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MU CORN GENE ZOO

Demonstration of corn history from domestication to modern hybrids

Primary Source: Smith, C. Wayne. 1995. Crop Production: Evolution, History, and Technology. John Wiley and Sons, Inc. New York, New York.

Teosinte

According to the most widely accepted theory of corn origin, teosinte is the wild progenitor of modern corn. The term teosinte refers to a group of five species within the genus Zea. Both annual and perennial species survive in Mexico and several neighboring countries as a wild grass.

Teosinte produces a small ear that contains five to ten hard, triangular or trapezoidal black kernels. The outer covering is hard and resistant to digestion by ruminants, which forage on teosinte and aid in seed distribution through their droppings.

Nearly all populations of teosinte are either threatened or endangered, and the Mexican government has attempted to protect wild teosinte populations. Wild teosinte plants may contain beneficial traits, such as disease and insect resistance that could be used to improve modern corn hybrids.

Corn Races and Movement from Origin

When Europeans began settling in what would become Mexico and the USA, many races of corn had been developed. These races had been selected by Native Americans for specific environments and uses, and corn had been adapted from its center of origin in southern Mexico to central and northeast North America. Examples of early Mexican varieties include: Chapalote, Harinoso de Ocho, Peptilla, Tabloncillo, Tuxpeno, and Olotillo.

Races found in what would become the United States and some of the Mexican varieties that gave rise to those races include:

Pima-Papago originated from Chapalote, a Mexican race and perhaps two other races. Examples of representatives of this race are Kokoma and Z01-010 .
Southwestern 12-Row is similar to the Northern Flints. No representatives of this race are grown in the MU Corn Gene Zoo.
Southwestern Semidents derived from a Mexican race, Tuxpeno, and flint and flour types growing in the area.
Great Plains Flint and Flours resulted from mixing of Northern Flints and varieties from the Southwest. An example representative of this race is Winnebago Mixed.
Northern Flints may have originated from a Mexican variety, Harinoso de Ocho. Examples of representatives of this race are Longfellow, Smut Nose, and Yellow Thompson.
Southern Dents were grown in what would become the southeastern part of the USA. The varieties were similar to Mexican varieties such as Pepitilla and Olotillo. Examples of representatives of this race are Gourdseed, Shoepeg, and Hickory King.
Southeastern Flints and Flours are divided into two subraces, Caribbean Flints, and Cherokee Flour. Examples of representatives of this race are Yellow Creole and Golden Prolific.
Derived Southern Dents may have resulted from crosses among Southern Dents, Southeastern Flints, and Corn Belt Dents. Examples of representatives of this race are Horsetooth and Latham’s Double.

Other examples of open-pollinated varieties:

The representatives of the described races were open-pollinated varieties. Open-pollinated varieties are not hybrids, but are mixtures of genotypes with less yield potential than modern hybrids. Open-pollinated varieties were grown by USA farmers until the advent of modern hybrids. Some open-pollinated varieties were used to derive inbreds for hybrid corn breeding. Some examples of open-pollinated varieties are: Bloody Butcher, Mandan Bride, Hopi Blue, Arkansas Red and White, and Boone County White.

Reid’s Yellow Dent, an open-pollinated variety, resulted from an unplanned, but fortuitous, crossing of a northern flint and southern dent. While living in Illinois, Robert Reid had planted ‘Gordon Hopkins Gourdseed’ (from Virginia). Because of poor germination some of the plants never emerged and Mr. Reid planted ‘Little Yellow’ (an old northern flint) into the missing hills. These two corn types crossed naturally. Robert and James Reid selected the best ears for several generations and by 1893 established the open-pollinated variety, Reid’s Yellow Dent. This variety was more productive than its contemporaries. Reid’s Yellow Dent was the dominant corn in the USA for several decades and the source for many popular inbreds, including B73.

Lancaster Sure Crop, an open-pollinated variety, was developed by Isaac Hershey in Lancaster County, Pennsylvania. He mixed more than six varieties together and began selecting for early maturity, disease resistance, easy harvest and uniformity. Many modern hybrids have at least one inbred that traces back to this open-pollinated variety.

