single gene controls more than one trait. Thus, ageing is caused by the combined effect of many pleiotropic genes that each had a beneficial effect in an animal’s youth but also had an adverse side effect in older age.

Prediction: Species with younger age of sexual maturity and more vigorous reproduction traits would tend to have shorter life spans.

Austad SN, Hoffman JM (2018) Is antagonistic pleiotropy ubiquitous in ageing biology? Evol Med Public Heal 2018:287-294

Antagonistic pleiotropy AP

Problems:

  • Ross experiment - the laboratory line of flies selected for longevity (lived twice as long as the the short-lived line) also laid more eggs than the short-lived flies.

  • Why this unavoidable side effects issue would only affect maintenance of the condition of an adult organisms when tasks have to be performed in the development, growth, and day-to-day operation of an organism?

MA and AP hypotheses - comparison

Fabian D, Flatt T (2011) The Evolution of ageing. Nature Education Knowledge 3(10):9

Disposable soma DS (Kirkwood 1977)

DEF: organisms age due to evolutionary trade-offs between growth, reproduction, and DNA repair maintenance

Kirkwood TBL (2002) Evolution of ageing. Mechanisms of ageing and Development 123:737-745

Disposable soma DS - problems

  • resource allocation - evidence against

Shanley DP, Kirkwood TBL (2000) Calorie restriction and ageing: A life-history analysis. Evolution 54:740-750.

  • simultanous reproduction and ageing process

Evolution of longevity (Cichoń 1997)

Cichoń M (1997) Evolution of longevity through optimal resource allocation. Proc R Soc B Biol Sci 264:1383-1388

Programmed ageing

Group/Kin Selection

DEF:benefit to survival of a group may offset some degree of individual fitness disadvantage and thus allow for the evolution of organism characteristics that are adverse to individual fitness such as programmed ageing

A lot of criticisms…

But see grandmother hypothesis