[Paleopsych] Possible evidence for immortal bacteria! (And why not?)

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This should be of particular interest to Howard.

---------- Forwarded message ----------
Date: Fri, 25 Feb 2005 08:41:59 +0100
From: Eugen Leitl <eugen at leitl.org>
Reply-To: transhumantech at yahoogroups.com
To: transhumantech at yahoogroups.com
Subject: [>Htech] Re: [GRG] Possible evidence for immortal bacteria! (And why
     not?) (fwd from ag24 at gen.cam.ac.uk)

----- Forwarded message from Aubrey de Grey <ag24 at gen.cam.ac.uk> -----

From: Aubrey de Grey <ag24 at gen.cam.ac.uk>
Date: Thu, 24 Feb 2005 18:17:33 +0000
To: grg at lists.ucla.edu
Subject: Re: [GRG] Possible evidence for immortal bacteria! (And why not?)
Reply-To: Gerontology Research Group <grg at lists.ucla.edu>


> This was the very study to which I was referring in my reply to Bob!

It's probably the most talked-about scientific publication of the month,
in fact.  It's certainly very exciting, but various details need to be
kept in mind at this stage:

1) The experiment was done for eight generations and showed a linear
decline in the growth rate with increasing numbers of generations in
which the old pole was inherited.  This is in contrast to the standard
pattern in aging where the functional decline accelerates with age.  It
is thus very important to extend this study to 20 or 30 generations to
see whether the trend eventually accelerates or levels off.  Of course
the entire lineage does not need to be followed -- one just needs some
cells at each point in the virtual lineage.

2) The authors measured "growth rate", and they really do mean rate of
increase in the size of the cells.  However, they note that new-pole
daughters are larger on average than old-pole ones, and additionally
that the new-pole daughter tends to divide first.  The latter is to be
expected, since increase of size is generally limiting for generation
time for bacteria in rich medium.  This merits a lot more discussion
(for example there is nothing about whether multi-generation-old-pole
cells are especially small), because the whole result may be because
smaller cells grow more slowly at first (not that that wouldn't be
interesting, of course).  The authors describe the above results as
showing that there is no "juvenile phase" whereby the new cell needs
to go through some initial maturation process before it gets going,
but they forget that it may be the daughter with the old pole that is
going through such a phase.

3) The authors allude in the discussion to the phenomenon constituting
a 2% "cost" of the aging process at the population level.  They don't
elaborate, but I think the meaning must be that the colony would grow
2% faster if all cells grew as fast as the new-pole ones.  But this is
not the right calculation if one wants to determine cost, because if the
divisions were precisely symmetrical then the old-pole cell would grow
faster but the new-pole cell would grow more slowly.  I haven't done
the maths but I strongly suspect that asymmetrical division (and hence
asymmetrical dilution of damage) can for some examples of the function
linking growth rate to damage levels confer a higher colony growth rate
than symmetrical division.

4) Possibly the main reason the above points matter so much, especially
the last one, is because of the effect on the validity of concluding
that the observed phenomenon is universal, inescapable etc.  There are
numerous circumstances in which an organism's optimum metabolic tactics
vary non-linearly with stress: for example the increase of maintenance
at the expense of reproduction in caloric restriction, or the fusion
response of mitochondria to high stress, or the senescence response
of cells to over-frequent double-strand breaks.  I suspect that under
a variety of alternative, reasonable assumptions, the colony growth rate
would switch from being best with symmetrical division to being best
with asymmetrical division depending on whether stress (e.g. oxidative)
and hence rate of damage accumulation was above or below some threshold.
If so, repeating this experiment under low oxygen might give different
results.

Aubrey de Grey
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