Genetic Diversity of Maize
When the average person thinks about the diversity of corn, they probably imagine something like this basket of sweet corn. Supermarkets often have three types of sweet corn for sale, yellow, white and bicolor. And of course everyone is familiar with popcorn- yellow, white, and sometimes red or blue. But the diversity of corn is vast. One of the major sites that preserve the genetic varieties of corn is the US Department of Agriculture's Plant Introduction Station in Ames, Iowa. They hold 19,780 different samples or "accessions" of corn from around the world. The diversity of corn found in those accessions reflects physical differences in seed color, shape and texture, but also the physiological differences that make some varieties suited to growing in desert environments or the wet tropics. These seeds hold the genes that have accumulated through natural and human selection over hundreds or even thousands of years.
A Sample of Genetic Diversity
Genetic diversity is sometimes easy to see. Corn seeds have some obvious different colors, such as red, white, or yellow. But corn seeds may have different shapes (variety Pencil Cob) and may have a surface indentation (dent corn seeds). Here is a sample of some of the variation in corn seeds. To learn more about genetic variation in corn, view the slide show.
|Pencil Cob is a variety that was and is grown in the Southeastern US. The seeds are long and narrow, with a dent. The cobs are skinny, like a pencil!|
|Oaxacan Green corn is used by the Zapotecs in Mexico to make green flour tamales.|
|The Acoma Pueblo is the oldest continuously inhabited community in North America. Corn (called Yaak'a) is a staple crop of this area. This accession has great diversity- the corn on a single ear may be blue, red, white or even mixed colors.|
|Blue corn was grown by several Native American groups, and has cultural traditions associated with it. The blue color is due to production of substances called "anthocyanins" in a layer of the seed (aleurone). There are 10 genetic factors that control this color (Betran et al.).|
Landraces, Heirlooms, Hybrids- What's the Difference?
We know that people have cultivated corn for thousands of years. The type of corn that people grew in a particular location was determined by how well it grew and what people wanted to do with the crop. In the agricultural world of pre-industrial societies, natural and human selection produced local types of corn that had a genetic make-up that was different than in other regions. In more modern times, scientists have used the term landraces to identify unique types of crop plants.
'a landrace is a dynamic population(s) of a cultivated plant that has historical origin, distinct identity and lacks formal crop improvement, as well as often being genetically diverse, locally adapted and associated with traditional farming systems'. Camacho Villa, 2006.
Some of the oldest varieties of corn are landraces from Mexico. Palomero Toluqueno is a type of popcorn that was grown in the highlands of Mexico, though it is not grown much today. Chapalote is another landrace with a long history of cultivation.
In the United States, the variety Gourdseed is sometimes called a landrace. Gourdseed is a narrow seeded dent corn, which was grown by settlers in Colonial America. Its genetic heritage is from Mexico. Although it is probably one of the earliest corn types grown by European settlers, the earliest written account of gourdseed is from John Lorain in 1825 (Anderson & Brown, 1952). Gourdseed is now sometimes sold commercially as an heirloom variety.
Seed catalogs often include listings of "heirloom" plants. These are varieties that were commercially available in the past, but fell out of use when hybrid types were developed and promoted. They are open pollinated- meaning that seed can be saved from one generation to the next without losing characteristics and performance of that variety. The term heirloom is evocative- an heirloom is an old treasure. These are "old, treasured varieties" that can still produce good quality crops. There are several heirloom sweet corn varieties, such as Stowell's Evergreen. This corn was developed by Nathan Stowell of New Jersey in 1848, but had even earlier use by Native Americans. Another heirloom corn variety is Country Gentleman.
Nearly all the corn grown in the US (either commercially or even in home gardens) is hybrid corn. Hybrid seed comes from the crossing of two pure lines. A common saying is that hybrids do not "Breed true"- meaning that if you try to save seed from a hybrid variety, your saved seed will not have the characteristics of the original hybrid seed you planted. Hybrid lines can have desirable, valuable characteristics. For example, some of the newest varieties of sweet corn are called supersweet and contain the shrunken-2 gene (sh2). Dazzle, Honey & Pearl, Butterfruit, Candy Corner, Illini Xtra Sweet or Snow White are some of the supersweet varieties. But growers must purchase new seed each year, and cannot save money by replanting from their harvest.
In contrast to hybrids, there are open-pollinated varieties. Before 1950, most of the corn grown in the US was "open pollinated". These varieties will breed true and farmers and gardeners can replant their fields with saved seed. Families could select the best seeds from their crop and continue to replant- adapting a variety to their local environment. Some examples of modern, open pollinated varieties of corn include Reid's Yellow Dent, Silver King, and Truckers Favorite.
Anderson, Edgar, and William L. Brown. 1952. The history of the common maize varieties of the United States Corn Belt. Agricultural History 26: 2-8.
Camacho Villa, Tania Carolina, Nigel Maxted, Maria Scholten and Brian Ford-Lloyd 2006. Defining and identifying crop landraces. Plant Genetic Resources 3(3): 373-384.
Gardening with Heirloom Seeds: Tried-and-True Flowers, Fruits, and Vegetables for a New Generation by Lynn Coulter.
Seed to Seed: Seed Saving and Growing Techniques for Vegetable Gardeners (Paperback).
A Celebration of Heirloom Vegetables: Growing and Cooking Old-Time Varieties (Hardcover) by Roger Yepsen.
100 Vegetables and Where They Came From by William Woys Weaver.
Seed Savers Exchange- Non-profit organization that promotes the preservation of old seed varieties and exchange of those varieities among members http://www.seedsavers.org/
United States Department of Agriculture- Agricultural Library. This page provides links for information about growing heirloom varieties of fruits, vegetables and herbs.
Save Our Seed- a non-profit organization dedicated to preserving and promoting organic and heirloom seeds in the Southeastern US http://www.savingourseeds.org
Suzanne P. DeMuth. 1999. Vegetables and Fruits. A Guide to Heirloom Varieties and Community-Based Stewardship. National Agricultural Library, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland. http://www.nal.usda.gov/afsic/AFSIC_pubs/heirloom/heirloom.htm