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The
Corn Palace in Mitchell, South Dakota, uses corncobs
to
make murals on the sides of the building - yes, the mural
on
the right is made of corncobs. Each year’s murals have a
different
theme, and they use 13 different shades of corn in
their
artwork, but after the drought of 2012 they only had 8
shades
to work with for 2013. This is a picture of the palace
as
it appeared in 1907. Notice the questionable decoration
on
the center minaret – of course this was 25 years before
the
rise of the Nazi party.
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One of the first misconceptions we have to get out of the
way is that corn is actually corn. The word corn doesn’t literally refer to the
stuff on the cob we eat in the summer and the stuff we pop on a
cold afternoon. What we call corn is much more accurately called maize.
The word "corn" comes from an old german/french word. In most
uses before the 1600’s, corn meant the major crop for one particular area or
region. In England, corn meant wheat; in Scotland or Ireland, it most likely
means oats. There is even mention of corn in the King James Bible. This was
translated several times and hundreds of years before maize arrived in Europe.
The “corn” of the Bible most likely means the wheat and barley that were grown
in the Middle East at the time.
When Columbus took maize (Zea mays) across the Atlantic to Europe, he might have referred to
it as the chief crop of the Indians; therefore, it was Indian corn. After a
while, domesticated maize became so ubiquitous that the word “Indian” was
dropped, and all maize became corn – like all facial tissue becoming Kleenex.
The history of maize is, well, a-maizing. The corn we know
today is the most domesticated of all crops. It can’t survive on its own; it
has to be managed by man. Rice and wheat have naturally wild versions of
themselves that still grow in nature, but there is no wild corn, it is
purely man-made.
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Today’s
“corn” is actually a selective breeding result from a
grass
called teosinte and a grass called gamagrass. Genetic
experiments
have confirmed that each of these grasses was
involved
in the evolution of maize. There was also some back
crossing
of early maize with the grasses again. You can see
how the kernels and plants have changed over time.
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The early plants were quite variable, growing from 2 to 20
feet tall. The ears, when they developed, were small and had only eight rows of
kernels. More breeding took place, especially when the plants were brought
north. At that time, ears grew near the top of the plant, and the growing
season in the north was too short to allow full development.
Maize is a grass, so it has the nodes and internodal growth as we discussed a few months ago. Corn grows about 1 node unit for each full moon;
the Indians needed a corn that would mature in just three moon cycles. So they planted
kernels from stalks that had the lowest ears, thereby selecting for plants they
could harvest before it got too cold. Their selection was for size and
production, but colors came along for the ride.
There are many color genes possible in maize. A new version,
called glass gem corn, shows just how many colors are possible (see picture). Indian corn, as we define it now,
can be found in most of these colors; sometimes ears are all one color,
sometimes they are combinations of colors. It all depends on who is growing
nearby, but we need to know a little more about corn in general to explain
this.
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This
Carl’s glass gem corn. The photographer swears there
was
no manipulation of this image. The corn is just this
pretty!
I’d hate to eat it. This strain was the result of many
years
of selective breeding, and the seeds were passed
down through a couple growers before they got this result.
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Maize is a food grain,
meaning that has small fruits with hard seeds, with or without the hulls or
fruit layers attached. More specifically, maize is a cereal grain, because it comes from a grass. Wheat is a grass, so
is barley, rice, and oats. Basically, these are the grains your morning cereal
is made from, so which came first, the breakfast “cereal” or the “cereal”
grain? The answer is out there.
And by the way - yes, grains are types of fruits. The fruit
is more precisely called a caryopsis (karyon means seed); a small fruit and
seed from a single ovary, which doesn’t split open when mature (indehiscent). One of the
characteristics of most grains is that the pericarp (the fruit) is fused to the
seed coat, so it is difficult to talk of the fruit without including the seed.
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The
point of this cartoon is to show you that there are
many
layers to the kernel. The whole thing is not the
embryonic
plant, just the germ. Some people say wheat
germ
is healthy to eat. It would take a lot of kernels to get
much
germ. You can see the hull is made up of several
layers
as well, this is here the color is expressed. The
endosperm
is what tastes food. It is many cells, all
storing the sugars.
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It's the hull that shows the color of a kernel of maize. You
can pop blue, red, or purple corn, but the popcorn will still be whitish
yellow. The color genes are present in all the cells of a kernel, but they are only expressed
in the epidermis or hull; this will be important in a minute or two.
So how can Indian corn have kernels of different colors? The
same way that you and your siblings look different. Each kernel is a different
seed, so each is a different potential plant. The male flowers of the corn
tassel send out grains of pollen to pollinate the female flowers. Each pollen
grain has a sperm cell, and each has undergone the same process of mitosis and
meiosis as human sperm – there is genetic variation there.
The female flowers are the silks on the ear of
corn. Each silk is connected to a different ovary (potential kernel). Again,
each egg is a different version of the maternal plant’s genome. Different silks
could be pollinated by different male plant pollens floating around in the air
– nothing says that all the kernels must have the same dad.
