varieties, hybrids may not show as much advantage as they do in cross-pollinated varieties,
because these species naturally tolerate inbreeding. Furthermore, it is more difficult to
manually enforce an adequate number of hybrid matings. However, hybrid varieties are fre-
quently used in high-value horticultural crops that produce a large amount of seed from a
single mating. Hybrids have also been used in some self-pollinated crops in which mech-
anisms of male fertility can be used to ensure cross-fertilization. Some of these crops
include sugarbeet and sunflower.
3.4.4. Clonally Propagated Species
Some crop plants are propagated naturally and/or artificially through vegetative propa-
gation rather than through sexually produced seeds. Globally, the most important
example is potato, but other examples include banana, strawberry, yam, sweet potato,
and many tree crops. Although the crossing behavior of clonal crops is not relevant to
propagation, it is still important in the breeding strategy. Most clonally propagated crops
are cross-pollinated, so breeding methods are most similar to those used in cross-pollinating
seed crops. However, the ability to maintain an “immortal” genotype makes selection of a
population less important, and selection of individual plants becomes far more relevant.
The selection of tree crops presents special challenges because of the long juvenile
period, so many fruit tree varieties have been identified by careful observation of hybrids
from serendipitous crosses that may have taken place many years ago.
3.5 Breeding Enhancements
This section provides a brief description of several of the most important techniques that
can be used to enhance the success of a breeding program. Many additional techniques
are discussed in other literature. Perhaps the most important modern breeding enhance-
ment is plant transformation: the ability to transform plants with DNA that originates
from different species. This topic is discussed in other chapters, but it is interesting to
note that the way in which genetic transformation can be incorporated into a breeding
program bears many resemblances to the use of mutation breeding, discussed in
Section 3.5.3. Furthermore, marker-assisted selection (Section 3.5.2) is often used as a
follow-up to genetic transformation in order to recombine a transformation event into
new breeding populations.
3.5.1 Doubled Haploidy
The derivation of pure lines (Fig. 3.3) is one of the most important steps in breeding self-
pollinated varieties. In the SSD method (Fig. 3.10), this step could be considered “wasted
time” if there were a shortcut to produce pure lines. This shortcut exists, and it is called
doubled haploidy.
The principle behind doubled haploidy is that every plant species produces haploid
gametes during meiosis. Haploid gametes are found in the female (egg) and in the male
(pollen) tissues. By forcing these gametes to double the chromosomes in their nuclei, we
can immediately produce a cell type that is both diploid and homozygous. There are
74 PLANT BREEDING