Principles of Plant Genetics and Breeding

62 CHAPTER 4
homozygous and therefore become expressed. However,
such depression is reversed upon cross-pollination.
Hybrid vigor (the increase in vigor of the hybrid over
its parents resulting from crossing unlike parents) is
exploited in hybrid seed production (see Chapter 18).
In addition to hybrid breeding, population-based
improvement methods (e.g., mass selection, recurrent
selection, synthetic cultivars) are common methods of
breeding cross-pollinated species.
Asexual reproduction
Asexual reproduction may be categorized into two –
vegetative propagation and apomixis. Asexual reproduction
is also called clonal propagation because the
products are genetically identical to the propagules.
Vegetative propagation
As previously indicated, certain species may be reproduced
by using various vegetative parts including bulbs,
corms, rhizomes, stems, and buds. Vegetative propagation
is widely used in the horticultural industry. Pieces
of vegetative materials called cuttings are obtained from
parts of the plant (e.g., root, stem, leaf ) for planting.
Potato, cassava, sugarcane, rose, grape, and some perennial
grasses are frequently propagated by stem cuttings.
Methods such as grafting and budding are used for
propagating tree crops, where two different plant parts
are united by attaching one to the other and securing
with a tape. Healing of the graft junction permanently
unites the two parts into one plant.
A variety of sophisticated techniques are used to vegetatively
propagate high value plants. Numerous plantlets
may be generated from a small piece of vegetative material
(e.g., a segment of a leaf) by the technique of micropropagation.
The tissue culture technique is used to
rapidly multiply planting material under aseptic conditions
(see Chapter 11 for more details). Perennial horticultural
species tend to be clonally propagated, whereas
annuals and biennials tend to be propagated by seed.
Clonally propagated crops may be divided into two
broad categories on the basis of economic use:
1 Those cultivated for a vegetative product. Important
species vegetatively cultivated for a vegetative product
include sweet potato, yam, cassava, sugarcane,
and Irish potato. These species tend to exhibit certain
reproductive abnormalities. For example, flowering
is reduced, and so is fertility. Some species such as
potatoes have cytoplasmic male sterility. Sometimes,
flowering is retarded (e.g., by chemicals) in production
(e.g., in sugarcane).
2 Those cultivated for a fruit or reproductive product.
Plants in this category include fruit trees, shrubs,
and cane fruits. Examples include apple, pear, grape,
strawberry, and banana.
Breeding implications of vegetative propagation
There are certain characteristics of clonal propagation
that have breeding implications.
1 Clonal species with viable seed and high pollen
fertility can be improved by hybridization.
2 Unlike hybridization of sexual species, which often
requires additional steps to fix the genetic variability
in a genotype for release as a cultivar (except for
hybrid cultivars), clonal cultivars can be released
immediately following a cross, provided a desirable
genotype combination has been achieved. Clonal
breeding is hence quick.
3 When improving species whose economic parts are
vegetative products, it is not important for the
hybrid to be fertile.
4 Because of the capacity to multiply from vegetative
material (through methods such as cuttings or
micropropagation), the breeder only needs to
obtain a single desirable plant to be used as stock.
5 Heterosis (hybrid vigor), if it occurs, is fixed in
the hybrid product. That is, unlike hybrid cultivars
in seed-producing species, there is no need to
reconstitute the hybrid. Once bred, heterozygosity
is maintained indefinitely.
6 It is more difficult to obtain large quantities of
planting material from clones in the short term.
7 Plant species that are vegetatively parthenocarpic (e.g.,
banana) cannot be improved by hybrid methodology.
8 Species such as mango and citrus produce polyembryonic
seedlings. This reproductive irregularity
complicates breeding because clones of the parent
are mixed with hybrid progeny.
9 Clonal crops are perennial outcrossers and intolerant
of inbreeding. They are highly heterozygous.
10 Unlike sexual crop breeding in which the genotype
of the cultivar is determined at the end of the breeding
process (because it changes with inbreeding),
the genotype of a family is fixed and determined at
the outset.
11 Both general combining ability (GCA) and specific
combining ability (SCA) (that is, performance in
crosses) can be fully exploited with appropriate
breeding method and population sizes.