Some breeding objectives can produce interesting challenges. Consider the objective of
reduced seed content in grapes or watermelons. Where does the seed come from to grow the
next crop? In these cases, breeders can “trick” the plant by producing hybrid varieties that
have an unbalanced number of chromosomes such that they cannot undergo proper meiosis
to produce viable seed. Another interesting challenge is the incorporation of traits that we
cannot measure directly. For example, it is desirable to incorporate multiple sources of
disease resistance in a single plant variety so that it is more difficult for a pathogen to
mutate and overcome the resistance. But if there are two genes at different loci that both
confer resistance, how do you know that they are both there? In this case, one solution is
to identify genetic markers that are linked to each resistance gene, so that the markers
can be selected instead of the resistance.
Breeding objectives can change suddenly and unpredictably. An example was the
sudden appearance in the 1990s of devastating levels of a fungal pathogen called
Fusariumin wheat. Now every major wheat breeding program is attempting to introduce
new sources of resistance toFusariumdisease. Another example is the development of a
USDA health claim in the late 1990s aroundb-glucan in oat. While breeders were aware
of this factor, and had initiated selections for higherb-glucan, it was not until the develop-
ment of this health claim that a particular value was placed on varieties that had elevated
levels of this trait. These points illustrate that breeders must be constantly aware of changing
market forces and agricultural conditions. Indeed, it is not unusual for breeders to help drive
some market forces through their knowledge of traits that are available, and through aware-
ness of economic factors affecting industry and producers.
3.4 Methods of Plant Breeding
It has been claimed that breeding is a continuous cycle of recurrent mating and selection.
There is rarely a startpoint or an endpoint in a breeding program; rather, it is a continuous
pipeline that must be kept filled for continual delivery of new and better plant varieties.
Breeders try to release improved varieties every year, but today’s varieties may be the
result of planning and crossing that began a decade ago. Add to this the fact that breeders
mix and match various breeding methods, depending on objectives, and that they constantly
modify and update their strategies, and you can understand why it is difficult to write down
a simple “recipe” for successful plant breeding. Nevertheless, several core strategies have
been developed, and most breeders adopt and adapt one or more of these strategies depend-
ing on plant characteristics, breeding objectives, resources, and personal preference. While
the breeding systems that are described below appear to have a beginning and an end, you
must remember that many cycles at different stages will be running simultaneously in a
given breeding program, and that material from one system can become starting material
for another system.
3.4.1 Methods of Hybridization
Most breeding methods incorporate sexual hybridization as a method of generating new
genetic variability. Hybridization may occur naturally, as in the case of out-crossing
species, or it may require tedious manipulation of flowers in the case of a self-pollinating
species. In special cases, sexual hybridization has been used as a method to combine
traits from species that are rarely cross-fertile. The methods for hybridizing most
3.4. METHODS OF PLANT BREEDING 63