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What
we call Indian corn is just corn that has not been bred
so
much as to have only color gene, and can be pollinated by
different
dads. You can see that Indian corn can have several
colors
or one major color. The interesting parts are those
spots
and streaks. Read on for more about them.
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So, it isn’t to difficult to see that different kernels
could be different colors, either from random assortment and mendelian
genetics, or from different pollens meeting different eggs. The reason we eat
yellow corn or white corn or yellow/white corn is because the color genes have
been selected for by breeding, and the pollination process is highly controlled.
This is not the case with Indian corn.
So that’s the story for corn color – or is there more? Look
closely at Indian corn above; some kernels have streaks or spots of color. How does
that happen?! This is completely different from having kernels of different
color, and relates to one of the great exceptions in DNA biology.
Barbara McClintock found that by observing the chromosomes
of maize very carefully, specifically chromosome nine, and by looking at the
resulting kernels from selective breedings, she could match changes in the
chromosome to changes in color streaks and spotting.
She noticed changes in the length of the arm in some cells,
and related this to the movement of genes along the chromosome. To this point,
all scientists believed that genes stayed in the same place on a chromosome
forever. McClintock saw genes jumping from one place to another. She called
them transposons.
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The
mechanism of transposon control in corn is a bit
complicated.
The C gene codes for pigment, but can be
disrupted
by the Ds transposon. (top). If Ds never moves
out,
then the kernel will be white in this example. If the Ds
gene
never moves in, the kernel will be completely purple.
If
it jumps out and in or in and out, then you get spots. The
bottom
image shows that the early the change, the larger the
spot,
because more daughter cells will have the functional
or dysfunctional gene.
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When Ds was located inside C, no color was produced, but
when it was not, the daughter cells could produce color. A kernel has many
cells that divide and divide, so some progeny could switch back and forth and
produce cells on the hull that may or may not be able to produce the color
protein (see picture). If the move to disrupt C occurred early, more
daughters would be produced and more of the surface would lack color. If it was
late, the spot would be smaller (see bottom image to left).
This idea of jumping genes was revolutionary …. and not well
accepted at first. Even though Barbara’s science was impeccable, others just
weren’t as good at spying the small changes in the chromosome. It took a while
for the laboratory techniques to catch up to Barb’s eyes – then they gave her
the Nobel Prize.
From our new knowledge of transposons have come many
discoveries – some not so savory. Some infectious agents, both bacterial and
eukaryotic, use jumping genes to escape our immune system. Neisseria gonorrhea was one of the first shown to do this. Our
immune system, given time, will find bacteria that have taken up residence
inside us; in gonorrhea's case, through sexual transmission.
N. gonorrhea has
found that if it can change its costume, our immune system must start over
looking for it. The proteins it has on its surface are what our immune cells
recognize, we call them antigens. Gonorrhea organisms can go through antigen variation; they have many
surface antigen genes, and can switch them out if they are detected.
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Variable
surface glycoproteins are like selecting for antibiotic resistant
bacteria.
One organism may switch its VSG for antigenic variation,
just
like one bacterium might pick up a resistance gene.
When
the immune system finds and mounts a response to the
organisms
with the “blue” VSG, they are killed, but now the “green”
VSG
organisms can proliferate. This is like when the antibiotics kill
off
the susceptible bacteria, the resistant ones (green) then
have more room and food to overgrow.
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In the case of Pneumocystis,
a 2009 study showed that there are over 73 major surface glycoprotein (MSG)
genes that can be switched in and out. They differ by an average of 19%, so the
protein sequence of each is markedly different. Even though we don’t know the
function of the MSG, it would appear that it is designed to increase the
variation of the organism, probably to avoid an immune response.
Still have that warm and fuzzy feeling about Indian corn as
a representative of Thanksgiving?
Next week, we start to look at the last of the four biomolecules - lipids. Can you believe some people can't carry any fat on their body, no matter how much they eat?
It just so happens that Barbara McClintock and her corn made up a portion of a recent exhibition at the Grolier Club in NYC, entitled, "Extraordinary Women of Science and Medicine: Four Centuries of Achievement." The exhibit included one of Barbara's ears of corn and some of her breeding materials. The catalogue is available from Oak Knoll Books. Thanks to Karen Reeds, independent curator and museum consultant for the heads up.
Next week, we start to look at the last of the four biomolecules - lipids. Can you believe some people can't carry any fat on their body, no matter how much they eat?
It just so happens that Barbara McClintock and her corn made up a portion of a recent exhibition at the Grolier Club in NYC, entitled, "Extraordinary Women of Science and Medicine: Four Centuries of Achievement." The exhibit included one of Barbara's ears of corn and some of her breeding materials. The catalogue is available from Oak Knoll Books. Thanks to Karen Reeds, independent curator and museum consultant for the heads up.
For
more information or classroom activities, see:
History
of maize –
Transposons
–
Antigenic
variation -
