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Are there "Identical Twin" plants?

Wayne Parrott wparrott at uga.edu
Sun Dec 17 15:36:25 EST 2000

Andrew Dalke wrote:

> >Hammond:  the question then: Is there direct evidence from
> >          crop yield and other agricultural data on parthogenetic
> >          (asexual-clonal) plants, which indicates a significant
> >          VARIANCE in adult size, from which we may conclude
> >          that there is such a thing as an "ADULT GROWTH DEFICIT"
> >          in these plants (in the actual environment), whereby
> >          they are generally always somewhat smaller than some
> >          "theoretical genetic maximum adult size"?
> >            And further, from the Normal Distribution Curve
> >          for the adult size of these (clonal) plants, can
> >          we ESTIMATE the "theoretical genetic size" of the
> >          plant, statistically.... by for instance assuming
> >          that it must be near "2-Standard Deviations" above
> >          the mean clonal size?
> That question isn't very interesting.  Because of variations in
> the environment (soil, water, light, pests, etc.) there will
> be variations in size even with with identical genes.  The
> largest of these sets a minimum value on the size for that
> genotype as a whole.  It is well neigh impossible to estimate
> what the maximum size would be for a species, only set a lower
> bound.

Actually, from an agricultural point of view, the question is very
interesting, as we always want to reach maximum crop yields.  Yields
obtained in variety trials-- under optimum conditions on university
experimental farms-- are usually much higher than those on real farms.
We want to know why yields on farms do not reach their potential, so
that some day, we may find a way to ensure that plants live up to their
full genetic potential for yield.

Yield of crops has risen dramatically the past half century.  In fact,
land area under cultivation has held fairly steady since the 60's, and
the increased food production that is feeding the increased population
has come from these increased yields.  About half of the yield gains are
from improved agricultural practices- better fertilizer, pesticides, and
irrigation.  The other half comes from genetics-- leading to the
development of crops with the ability to reach a greater size or produce
more harvestable tissue.

Theoretically, genetically identical plants should be identical
regardless of where they are planted.  The failure of genetically
identical plants to be the same in different environments allows crop
scientists to estimate what portion of a plant's size or yield is caused
by genetics, and what portion is due to environmental factors.  One can
extrapolate from the genetic portion, and estimate the maximum yield.
Keep in mind that that estimate is only valid for that group of
genetically identical plants.  Change the genetics, and you change the
maximum growth potential.

> In other words, it is tautological that all plants must be
> smaller than their "theoretical genetic maximum adult size."
> You will never find a plant larger than that limit.  At best
> all you can do is, as you say, make observations to the
> distribution of sizes and apply some arbitray cutoff in
> the extrapolation to determine what that size might be.
> So the answer to your question '[may we] conclude that there
> is such a thing as an "ADULT GROWTH DEFICIT" is "yes" but
> it isn't interesting or useful.
> The environment does have a big impact on the size.  For
> example, some plants would do much better in a CO2 rich
> environment compared to what there is now.  Others are
> affected by the timings of the rains.  I'll bet even more
> are limited by being in a 1g gravity field and would grow
> a lot bigger at 1/2 g.  So the environment needed to produce
> the "maximum adult size" might not even be available on
> this planet and any extrapolation you make must necessarily -
> if only implicitly - include that environmental limitation
> in its qualifiers.
> Why is your concern on the adult plant size?  Wouldn't other
> attributes be more important, like ability to produce food
> or to reproduce?  Or drought or pest resistance?  It isn't
> good to be a big plant if you can't survive seasonal droughts.
> Hmm.  Thought of a related issue to point out why I don't
> think the question is all that intesting.  Consider viruses.
> Nice, simple reproductive factories.  You can apply your
> same arguments to them to an even higher degree - after
> all, there is a much smaller set of genes needed to make
> the virus and hence less possible variation called by
> environmental effects on gene expression.
> Take HIV as an example virus.  It's an enveloped virus and
> the size of the envelope can vary somewhat.  Again, by
> definition, any HIV virus can be no bigger than the biggest
> possible HIV virus.  Given the distribution of observed sizes
> it's a pretty safe bet that we've never seen the largest HIV
> virus which has ever been produced, much less the biggest
> which could be produced.  Hence, viruses also suffer from
> an "ADULT GROWTH DEFICIT"!  (Because of differences in
> human cells, perhaps a better choice would be a bacteriophage
> virus. I don't know enough about that field to pick a good
> example.)
> Since your statement can be applied to almost every species
> and for every possible variable characteristic, I can only
> conclude that it is not a very useful concept.
> > I'm not advising you to take a graduate degree in theoretical
> > physics either
> Too late.  Already did that.  :)
>                     Andrew
>                     dalke at acm.org

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