Subject: Correlation vs causation: a special problem for symbiologists

From: Dennis <[email-masked]>
Date: Thu, 13 Apr 1995 14:39:37 -0700 (MST)

I've read two papers this past two weeks (that about wears me out) and
both attribute the cause of the effects they observed to a symbiont (actually
endophytes, but I'm trying to be general here).  BUT, in neither case
were any manipulative experiments performed to tease out the cause from
a correlation.  This drives me nuts (even more so than my usual disposition)
as these types of correlative studies are EASY to do compared to manipulative
experiments.
 
For example:  Imagine platyhelminth A with symbiotic green algal cells in
there ectoderm vs platyhelminth B without (both same species) and one grows
twice as fast as the other.  Can you attribute the this growth difference
to the symbiont???  Well many times yes it is.  But erroneously.  The presence
of the symbiont might simply be correlated with the growth difference.
This reasoning should be quite straight forward.  If it is, why do you so
often see examples where the symbiont is considered the cause???
 
I don't want to loose sight of the biology, nor say that it is wrong to
suggest the symbiont is responsible, but time and time again papers are
published which loose sight of this correlation vs causation issue.
 
I fully understand that it might be very difficult to manipulate symbionts
in many systems and that it is difficult and time consuming.  However, I
feel that causation is the exciting and often time the goal of the study
so manipulative studies are mandatory.
 
Another situation which drives me nuts:  Organism A is found dead with
fungus B or bacteria C growing all over it.  The worker states organism A
was killed by fungus B (or bacteria C).  However, without experimental
evidense to support this, how can these conclusions be reached??  In
some instances, if the fungus B or bacteria C is a known pathogen,
perhapps this conclusion can be reached.
 
Ok thats my "peave" for the week.
Any comments?
 
Cheers Dennis Wilson

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From: [email-masked]
Date: Fri, 14 Apr 1995 17:36:46 -0500 (CDT)

> Any comments?

Yes--I agree.  Plant pathologists and medical types have a set of rules 
for determination of causation.  This is the Koch's Postulates.  Here an 
organism associated with a disease can be determined to cause that 
disease only if:  1. it is repeatedly associated with disease; 2. when 
inoculated into a host it causes symptoms of the previously observed 
disease; and 3. It can be reisolated from the diseased plants.  It seems 
to me that as a bare minimum studies to determine effects of a symbiont on 
hosts should be approached with similar conservatism and more or less 
the same experimental approach.

Jim White
[email-masked]

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From: Peter Herman x5495 <[email-masked]>
Date: Sun, 16 Apr 1995 17:27:20 -0600 (MDT)

> the same experimental approach.
Hi All;

	While I agree in general about the causation problem, fulfilling
Koch's Postulates is not always possible.  In some cases the symbiont cant
be cultred outside the host.  In others, there seems to be an inoculation
problem.  I have some experience with the latter.  In a gall midge
(Lasioptera ephdricola), shrub (Ephedra trifurca), fungus (Aureobasidium
pululans) system we have the following set up.  The fungus is always (and
I do mean always, ~10,000 galls looked at for one reason or another and
never a one without the fungus ring which lines the gall chamber)
associated with the gall chamber.  The chamber is located inbetween the
vascular tissue and the photosynthetic parenchyma of the stem.  The gall
"looks fungal" and we tried a Koch's postulates experiment.  We got a lot
of "almost" galls in the experimentals and nothing more than a little
necrotic lession in some of the controls.  I am not convinced that the
insect imports the fungus to make the galls, but I don't feel that the
possibility is eliminated either.  Our results can be interpreted at least
3 ways. 1) the insect is the sole source of gall formation and our "almost
galls" are mearly coincedental, 2) the fungus and insect must interact
developmentally or biochemically in gall formation, or 3) the fungus is
really the gall former but the insect is a lot better at placing the
inoculum when the 1st instar chews in than I am with a microlier
syringe. 

I guess my middle ground position is you are obligated to TRY Koch's 
Postulates experiments and then go from there depending on what your 
results tell you.

Peter

*   Peter Herman, Dept. of Biology	Phone: +1 505 646 4532    *
*   New Mexico State University		Fax:   +1 505 646 5665    *
*   Box 30001, Dept. 3AF					  *
*   Las Cruces, NM 88003 USA		e-mail [email-masked]    *