Corn Hybrids

One of the earliest documented examples of controlled breeding of corn was conducted by W.J. Beal in 1877. Lester Pfister, and others, began developing inbreds for use in corn breeding in the early 1900s. Inbreds are derived by forcing corn plants to self-pollinate. The percentage of homozygosity increases with each generation of self-pollination (inbreeding). Two examples of commonly used inbreds are B73 and Mo17. B73 was developed by Dr. Wilbert Russell at Iowa State University and was released in 1972. Mo17 was released in 1964 by Dr. Marcus Zuber, a USDA researcher located at the University of Missouri. The cross between B73 and Mo17 is used in the MU Gene Zoo as a demonstration of hybrid vigor. The hybrid is more vigorous and yields substantially more than either inbred parent.

The first commercially successful hybrids were double (sometimes called 4-way) crosses. This hybrid involves four inbreds and requires two years to complete. In the first year, two single cross hybrids (see below) are made by crossing pairs of inbreds. In the second year, the two single cross hybrids are crossed. The seed that results from this second cross is sold to farmers as a double cross corn hybrid. Double cross corn hybrids were more economical to produce than single cross hybrids because the female parent (producer of seeds) exhibited hybrid vigor and produced many seeds. Double cross corn hybrids were commonly grown in the USA during the 1940s, 1950s, and 1960s. An example of an early double cross hybrid from one of several companies is on display in the MU Corn Gene Zoo.

Single cross corn hybrids, on average, yield more than double cross corn hybrids. They possess less inbreeding depression and more hybrid vigor. Seed of single cross corn hybrids was more expensive to produce and purchase because the female parent was an inbred that produced fewer seeds than hybrids. Modern, high yielding inbred lines allowed for reasonably priced single cross hybrid seed. Today more than 95% of the corn acres planted in the USA is planted with single cross corn hybrids. An example of an early single cross hybrid from one of several companies is on display in the MU Corn Gene Zoo, along with an example of a modern hybrid.

Demonstration of modern hybrids and use of biotechnology

Corn hybrids that contain one or more biotechnology traits became commercially available in the late 1990’s. Two herbicide resistant traits, Roundup Ready™ (resistant to glyphosate) and Liberty Link™ (resistant to glufosinate), are currently available to corn growers in the USA. The gene that confers glyphosate resistance was derived from an Agrobacterium species. The gene that confers resistance to glufosinate came from Streptomyces hygroscopius.

Biotechnology traits that confer insect resistance originated from several biotypes of Bacillus thuringiesis. Traits derived from this bacterium are often referred to as Bt traits. The first commonly available Bt trait conferred resistance to feeding by Lepidoptera (moths and butterflies) larva. The primary target insect species is the European corn borer. A second Bt trait confers resistance to feeding by Coleoptera (beetles) larva. The primary target insect species is corn rootworm. Both Bt traits confer resistance to several insects other than the primary target species.

Many modern hybrids contain more than one biotech trait. These are referred to as stacked traits. Some hybrids contain four or more traits of version of similar traits. Specific, trademarked, traits can also be developed without the use of biotechnology. An example is increased drought tolerance in the hybrids with the AQUAmax™ trait.

Demonstration of adaptation

Modern corn hybrids are nearly day-neutral, meaning that photoperiod length has little effect on the timing of flowering and other stages. Corn development is controlled by exposure to heat units. Hybrids adapted to the southern portion of the USA require more heat units to mature than hybrids adapted in the northern Corn Belt.

Growing degree days (GDD) are a measure of accumulated heat units. The formula for GDD of one day is:
[(Tmax + Tmin)/2] - 50 where:
Tmax is the high temperature for the day.
Tmin is the minimum temperature for the same day.

For corn, if Tmin is less than 50, then 50 is entered. If Tmax is greater than 86, then 86 is entered. GDDs are accumulated from planting date through a specific ending date or stage. All temperatures are Fahrenheit.

Corn Relative Maturity (CRM) uses the unit “days”, but is an estimation of relative maturity and does not mean calendar days. Most corn hybrids are sold with a CRM designation. Typical CRMs for Missouri hybrids range from 105 to 116.

Demonstration of special use corn types

Dent corn
More than 90% of the corn produced in the USA is classified as dent corn. Kernels of these hybrids have a central core of soft, floury endosperm surrounded on the sides by hard, flinty endosperm. The hard endosperm does not cover the entire crown of the kernel. As the kernel dries, the soft endosperm shrinks more than the hard endosperm, creating an indentation in the center of the top of the kernel. Primary uses of dent corn are animal feed and ethanol production.

