Principles of Plant Genetics and Breeding

172 CHAPTER 10
The purpose of these barriers is to maintain the genetic
integrity of the species by excluding gene transfer from
outside species. Some barriers occur before fertilization,
some after fertilization. These barriers vary in degree of
difficulty to overcome through breeding manipulations.
Spatial isolation
Spatial isolation mechanisms are usually easy to overcome.
Plants that have been geographically isolated may
differ only in photoperiod response. In which case, the
breeder can cross the plants in a controlled environment
(e.g., greenhouse) by manipulating the growing environment
to provide the proper duration of day length
needed to induce flowering.
Prefertilization reproductive barriers
These barriers occur between parents in a cross. Crops
such as wheat have different types that are ecologically
isolated – there are spring wheat types and winter wheat
types. Flowering can be synchronized between the two
groups by, for example, vernalization (a cold temperature
treatment that exposes plants to temperatures of
about 3–4°C) of the winter wheat to induce flowering
(normally accomplished by exposure to winter conditions).
Mechanical isolation may take the form of differences
in floral morphology that prohibit the same
pollinating agent (e.g., insect) from pollinating different
species. A more serious barrier to gene transfer is
gametic incompatibility whereby fertilization is prevented.
This mechanism is a kind of self-incompatibility
(see Chapter 4). The mechanism is controlled by a complex
of multiple allelic systems of S genes that prohibit
gametic union. The breeder has no control over this
barrier.
Postfertilization reproductive barriers
These barriers occur between hybrids. After fertilization,
various hindrances to proper development of
the embryo (hybrid) may arise, sometimes resulting in
abortion of the embryo, or even formation of a haploid
(rather than a diploid). The breeder may use embryo
rescue techniques to remove the embryo and culture
it to full plant. Should the embryo develop naturally,
the resulting plant may be unusable as a parent in
future breeding endeavors because of a condition called
hybrid weakness. This condition is caused by factors
such as disharmony between the united genomes. Some
hybrid plants may fail to flower because of hybrid
sterility (F 1 sterility) resulting from meiotic abnormalities.
On some occasions, the hybrid weakness and infertility
manifest in the F 2 and later generations (called
hybrid breakdown).
Wide crosses
The first choice of parents for use in a breeding program
are cultivars and experimental materials with desirable
traits of interest. Most of the time, plant breeders make
elite × elite crosses (they use adapted and improved
materials). Even though genetic gains from such crosses
may not always be dramatic, they are nonetheless
significant enough to warrant the practice. After
exhausting the variability in the elite germplasm as
well as in the cultivated species, the breeder may look
elsewhere, following the recommendation by Harlan
and de Wet, as previously noted. These researchers
proposed that the search for desired genes should start
from among materials in the primary gene pool (related
species), then proceed to the secondary gene pool, and if
necessary, to the tertiary gene pool. Crosses involving
materials outside the cultivated species are collectively
described as wide crosses. When the wide cross involves
another species, it is called an interspecific cross (e.g.,
kale). When it involves a plant from another genus, it is
called an intergeneric cross (e.g., wheat).
Objectives of wide crosses
Wide crosses may be undertaken for practical and economic
reasons, research purposes, or to satisfy curiosity.
Specific reasons for wide crosses include the following:
1 Economic crop improvement. The primary purpose
of wide crosses is to improve a species for economic
production by transferring one or a few genes, or
segment of chromosomes or whole chromosomes,
across interspecific or intergeneric boundaries. The
genes may condition a specific disease or pest resistance,
or may be a product quality trait, amongst
other traits. In some species such as sugarcane,
cotton, sorghum, and potato, hybrid vigor is known
to have accompanied certain crosses.
2 New character expression. Novelty is highly desirable
in the ornamental industry. Combining genomes from
diverse backgrounds may trigger a complementary
gene action or even introduce a few genes that could
produce previously unobserved phenotypes that may
be superior to the parental expression of both qualitative
and quantitative traits.