Principles of Plant Genetics and Breeding

F. J. Betrán
Texas A&M University, College Station, TX 77843, USA
Figure 1 Heterosis in maize: A × B hybrid ears in the middle
and corresponding parental inbreds A and B in both sides.
Figure 2 Relative grain yields of an open-pollinated population
and a single-cross hybrid developed from selected inbreds from
the same population.
BREEDING CORN 491
Industry highlights
Hybrid breeding in maize
Introduction
Maize is a cross-pollinated species that shows high
heterosis (i.e., superior performance of crosses relative
to their parents) for grain yield (Figure 1). This high
expression of heterosis is exploited in maize hybrids
and constitutes the foundation of the maize seed
industry. Maize hybrids were first developed in the
United States in the mid-1930s and by the early 1960s
practically all the maize area in the US was planted to
hybrids (Duvick & Cassman 1999). Improved productivity
and selection gain with the use of hybrids has
stimulated increased investment in hybrid development,
resulting in impressive genetic progress (Figure
2). Shull (1909) outlined the pure-line method in
maize breeding suggesting the use of self-fertilization
to develop homozygous lines that would be of use
in hybrid production. This combination of inbreeding
and hybridization constitutes the basis of maize
improvement. The general process to develop maize
hybrids starts with the creation of a source segregating
breeding population that it is used to develop inbred
lines through inbreeding and selection (Figure 3).
Selected inbreds are then evaluated in hybrid combinations
across locations to select superior hybrids
and to estimate their combining abilities. The following
is a brief description of the main components of
this process from the starting breeding population to
the commercial hybrids.
Source breeding populations
Different type of segregating populations can be used
as the source in line development: open-pollinated
cultivars (OPC), synthetic cultivars, single crosses,
backcrosses, double crosses, related line crosses, and
exotic germplasm. Overall, major emphasis goes to
the use of breeding populations created by hybridization
of complementary inbreds and the selection of
progenies possessing the desirable traits from both parents
(Hallauer 1990). Selection within F 2 and backcross
populations using pedigree breeding is the most
important breeding method to develop maize inbreds.
Breeding programs that emphasize pedigree selection
within populations developed from elite inbred lines
are therefore cyclical creating second-, third-, fourth-, etc. generation recycled improved inbreds (Figure 3). The incorporation
and introgression of exotic germplasm brings new desirable alleles and genetic diversity to this recycling of elite lines (Goodman
et al. 2000). A backcross or multiple backcross to the best parental inbred is used commonly to increase the probability of maintaining
favorable combinations of alleles (Troyer 2001). Maize breeders use multiple trait, multistage, and multiple environment
selection methods (Betrán et al. 2003). Multiple environment and multiyear inbred general combining ability values with