Tuesday, November 24, 2015

Corn Color Concepts

Biology concepts – maize, transposon, antigenic variation, cereal grain, food grain, caryopsis

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.
Thanksgiving decorations typically include some colorful earns of dried corn, commonly referred to as “Indian corn.” However, this corn has a history much more involved than mere decoration. People might be less inclined to hang it around their house if they knew how much it has in common with the organisms that cause gonorrhea, Lyme disease, and Pneumocystis pneumonia.

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.

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.
The earliest corn-like plant was called teosinte. It's a grain plant with very small, vertical kernels. This plant was bred with something else, maybe gamagrass, and over time became early maize. Early maize was then bred back to teosinte, and the cob emerged. A recent article from Florida State shows that corn was being bred and harvested as early as 5300 BCE.

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.

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.

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.

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.
The hull is a little more vague. Corn has a husk (the leaves that surround the ear), which is often considered the same thing as a hull. But each kernel on the ear also has a hull, the epidermis that is more brittle when dried. In other plants, husk and hull mean the same thing.

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.

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.

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.

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.
But this jumping is not haphazard. It was under the control of another gene. When one gene (Ds) was activated to jump by another gene (Ac), its new position disrupted a third gene’s (C) sequence (Ds = disrupter, Ac = activator, and C = color).

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.

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.
They do this by moving different surface antigen genes in and out of an expression site. Only the surface antigen gene in the expression site is transcribed and translated to protein, but they can jump in and jump out when needed. Antigenic variation also occurs with Borrelia burgdorferi, the causative agent of Lyme disease, the Plasmodium falciparum of malaria, and Pneymocystis jirovecii, a eukaryote that causes the pneumonia most AIDS patients contract.

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.

Pohl ME, Piperno DR, Pope KO, Jones JG. (2007). Microfossil evidence for pre-Columbian maize dispersals in the neotropics from San Andres, Tabasco, Mexico. Proc Natl Acad Sci U S A. , 104 (16), 6870-6875 DOI: 10.1073/pnas.0701425104

Keely SP, & Stringer JR (2009). Complexity of the MSG gene family of Pneumocystis carinii. BMC genomics, 10 PMID: 19664205

For more information or classroom activities, see:

History of maize –

Transposons –

Antigenic variation -