Principles of Plant Genetics and Breeding

102 CHAPTER 6
difficult for users to identify accessions with promise for
breeding.
Concept of prebreeding
Plant breeders usually make elite × elite crosses in a
breeding program. This practice coupled with the fact
that modern crop production is restricted to the use
of highly favored cultivars, has reduced crop genetic
diversity and predisposed crop plants to disease and
pest epidemics. To reverse this trend, plant breeders
need to make deliberate efforts to diversify the gene
pools of their crops to reduce genetic vulnerability.
Furthermore, there are occasions when breeders are
compelled to look beyond the advanced germplasm
pool to find desirable genes. The desired genes may
reside in unadapted gene pools. As previously discussed,
breeders are frequently reluctant to use such materials
because the desired genes are often associated with
undesirable effects (unadapted, unreproductive, yieldreducing
factors). Hence, these exotic materials often
cannot be used directly in cultivar development.
Instead, the materials are gradually introduced into the
cultivar development program through crossing and
selecting for intermediates with new traits, while maintaining
a great amount of the adapted traits.
To use wild germplasm, the unadapted material is put
through a preliminary breeding program to transfer the
desirable genes into adapted genetic backgrounds. The
process of the initial introgression of a trait from an
undomesticated source (wild) or agronomically inferior
source, to a domesticated or adapted genotype is called
prebreeding or germplasm enhancement. The process
varies in complexity and duration, depending on the
source, the type of trait, and presence of reproductive
barriers. It may be argued that prebreeding is not an
entirely new undertaking, considering the fact that all
modern crops were domesticated through this process.
The difference between then and now, as D. N. Duvick
pointed out, is one of demarcation between gene pools.
In the beginning of agriculture, there were no discernible
differences between highly domesticated and
highly selected elite cultivars being deliberately infused
with genes from highly undomesticated germplasm. In
other words, the early farmer-selectors did what came
naturally, discriminating among natural variation without
deliberately hybridizing genotypes, and gradually
moving them from the wild to adapted domesticated
domain.
The traditional techniques used are hybridization followed
by backcrossing to the elite parent, or the use of
cyclical population improvement techniques. The issues
associated with wide crossing are applicable (e.g., infertility,
negative linkage drag, incompatibility), requiring
techniques such as embryo rescue to be successful. The
modern tools of molecular genetics and other biotechnological
procedures are enabling radical gene transfer
to be made into elite lines without linkage drag (e.g.,
transfer of genes from bacteria into plants; see Chapter
14). This new approach to the development of
new breeding materials is more attractive and profitable
to private investors (for-profit breeding programs).
Such creations can be readily protected by patents for
commercial exploitation. Further, these technologies are
enabling plant breeders not only to develop new and
improved highly productive cultivars but also to assign
new roles to cultivars (e.g., plants can now be used as
bioreactors for producing novel traits such as specialized
oils, proteins, and pharmaceuticals).
The major uses of germplasm enhancement may be
summarized as follows:
1 Preventions of genetic uniformity and the consequences
of genetic vulnerability.
2 Potential crop yield augmentation. History teaches
us that some of the dramatic yield increases in major
world food crops, such as rice, wheat, and sorghum,
were accomplished through introgression of unadapted
genes (e.g., dwarf genes).
3 Introduction of new quality traits (e.g., starch, protein).
4 Introduction of disease- and insect-resistance genes.
5 Introduction of environment-resistance genes (e.g.,
drought resistance).
Prebreeding can be expensive to conduct and timeconsuming
as well. With the exception of high value
crops, most prebreeding is conducted in the public sector.
The Plant Variety Protection Act (see Chapter 15) does
not provide adequate financial incentive for for-profit
(commercial) breeders to invest resources in germplasm
enhancement.
Plant explorations and introductions and
their impact on agriculture
Plant explorations
Plant exploration is an international activity. Recent
political developments are making germplasm collections
less of an open access activity. Explorers must
obtain permission to enter a country to collect plant
material. Most of these germplasm-rich places are