Principles of Plant Genetics and Breeding

such as sugarcane (most commercial cultivars), turfgrass
(e.g., “Tifway”), and forage crops.
Apomixis
Apomixis is vegetative propagation through the seed.
The seed in this instance is genetically identical to the
female plant. Using the apomictic propagule is similar to
reproducing the plant by other vegetative means as previously
described, only more convenient. Hybrid vigor
is fixed and expressed indefinitely through vegetative
propagation. The genetics and mechanisms of apomixis
are described in detail under breeding apomictic species
(see Chapter 11).
Monoecy and dioecy
The reproductive biology of monoecy and dioecy has
been previously described. Monoecious species bear
male and female flowers (imperfect) on the same plants
but on different parts of the plant. Environmental conditions
(e.g., photoperiod, temperature) can influence
sex expression by making one plant more female or
more male. In cucumber, short day and low night
temperatures promote femaleness, while the application
of gibberellic acid promotes maleness. Breeders may
manipulate the environment to produce hybrid seed.
Prerequisites for successful commercial
hybrid seed production
As briefly reviewed, commercial hybrid seed production
is undertaken for a wide variety of species including
Burton, G.W. 1983. Utilization of hybrid vigor. In:
Crop breeding (Wood, D.R., ed.). American Society of
Agronomy and Crop Science Society of America, Madison,
WI.
Crow, J.F. 1998. 90 years ago: The beginning of hybrid
maize. Genetics 148:923–928.
Hallauer, A.R., and J.B. Miranda. 1988. Quantitative genetics
in maize breeding. Iowa State University Press, Ames, IA.
Janick, J. 1996. Hybrids in horticultural crops. CSSA/ASHS
Workshop on Heterosis, Indianapolis, IN, November 3,
1996. Department of Horticulture, Purdue University,
West Lafayette, IN.
Melchinger, A.E., and R.K. Gumber. 2000. Overview of
BREEDING HYBRID CULTIVARS 349
References and suggested reading
field crops and horticultural species. Some species are
more suited to commercial hybrid seed production than
others. Generally, the following are needed for a successful
commercial hybrid seed production venture.
1 High heterosis. Just as plant breeding can not be
conducted without variability, hybrid seed production
is not meaningful without heterosis. The F 1 should
exhibit superior performance over both parents. The
degree of heterosis is higher in some species (e.g.,
corn) than others (e.g., wheat).
2 Pollen control and fertility-restoration system.
An efficient, effective, reliable, and economic system
should exist for pollen control to exclude unwanted
pollen from a cross. Some species have natural pollination
control mechanisms (S 1 , male sterility) or reproductive
behaviors (monoecy, dioecy) that can be
exploited to facilitate the crossing program. A sterility
system should have a fertility-restoration system to
restore fertility to the commercial seed. In the absence
of natural pollen control mechanisms, mechanical or
hand emasculation should be feasible on a large scale.
3 High F 1 yield. The F 1 seed is the commercial product.
Species such as corn that bear a large number
of seed per F 1 plant are more suited to hybrid seed
production than species that produce small amounts
of seed on an F 1 plant (e.g., wheat).
4 Economic seed production. Hybrid seed production
is more expensive overall than conventional seed
production. The cost of seed production may be
significantly higher when hand emasculation is the
method used in the crossing process. In this latter
scenario, hybrid seed production would be economic
only in high priced crops (e.g., tomato), or where
labor is cheap (e.g., cotton production in India).
heterosis and heterotic groups in agronomic crops. In:
Concepts and breeding of heterosis in crop plants (K.R.
Lamkey, and J.E. Staub, eds), pp. 29–44. Proceedings of
the Plant Breeding Symposium sponsored by the CSSA and
American Society of Horticultural Science, November
1996. Crop Science Society of America, Madison, WI.
Norskog, C. 1995. Hybrid seed corn enterprises. A brief history.
Curtis Norskog, 2901 15th Street SW, Willmar, MN.
Riday, H., and E.C. Brummer. 2002. Heterosis of agronomic
traits in alfalfa. Crop Sci. 42:1081–1087.
Sinobas, J., and I. Monteagudo. 1996. Heterotic patterns
among US Corn Belt and Spanish maize populations.
Maydica 41:143–148.