This assignment is all about corn. The word "corn" is actually from the Old World and means "grain." When the New World was "discovered" by the Europeans, corn was widely grown by the peoples of both North and South America. The Latin name Zea mays L. was assigned by Linnaeus (hence the authority "L.") in 1737. "Zea" is ancient Greek, meaning grain or cereal, and mays is from "mahiz," meaning "corn" in the Taino language of the indigenous peoples of the Bahamas and Greater Antilles. Maize continues to be an important crop in Central and South America and has now become a staple in many parts of Africa. In fact, maize has become so important as to achieve religious significance within the short time since its introduction.
In most cases, the ancestry of crop plants has been straightforward, with wild species remaining in the vicinity and resembling the crop plant. Corn, on the other hand, is so different from any wild species that its origins have been very elusive. The oldest archaeological evidence is from 6250 years ago. This supports molecular clock estimates. The molecular clock is a technique that molecular biologists use to determine approximately how long ago two species diverged. Scientists can deduce the amount of time that has elapsed based on how many minor differences there are between each of the species' DNA sequences. Genes are thought to change at a consistent rate, and so can provide a timeline for evolution.
Archaeological data supports the notion that corn was not domesticated over a long period of time. There are no intermediates between a progenitor and the corns that were grown as crops in the Americas. Another great mystery is the progenitor. Unlike other crop plants, no similar wild plant has been found to provide insight into the origins of corn. As you will learn during your investigation for this assignment, maize makes a very unlikely wild plant. It is unable to survive without the intervention of humans. As a result, a large, unanswered question has been: where did maize come from? In the twentieth century the origin of corn was the subject of heated debate. There were two main schools of thought: the Tripartite hypothesis and the Teosinte theory.
According to the Tripartite hypothesis, corn resulted from the hybridization between wild maize and a species of Tripsacum (intergeneric hybridization). This theory was proposed by Mangelsdorf (Harvard). The Teosinte theory (Beadle) proposes that teosinte was the progenitor of maize. This seems unlikely because of its many differences from maize. The modifications that gave rise to corn would have been rapid. Today, most researchers support the Teosinte hypothesis (see Doebley, 1990 in supplemental resources). Eubanks is one of only a few scientists who currently support the role of Tripsacum in maize domestication. Doebely, 1990 has suggested that it may take changes to as few as five genes for morphological changes to have occurred. This research is ongoing. Galinat (2001) offers a scenario that, whether accurate or not, illustrates the role that humans must have played in the development of corn as a crop. A beneficial mutation must have been recognized and, through selection and breeding, brought into cultivation. Indeed, corn, which recent research indicates resulted from a single domestication event, yielded much variation, as reflected in the over 200 landraces identified.
The article by Eubanks was selected for this assignment because, in addition to providing historical background to the investigations of the origins of this most elusive crop plant, it also provides ethnobotanical information.
The assignment is divided into two parts:
Part A: Short-Answer Questions
Part B: Essay
1. Compare and contrast the structure of teosinte with that of corn. Why did people believe that teosinte could not be the ancestor to corn? Why were its kernels probably not used to a great extent as food?
2. The movement of corn as a crop throughout the Americas was very slow. What accounts for this?
3. What archaeological data was used to provide information on the domestication of corn? What did each type of information reveal?
4. How did the domestication of corn differ from that of wheat? Could Galinat's scenerio for corn be applied to the domestication of wheat? Explain your answer.
5. Explain why corn seed is most commonly purchased every year rather than saved from a previous year's crop.
6. What are the main differences between the Tripartite and the Teosinte theories? What type of information would provide evidence that would put to rest the controversy?
Part B: Essay
Bt corn is one of the largest modern grain crops in North America. Explain how corn plants are transformed with the Bacillus thuringiensis gene. What are the potential impacts of Bt corn on the individual agricultural ecosystem and on the broader global ecosystem?
Write an essay on this topic of 500–750 words (2–3 double spaced word-processed pages). Be sure that you are presenting the concepts in your own words and that you include current information.