Sweet corn
Sweet corn hybrids possess one or more genes that slow the conversion rate of sugars to starch in the kernels. These sugars (e.g. sucrose) have a much sweeter taste than starch. Sweet corn is harvested before it matures (milk stage). Sweet corn genes are su (sugary), se (sugary enhanced; increases sweetness and increases shelf life), and sh2 (shrunken-2; increases sugar content fourfold or more). Because kernels with high amounts of sugar (less starch) have less solid support provided to normal corn kernels by stored starch, sweet corn seeds often shrink and shrivel upon maturity and subsequent drying.

Food grade dent corn
Although most dent corn produced in the USA is fed to animals, some hybrids have kernel characteristics that make them desirable for human food use, such as food grade dent corn. Typically, kernels of these hybrids are harder than normal and resist cracking. These traits, among others, are desired by the food industry.

White dent corn
White dent corn is used in various food products. Kernels of white corn do not produce beta-carotene, so they do not have the normal yellow color of most dent corn hybrids.

Flint corn
Kernels of flint dent corn types have a central core of soft, floury endosperm surrounded by hard, flinty endosperm. Unlike dent corn kernels, the hard endosperm covers the entire crown of the kernel. As the kernel dries, the hard endosperm prevents the kernel tip from indenting.

Popcorn
Popcorn is a type of flint corn. When kernels are heated, water in the kernels is vaporized to steam. The outside layer of hard endosperm traps the vapor inside the kernel until the pressure is high enough to rupture the kernel. As the pressure is released the kernel is literally turned inside out. A thin pericarp (outer covering sometimes called the hull) is desired in popcorn hybrids. Strawberry popcorn is both functional and ornamental. It is a true popcorn and will pop like other popcorn types. Kernels are bright red and borne on short cobs. The color and arrangement of kernels cause the ears to appear similar to strawberry fruit.

Flour corn
Kernels of these corn types produce almost no hard endosperm. The soft, floury endosperm grinds easily into meal.

Opaque (High Lysine)
Proteins stored in the endosperm of corn kernels are primarily in a class of protein called zein. Zein contains almost no lysine, an essential amino acid for humans and animals. Hybrids known as opaque produce less zein than normal and, instead, produce classes of protein that contain lysine. Normal dent corn kernels are translucent. Opaque corn kernels are not translucent and appear opaque. Foods and feeds produced with high lysine corn require less supplementation to balance amino acid ratios. Few high lysine hybrids currently exist.

High oil
Kernels of normal corn hybrids are about 3.5 to 4% oil. Nearly all of the oil is stored in the embryo. High oil hybrids produce twice this amount of oil because embryo size is increased. Feeds using high oil corn have greater energy values because of the increased oil content.

Waxy and Amylomaize
Kernels of dent corn produce two types of starch. Amylose is composed of long straight chains of glucose. Amylopectin contains branched chains of glucose and is similar to glycogen in animals. Starch from normal corn hybrids is 27% amylose and 73% amylopectin. Kernels from waxy corn hybrids produce very little amylose and nearly 100% of the starch is amylopectin. Kernels from amylomaize corn hybrids produce starch that is more than 50% amylose. Starches extracted from both hybrids are used to make several industrial and food products.

Ornamental
Most corn produced in the USA is yellow in color. White corn occurs when production of beta-carotene is blocked in the kernel endosperm. However, corn kernels can be red, purple or other colors. The red and purple pigments are made in the aleurone layer, a thin layer of cells that surrounds the endosperm. Kernels of ornamental corn possess many different colors, often on the same ear. The kernels can appear mottled with uneven expression of color in the kernel. These ears are commonly used for fall decoration. Kernels that possess colors other than yellow are possible in all types of corn including dent, flint, floury, and popcorn.

Oddities

Several corn types that exhibit unique features include the oddities Pencil Ear and Pod Corn. Pencil Ear corn plants produce a small cob that is not much larger in diameter than a large pencil. The glumes of associated with each kernel are very much reduced and not easily seen on ears of normal corn hybrids. A single gene in Pod Corn types causes the glumes to expand greatly and cover the entire kernel. Each kernel appears to possess its own husk.

The corn cob pipe industry began in Missouri in 1869 and corn cob pipes have been manufactured in Washington, Mo., ever since. Any corn cob can be used to make pipes except the cobs of modern hybrids are often too small in diameter. The corn cob pipe industry commissioned the University of Missouri to develop a corn type that produces a large diameter cob. The example of this corn type on display at the MU Corn Gene Zoo is Missouri Corn Cob Pipe.

