Date: Mon, 5 Jan 1998 08:38:00 -0500 From: "Mellard, David" <dam7@cdc.gov> To: cp@opus.hpl.hp.com Message-Id: <aabcdefg54$foo@default> Subject: Re: Sexual Reproduction of cultivars
Hi Carl,
>Okay, time for me to jump in.
Oh, boy, this ought to be fun.....
>...... like S. flava for instance, don't the seedlings of that S.
>flava x self have the same genetic makeup of the original plant? If
this is
>the case wouldn't any plant selfed reproduce an exact copy of itself?
I was
>always under the impression that if you created a nice cross you could
self
>it to maintain its identity. If this is not the case, a red vft x a
second
>of the same clone = a genetically new plant? Can someone straighten me
out?
This should be interesting. I'll be digging deep into the past to all
those microbiology and genetic courses and will probably give you more
details than you care to know.
Most people are familiar with the idea of X chromosomes and Y
chromosomes that determine sex in mammals, with XX being females and XY
being males. ((FYI, it's the opposite in birds; XX are males and XY
are females if you're a bird.))
Normal cells in the body are diploid, having two sets of each chromosome
while the sex cells (gametes) have undergone a reductive division so
that they have only 1 set of each chromosome. For instance, if diploid
cells have 46 chromosomes, then haploid cells will have only 23
chromosomes. When gametes (in mammals its the sperm and the egg) get
together, each with 23 chromosomes, the resulting diploid fertilized egg
is now back to the orgininal 46. The same thing happens in plants (I
hope).
But nature has added a fail safe system to increase diversity. Think of
an X chromosome as a ladder that has been twisted at both ends. This is
the double helix you've heard about. The X chromosome, though, is
actually two twisted ladders that are joined together for a short space
(technically called the centromere.) Now go back to that one twisted
ladder and lets look at the genetics that there. The ladder is actually
several miles long and each rung on the ladder is a base pair (thymine,
cytidine, adenine, and guanine -- I'm really digging deep now.) It
takes hundred or thousands of rungs (base pairs) to make up a genetic
trait. A single sex chromosome can have hundred or thousands of genetic
traits encoded along the length of the ladder. Now the other ladder
(the other sex chromosome) is in one sense the same as the first ladder.
By this I mean that along the length of the ladder it will encode for
the same set of genetic traits at the same locations along the ladder.
Now here is where genetic diversity comes in. I'll use birds as an
example because the genetics are simple and familiar for me. We have
two chromosomes and the trait we're looking at is feather color. A
normal bird will have two chromosome and the same two genes that encode
for feather color (normal) will lead to gray feathers. Another bird will
have the color cinnamon for its feathers. For this to happen, each of
the two chromosomes now has a mutation in each chromosome that causes
the cinnamon color to occur. But we can also have another bird that is
gray but is split for cinnamon. The two chromosome are now slightly
different. One chromosome has the gene for the normal trait and the
other chromosome has the gene for the cinnamon color. The normal gene
is capable of producing the biochemical reactions that allow for normal
gray coloration. The normal gene is dominant and the cinnamon gene is
recessive.
How did this happen. It happened when the diploid sex cells divided and
separated those two chromosome, one going to 1 gamete, the other going
to the other gamete. In the case of the normal cockatiel split to
cinnamon, one gamete had the gene normal the normal gray trait and the
other gamete that it combined with contained the gene for the cinnamon
trait.
So what does this have to do with whether or not a selfing can produce
the same plant. A chromosome has hundred of traits on it and its
companion chromosome has the same or slightly different set of the "same
traits." When the two companion chromosomes along with other
chromosomes in the diploid cell separate into gamete cells, it's like
shuffling two decks of cards and dividing them half. Each time you do
it, you get a different set of cards. Plants have hundreds or even
millions of diploid cells that have separated to form haploid gametes
(pollen and ovary -- right terms here?) and when the pollen and ovary
combine to form seeds, the genetic diversity can be immense, even when
it's a selfing because there's two sets of the same chromosome to begin
with. What's likely to happen when you self a plant is that you will
get a lot of plants that are phenotypically like the parent plant and a
smaller portion of plants that are phenotypically different. We won't
talk about genotypic diversity. Plants can be phenotypically similar
but genotically different, again because of all the shuffling that goes
on in traits (for instance, biochemical reactions, enzyme production)
that we can't see by looking at the plant.
If you got lost in the above (and sometimes I wonder if I got lost),
think of it as each plant has two genes for every trait. Those genes
can be the same or the genes can be slightly different. When sex cells
are formed, the genes separate into pollen and ovary; when the pollen
and ovary unite,the resulting two traits may end up in different seeds
and the plant from that seed may be similar or different from the parent
plant.
We won't get into the idea that several genes may be require for a
single trait. That would be like shuffling 5 decks of cards together
and then dividing them into several piles. An example for several genes
is probably color variation in plants but I'll let a botanist confirm
that notion.
Hope this was clear,
David
Best Regards,
Carl J. Mazur
Cherryhill Carnivorous Plants
Ontario Canada
http://www.vaxxine.com/ccphome
>Hi,
>
> Selfing of select, superior plants that are named cultivars is one
=
>very good way of possibly creating another superior plant. The
parental =
>name cannot be used on the seedlings derived from this method, but the
=
>opportunity might exist to name one of them ourselves. Orchid growers
=
>do it all the time. Cloning of select plants is a wonderful way to =
>share the wealth. Selfing or sibbing is an exciting way to improve on
=
>what mother nature has given us. Its fun to see what develops.=20
>
> Sincerely
>
> Jim Farrelly
>
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