Bart - in response to your post on Sanet # 1809 (5/1/00):
You wrote:
" Generally mis-placed fear. Apples develop from one ovary of the mother
plant (ie the tree) while the pollen from the paternal tree is
incorporated only in the other ovary (which becomes the seeds). Unlike
maize and such, the seeds aren't what you eat"
Your conclusion is correct (and the British Columbia organic apple
farmers need not lose any sleep over this), but your botany needs a bit
of help. When a flower is pollinated, one pollen grain actually contains
2 sperm (male parent) nuclei. One sperm nucleus will fertilize the
female egg (also called an ovule). The result will be a diploid zygote,
which then divides to become the embryo. The other sperm nucleus unites
with 2 female nuclei in an adjacent cell to become a triploid endosperm
cell. This will divide further to become the endosperm that will nourish
the developing embryo and in some cases, the germinating seed. This
process is called Double Fertilization and is found in pretty much all
flowering plants (angiosperms), though there are variations on the basic
theme.
Note please that there is a difference between pollination (the
germination of pollen grains on the female sigma) and fertilization (the
union of sperm and egg/endosperm nuclei). Contrary to popular belief,
they are not the same thing and do not occur simultaneously.
Why is this important in GM discussions? Corn is a cross pollinated crop
- the pollen is lightweight and designed to travel. Generally the pollen
comes from a different plant, carrying different genetics. Because of
double fertilization, the zygote contains a combination of the DNA from
both parents, and so does the endosperm. Genes from both parents are
expressed in the endosperm. This is important because the endosperm is
the starchy kernel that we eat. Therefore, if Bt corn pollen pollinates
non-Bt corn, both the embryo and the endosperm will both carry Bt genes,
and the endosperm will express them, producing Bt toxin. Anyone who has
ever grown sweet corn a little too near field corn knows the effect that
field corn pollen can have the sweet corn endosperm! Canola is also
cross pollinated, the pollen is carried by bees. The resulting seed will
contain male and female parent genes in both the embryo and the oily
endosperm. When Roundup Ready alfalfa hits the market, this too is cross
pollinated by bees. While the original plant itself won't express genes
from foreign pollen, if the alfalfa plant sets seed and if volunteer
plants grow in the field, the volunteer plants will express both the GM
and non-GM characteristics.
Why is this not as much of a concern with other types of plants?
Example 1 - The starchy portion of a wheat kernel ('white flour') is also
endosperm, but wheat is self pollinated, the pollen almost always comes
from the same flower. Therefore, even when there are GM wheat varieties,
the possibility of cross pollination and therefore the expression of GM
genes in the endosperm is fairly remote - not absolutely impossible, for
there is often a very low natural level of cross pollination, but not
very likely on a large scale. Soybeans are also self pollinated, and
like many legumes, pollination has usually already occured before the
flower even opens. Therefore we aren't as worried about pollen
contamination between Roundup Ready and non-Roundup Ready - again, not
totally impossible, but not a big risk. Seed contamination from
equipment and handling is much more of a concern here.
Example 2 - In a fruit, such as apple, the embryo and endosperm both
develop within the seed. The fleshy fruit is actually the swelling of
maternal ovary and receptable tissue, containing and expressing genes
only from the maternal parent - that is, the tree. Therefore, unless you
planted the seed of the apple and grew it out into a new tree, there
should be little effect from GM pollen that bees carried in. Asexually
propagated cuttings from the original tree would be unaffected and
unchanged by the introduced pollen. Cuttings from a Macintosh apple tree
will produce more Macintosh trees (dare we call them 'clones'?).
So - while there is plenty to be concerned about with GM contamination of
non-GM crops, at least there are a few things we don't have to worry
about quite so much! Mary-Howell Martens
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