Demonstration of the effects of single genes on plant development and morphology

Primary Source: Neuffer, M.G., E.H. Coe, and S.R. Wessler. 1997. Mutants of Maize. Cold Spring Harbor Laboratory Press.

Corn plants have more than 40,000 genes. Genes contain codes for structural proteins and enzymes. The science and art of corn breeding is combining the forms of these genes into new combinations that result in a hybrid superior to its contemporaries. Many genes affect the plant in ways that may not be obvious to the casual observer. However, some genes can have dramatic effects.

The following is a list of examples of these genes. The phenotypes they produce are displayed at the MU Gene Zoo.

Gene	Name	Description
al1	albescent plant	variable cross bands on leaves
an1	anther ear	anthers occur on ear
ar1	argentia	virescent seedlings with leaves that green rapidly along veins
ba2	barren stalk	ear shoots and most tassel branches are absent
bd1	branched silkless	ears produce branches but no silk; tassels are often bushy in appearance
bif1	barren inflorescence	majority of spikelets missing on both ear and tassel
bk2	brittle stalk	brittle plant parts are easily broken
blh1	bleached leaf	pale green midveins in upper leaves
bm2	brown midrib	midrib of leaf is brown
br1	brachytic	stem internodes are shortened and plants appear compressed
cg1	corngrass	plants produce many tillers and appear grass like
clt1	clumped tassel	short plants with compact tassel
cr1	crinkly leaves	short plant; leaves are broad and crinkled
ct1	compact plant	short stem internodes
d8	dwarf	miniature corn plants less than 12 inches tall
eg1	elongated glumes	glumes on tassel open at right angles and stay open
f1	fine striped	leaves have fine white stripes
g2	golden plant	plant parts have yellow cast
gs2	green stripe	pale green stripes between veins on leaves
hs1	hairy sheath	many hairs on leaf sheath
hsf1	hairy sheath frayed	hairs and leaf margins and sheaths, outgrowth on leaf margins
id1	indeterminate growth	extended growth without flowering
ij1	iojap striped	variable white stripes on leaves
j1	japonica striped	white striping on leaves
kn1	knotted	localized proliferation of tissue at vascular bundles on leaves
la1	lazy plants	plants grow flat along the soil after about V5 stage of development
les6	lesion	many small to medium chlorotic and necrotic spots on leaves
lg3	liguleless	displaced and fragmented ligule. Leaves are upright
Li1	lineate leaves	fine stripes on portion of leaves closest to stem
lw2	lemon white	white seedlings
lxm1	lax midrib	leaves have wide, flexible midribs
ms1	male sterile	tassel does not produce pollen
na1	nana	short, erect, dwarf plants
nl2	narrow leaf	leaves are narrower than normal and often slightly distorted
og1	old gold stripe	leaves are yellow striped
pl1	purple plant	purple pigment in leaves
ra1	ramosa	tassel is Christmas tree shaped and ears are branched
rg1	ragged leaves	leaves have defective tissue between veins, causing holes and tearing.
rs1	rough sheath	rough sheath and extreme ligule disorientation
sbd1	sunburn	leaves exposed to sunlight develop greyish surface
sdw1	semidwarf plants	shortened internodes and erect leaves
sk1	silkless	no silk on ears, pistils abort
sl1	slashed	leaves have longitudinal necrotic areas that open into slits
sr1	striate leaves	many white striations on leaves
tb1	teosinte branched	many tillers
te1	terminal ear	stalked ear appendages at tip
tlr1	tillered	extreme tillering; small ears
tp1	teopod	many tillers; many small podded ears
ts1	tassel seed	plants produce tassels in which the female parts develop and are fertile
tu1	tunicate	kernels on ear are enclosed with elongated glumes
v5	virescent	pale longitudinal stripes on leaves
vsr1	virescent striped	leaves possess light green to yellow longitudinal stripes
wd1	white deficiency	white stripes
wi2	wilted	leaves wilt under stress
wrp1	wrinkled plant	short plant with longitudinally corrugated leaves
ws3	white sheath	white leaf sheath and stem
wt2	white tips	white leaf tip, yellow banding
yg1	yellow green	bright yellow green leaves
ys3	yellow stripe	yellow tissue between leaf veins like iron deficiency symptoms
ysk1	yellow streaked	longitudinal yellow streaks on upper third of leaves
tu1	tunicate	kernels are enclosed in glumes
zb4	zebra cross bands	regularly spaced lighter green cross bands on earlier leaves
zb8	zebra necrotic	yellow green cross bands on older leaves; purple leaf tip