Is aging and dying from old age linked to sexual reproduction?
I couldn’t remember who had written about this matter when posting the quesion. William R. clarck has a book that seems to be interesting where he explains that bacterias don’t have events of programmed death, whileas mulcticelullar organisms do have it. This is due to the fact that (in his argument) that our eukaryotic cells have a limited number of divisions ahead, and maintaining a body with specialized cells requires that these cells be functional and sane or in other case die and leave room for another cell.
Sexual organisms have different lines of cells for somatic and reproductive matters, hence :
When sexual reproduction evolved, it became the dominant form of reproduction on the planet, in part because mixing DNA from two individuals corrects errors that have crept into the code. But this improved DNA made DNA in the other (somatic) cells not only superfluous, but dangerous, because somatic DNA might harbor mutations. Nature's solution to this danger, Clark concludes, was programmed death--the somatic cells must die. Unfortunately, we are the somatic cells. Death is necessary to exploit to the fullest the advantages of sexual reproduction.
(Excerpted from the abstract in amazon, Sex and the Origins of Death
).
To me it is an interesting argument that have some sensible points but is mistaken in its main proposition, ageing did not come after sexual reproduction, there are studies that show that at least some bacterias do age, they don’t have a programmed dead in their code but it is a fact that they age:
The fundamental cause of aging in bacteria is thought to be the accumulation of deleterious components (aging factors). Asymmetrically dividing bacteria, such as Caulobacter crescentus
, show signs of replicative aging.
The results for symmetrically dividing bacteria are more nuanced. For example, Escherichia coli, under certain experimental conditions, may exhibit signs of replicative aging caused by subtle asymmetries in its division.
But there seems to be a relationship between the potential longevity of a species and the onset of reproductive age. Studies with Drosophila
However the connection between them two is not direct but selected by the type of enviroment in which the species lives, as some studies with Drosophila melanogaster have pointed. For example in this interesting one Experimental evolution of aging, growth, and reproduction in fruitflies
(PNAS March 28, 2000. 97 (7) 3309-3313; https://doi.org/10.1073/pnas.97.7.3309), designed to test predictions of life-history theory and the evolutionary theory of ageing , they found that, as theory of history of life and ageing predict, higher extrinsic mortality rates -i.e hostile enviroments- did lead to the evolution of higher intrinsic mortality rates -i.e to shorter lifespans- and to decreased age and size at eclosion; peak fecundity also shifted earlier in life.
Interestingly, safe enviroments in other studies lead to more longeve flies, later onset of fetility but not to the extention of fertility to older ages, as it is reported in this same study.
There are some long lasting experiments bein carried out with guppies in Trinidad that seem to confirm these same observations.
Catastrophic events of massive reproduction and death
Another interesting relationship between sex and death has to do with those animals that undergo dramatic (and perhaps catastrophic) events of massive reproduction and then programmed death (often preceded by fast decay that resembles almost instant senescence). They are the semelparous animals:
organisms and others that die suddenly following reproduction (e.g. salmon, octopus, marsupial mouse (brown antechinus), etc.) also represent instances of organisms who incorporate a lifespan-limiting feature. Sudden death is more obviously an instance of programmed death or a purposeful adaptation than gradual ageing. Biological elements clearly associated with evolved mechanisms such as hormone signalling have been identified in the death mechanisms of organisms such as the octopus
.
All in all, there seems to be a genetic (and probably epigenetic too) program to maintain an orgahism healthy until some stage, after the “reproduction time” has had its time, but the phenomenon of senescence is not well understood yet, and its connection with sexual reproduction has to do with the ability of an organism to repair itself, while it can.
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