Proceedings of the Fifth Asian Regional Maize Workshop - Search ...

MAIZE BREEDUG
Arnel R. Hallauer-L!
Abstract
SUccessful maize (Zea mays L.) breeding programs include two equally important components:
identification and improvement of source germplasm and effective methods for the development of
inbred lines and hybrids. Both components are necessary to maintain consistent genetic advance
either for genetically broad-based cultivars (i.e., open-pollinated varieties and synthetic
varieties) or for hybrids produced from inbred lines (i.e., single crosses, three-way crosses,
and double crosses). The two components should not be considered separately, but as integral
components to realize maximum benefits of the two components.
Applied maize breeding programs usually are concerned with the improvement of traits that
are controlled by an unknown number of ,genes whose expressions are affected by environmental
effects. Effective selection requires the evaluation of progenies in different environments to
determine the breeding values of the progenies for both components. Recurrent selection methods,
in the broadest sense, are used for ~th components of maize breeding. Development and genetic
improvement of germplasm are continuous and long-term. Germplasm is provided for developing
inbred lines and hybrids from each cycle of germplasm improvement. Elite lines can be recycled
by pedigree selection methods to develop second-cycle lines for hybrids. Thus, the development
of progenies, evaluation of progenies in replicated trials repeated over environments, and
intermating of elite progenies are conducted cyclically for both components to gradually increase
the frequency of desirable alleles for the traits emphasized in selection.
Introduction
Maize (Zea mays L.) is the most widely and intensively grown crop species in the world and
ranks third, behind rice and wheat, in total production. Maize originated in southern Mexico or
northern Guatemala 5,000 to 10,000 years ago. Maize is extremely variable genetically, and
selection pressures were effective in developing maize strains to meet the needs of the native
inhabitants. Most of the genetic variability in maize was present before the European colonists
arrived in the Western Hemisphere. Native Americans had developed races that were being grown in
present-day southern canada, the United States, Mexico, central America, islands of the Caribbean,
and throughout South America by 1492. Columbus collected maize on the northern coast of
CUba on his first trip. After maize was introduced in Spain in 1493, within two generations it
became distributed throughout the world in areas where it could be grown and cultivated (Manglesdorf,1974).
Maize is a cross-pollinated crop species that has been studied extensively since the
European colonization of the Western Hemisphere. Several features of maize made it amenable to
genetic and breeding studies: male (tassel) and female (ear) flowers are separate which permit
cross and self-pollinations with equal facility; 300 to 500 seeds can be produced on each ear to
permit adequate evaluation and analysis of plants and progenies; the genome (2n = 20) of corn is
relative simple permiting extensive genetic and cytogenetic analysis; and studies on the effects
of inbreeding from self-pollination and the heterosis effects expressed in crosses have made
maize a model crop species for plant breeding method studies. Breeding and selection methods
developed and evaluated for improvement of germplasm and development of lines to produce hybrids
in maize are studied by plant breeders for use in other crop species.
JLI Professor, Dept. of Agronomy, Iowa State University, Ames, IA 50011, USA.
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