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Re: seed genetics

On Thu, 5 Mar 1998, Daniel Segal wrote:

> though i am no geneticist to say the least, it is obvious that by virtue
> of sexual reproduction each individual in a population is genetically
> unique.  The same is true of populations within a meta-population or
> region--each population is genetically distinct.  The question really is
> one of degree--how distinct are they from eachother?  Though it can never
> hurt to use site-specific seed and plant material, more often than not for
> any given species, the severity of this genetic differentiation is
> unknown.  For restoration I think the attitude has been "better safe than
> sorry", and this is good but clearly not scientific. 

The problem is, that by every measure of genetic differentiation that's
been tried, the amount of difference from one population to another varies
greatly.  And if you use isozymes to estimate genetic distance, you come
up with an entirely different set of numbers than if you use mitochondrial
DNA.  Or genomic DNA.  Or chloroplast DNA.   Or RNA.  And these sorts of
studies generally require at least a few hundred thousand for funding,
and several years time.  And most botanists have neither.  :-(

The information that comes out of these sorts of studies is often
equivocal, anyhow, unless you've got some populations that are clearly
in decline, and others that are doing well.  If you can show that
the populations that are no longer producing viable seed have less
genetic diversity than the ones that are, you can make a fair tentative
conclusion that genetic diversity may be involved.  Then you get into
the whole debate on whether natural populations should be "rescued"
by introducing more genes.  (Generally accepted conclusion: if the
low genetic diversity is due to human action, it's ok to do something;
if it's naturally occuring, don't mess with nature.)

We certainly have examples of plants that have almost no genetic
diversity: all Franklinia altamaha descend from a few seeds transported
to England in the 16/1700's.  And the grove of Metasequoia
glyptostroboides in China has only one individual with butressed roots.
Guess what happens to ALL of the trees descended from the seed brought
back in the 1940's?  Yep.  Buttressed roots.

> Furthermore, regarding the northernmost Big Tree groves in Placer Big
> Trees, there are several young individuals which were planted by an
> ambitious and well-meaning group years ago, but the seed was from southern
> groves of Sequoiadendron.  Lately there has been a lot of noise about
> removing the imports, so that their pollen does not sire north-south Big
> Tree offspring.  Again, probably better safe than sorry, but does anyone
> know for sure that the infusion of some slightly different genome into
> this grove will be detrimental?  Is nature as simple as that?  The
> variables are so many, not just mean annual temp. or mean July temp., that
> we can't possibly know until we try.  I don't advocate vigilanteism with
> regard to the genetics of California's populations of all species, but
> gene flow is natural, even if we sometimes drive it...

Gene flow is natural, but the question remains how much, whether we know
enough to be tinkering.  I tend to be pretty conservative on this... 
let's see what happens when we greatly increase genetic diversity in
species in a nice botanic garden well away from the natural populations,
and only introduce new genes into the naturally occuring populations when
it's clear the low diversity populations are pretty well on their way out 
otherwise.t

One of my major reasons is rather selfish: if we start making major
genetic transplants from one area to another, it's potentially going
to screw up all future genetic research from those populations.  

Population genetics gets pretty theoretical, and pretty mathematical
at times, but there are some good books available to get you started
reading the research papers.

I personally like Daniel Hartl's 1997 Principles of Population
Genetics, and Brian Manly's 1985 Statistics of Natural Selection.

Kay Lancaster    kay@fern.com
just west of Portland, OR; USDA zone 8
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