Walton C. Galinat

Evolution of Corn

Publisher Summary This chapter presents the key trait differences between teosinte and corn in isogeneic backgrounds that enables to determine the minimum number of genetic changes that are essential to convert teosinte into corn, to determine the inheritance and chromosomal location of these genes, to determine the modifying effects of background genes in shaping the expression of the key trait genes, and to determine from the past and current direction of corns evolution as to where it may be going from here and which loci and genes will be useful in directing the future extension of this evolution. Only four or five inherited key trait units separate teosinte from corn and is based on the recovery rate of parental types in F 2 segregations. One viewpoint holds that these inherited units are clusters of linked genes that eventually evolved their partial isolating mechanisms through tight linkage, cross-over suppressors—such as cryptic rearrangements—, and chromosome knobs or by close linkage with gametophyte genes.

Corn's evolution and its significance for Breeding

SummaryExamples of two ways in which studies of corns evolution may contribute to its breeding are given. First, the association of increased pollen grain size with increased ear length in nature indicates the necessity of maintaining this association during artificial selection for increased ear length. Second, by understanding the role of introgression (or flow of germplasm from corns wild relatives, teosinte andTripsacum), we might be able to increase our pool of germplasm and be able to control combining ability, as used in hybrid corn breeding. The use of experimentally controlled introgression obtained directly from teosinte andTripsacum to further improve corn should make it possible to increase the yield and utility of this basic food plant to higher levels than ever would have been attained otherwise.

The domestication and genetic erosion of maize

The genetic variability available for selection is the raw material for crop improvement. However, the process of domestication itself restricts the range of genetic variation, which eventually limits the crops selective response to small gains and increases the crops vulnerability to parasites. The variability in primitive cultivars and their wild relatives remains a largely unexploited genetic resource some of which has been lost. Therefore, we must preserve and understand the variation that still remains in our cultivars and their wild relatives, because it is upon this variation that mankinds survival depends (Galinat, 1972). The seemingly wide variation in cultivars in comparison to that in the ancestral wild species is more apparent than real. It results from a vast increase in population size, a transport to new habitats and a selection by man of traits that are not only lethal in the wild but that may be added up or assembled in combinations so bizarre that the original wild form may appear to be unrelated or unrecognizable. A full exploitation of the variation that lingers from the past requires an understanding of its role in the crops evolution. The emergence of maize as a domestic plant from the wild grasses of Central America is shrouded in mystery because

The evolution of corn and culture in North America

Corn (Zea mays L.) has been recognized as the basic food plant of all the more advanced New World cultures, such as those of the Inca, Aztec, Maya and Pueblo Indians, as well as of our own. The developinent of these cultures was permitted not only by corns great productivity and its adaptability to a wide range of growing conditions but also from an adaptation in structure which facilitates its utilization by man. Although the grain-bearing axis of the other cereals is like that of corn in having acquired the essential non-shattering characteristic, only corn has evolved the additional advantages of the familiar cob with its over-all enclosure of husks. The co1 is a structure convenient for harvesting, storage and shelling by hand, as well as for chewing-off the sweet immature grain from roasting ears. Although the structure of the modern corn plant is well known, the extinction of wild corn has created a mystery as to how its form becamue a function of serving man (Mangelsdorf et al, 1964). Early in domestication, the tassel, an inflorescence which terminates the main stalk, became entirely male, and its structure became that of a lax plume whose waving branches shed their pollen into the wind. The ear, which terminates the lateral branches, became entirely female, and its structure became highly specialized to the benefaction of man. Thereby,

El origen del maiz: El grano de la humanidad

Queridos amigos, me da gran placer a hablar con ustedes, aqui en M6xico, acerca del maiz. Es un placer, pues, porque son ustedes la noble gente cuyos antepasados de hace ocho mil anos crearon el maiz aqui en este sagrado lugar. Ellos desarrollaron el maiz de una planta silvestre nativa, l lamada teosinte, 1o que significa el grano de Di6s. Estos antepasados de ustedes tuvieron la inteligencia, la visi6n y la maravil losa bendici6n de seleccionar de entre mutaciones de grupos silvestres, dos tipos de plantas de teosinte con cuatro hileras de granos en cada mazorca, y de cultivar estos dos tipos juntos y aisladamente. E1 hibrido derivado de eUos llego a ser el pr imer maiz, nombre que significa el grano de la humanidad. Yo no hablo espafiol, and so please allow me to continue in my native tongue.

The evolution of glumeless sweet corn

After some 7,000 years of changes during domestication, the ear of corn may now reach a higher level of utility to man by acquiring a glumeless cob. From meager beginnings as an unpromising food plant, corn has already evolved an ear with so many advantages for harvesting, storage and shelling that it is now the basic food plant of this hemisphere. The ear of wild corn, as reconstructed from its prehistoric remaiins from Tehuacan Mexico (Mangelsdorf et al. 1964), was a tiny, slender spike divided into male (staminate) and female (pistillate) regions, and its few kernels were partly enclosed by long chaff (glumes) around individual kernels, as well as partly enclosed by a few husks arising on the shank below. Then, following domestication, a separation of the sexes into two different inflorescences borne on different parts of the plant permitted divergence into the familiar tassel and ear. When the tassel, which now terminates the main stalk, became entirely male, its structure became that of a lax plume with waving branches adapted for shedding pollen into the wind. Meanwhile, an all-female ear developed at the tip of the lateral branch, and its axis became a massive cob beset with hundreds of large, tightly packed kernels. The husks increased in length and number as they formed an enclosure which protected the grain from the feeding of birds. Thus, the ear glumes became obsolete for protective purposes. As with other structures which have lost their original function, the ear glumes became somewhat reduced. This, in turn, allowed the evolution of a more efficient type of cob because of less competition between the grain and glumes for energy (Mangelsdorf and Reeves 1959).

The Society for Economic Botany 1994 Distinguished economic botanist

Walton C. Galinat, we welcome you to the ranks o f Distinguisbed Economic Botanists. More than 50 years ago, in Connecticut as a high school student, you began your work on the genetics of Zea maize, a plant unique among the cereal grains. You served in the United States Coast Guard during the Second World War then entered the Universi ty of Connecticut, and as an undergraduate, resumed your work on the genetics o f sweet corn with Ralph Singleton and Donald Jones at the Connecticut Experiment Station. You received your doctoral degree at the Universi ty of Wisconsin and returned to the Connecticut Experiment Station. Later, at Harvard Universi ty s Bussey Institute, in collaboration with Paul C. Mangelsdorf, also a Distinguished Economic Botanist o f this Society, you continued your work on maize genetics and morphology. Together with Mangelsdorf, your interest in the origin and evolution of maize expanded and you began a long and fruitful collaboration with noted students of the prehistory and origins of agriculture in the Americas. W h e n you j o i n e d the U n i v e r s i t y o f Massachusetts in 1963, you continued your collaborations in the study o f maize and human culture and you were increasingly sought out by graduate students in genetics as an advisor and by young archaeologists for analysis and interpretation of their prehistoric maize collections. Botanists o f varied interests found their way out to the Field Station in Waltham, Massachusetts, and spent hours with you in the red brick lab out in the middle of the corn field. These were mem-