Path: utzoo!attcan!uunet!husc6!rutgers!iuvax!silver!chiaravi
From: chiaravi@silver.bacs.indiana.edu (Lucius Chiaraviglio)
Newsgroups: sci.bio,sci.med
Subject: Re: Mortality
Summary: DNA damage theory of aging still doesn't work
Keywords: aging, DNA damage, fertility, evolution of external testicles
Message-ID: <1898@silver.bacs.indiana.edu>
Date: 12 Jun 88 19:21:06 GMT
References: <36@feedme.UUCP> <1894@silver.bacs.indiana.edu> <31342@yale-celray.yale.UUCP>
Reply-To: chiaravi@silver.UUCP (Lucius Chiaraviglio)
Organization: Department of Molecular, Cellular, and Developmental Biology at Indiana University, Bloomington
Lines: 78
Xref: utzoo sci.bio:1235 sci.med:5738

In article <31342@yale-celray.yale.UUCP> briscoe-duke@CS.YALE.EDU (Duke
Briscoe) writes:
|In article <1894@silver.bacs.indiana.edu> chiaravi@silver.UUCP (Lucius
|Chiaraviglio) writes: 
|
|>No, these mechanisms have not been determined, although it is definite
|>that aging occurs at the cellular level.  Many people have thought
|>that aging is the result of accumulated DNA damage, but this theory
|>absolutely fails to explain how accumulation of DNA damage can lead to
|>aging of individuals yet allow them to give rise to perfectly young
|>offspring.  ...
|
|I don't think this is a valid point against the DNA damage
|accumulation theory.  Some damage does occur to eggs and sperm as
|individuals age.  People do become sterile with age (for various
|reasons, some of which may be related to DNA damage to germ cells).

	This appears not to be due to DNA damage, although certain kinds of
damage (such as chromosomes getting stuck to each other so that they
nondisjoin) do hit progressively more eggs (because no more are made) as time
passes.  Animals become less fertile or infertile with age as the gametes quit
functioning due to somatic aging, and also in females because of running out
of eggs (no more eggs are made after the early part of life (before birth in
humans)).

|Germ cells with too much damage may result in miscarriages.  Less
|damage may result in viable mutations, which may be harmful, neutral,
|or beneficial.

	The miscarriage selection mechanism does take place, but it wouldn't
work quite well enough to prevent pre-aged offspring from being born --
offspring that are much sicker than mere aging (unless it were extreme) would
do can be born, even if their genome is significantly damaged, such as
aneuploidy of the sex chromosomes (including but not limited to Turner's
Syndrome and Klinefelter's Syndrome), trisomies of chromosomes 13, 18, and 21
(Down's Syndrome), possibly trisomy 22, and monosomy of a piece of the short
arm of chromosome 5 (Cri du Chat syndrome), as well as various mutations
damaging much smaller pieces of the genome.

|                Also, note that testicles are kept outside of the main
|body cavity where the temperature is slightly lower, which I believe
|is an adaptation to lower the rate of chemical mutagenic reactions.

	The lowering of temperature that testicles experience relative to the
body (a couple of degrees Centigrade) is not enough to lower the rate of
chemical mutagenesis greatly.  Also, if that were a significant factor,
female mammals would likely have external ovaries.

|Perhaps this is related to the extended fertility of men versus women
|(once again, there can be other factors involved in this phenomenon).

	No, females of diverse animals become infertile first because they run
out of (or at least run low on) eggs (no more are being made, as stated
above) and due to accompanying hormonal changes that are also caused by
somatic senescence (which may stop ovulation before every last egg has been
depleted).  Incidentally, a lot of animals die of old age before either males
or females become sterile (although in nature something else gets them before
old age does).

|Has anyone seen this theory for the evolution of external testicles
|before?

	I haven't seen this theory for the evolution of external testicles
before, but it doesn't seem to wash in light of the above information.
However, it is known that some step in spermatogenesis is significantly more
heat-sensitive than everything else.  Therefore, it seems that external
testicles evolved when mammals (and maybe their immediate precursors) became
warm-blooded, as a kludgy solution to this problem (would have been better to
just fix the process to be less sensitive to heat, but evolution does not
always produce the best possible result -- but what do you expect from
something that depends on random mutations?).

	-- Lucius Chiaraviglio
	   chiaravi@silver.bacs.indiana.edu
	   lucius@tardis.harvard.edu	(in case the first one doesn't work)
	Better active today than radioactive tomorrow. . .
	. . .but better radioactive today than inactive tomorrow.
	:-)