Principles of Plant Genetics and Breeding

194 CHAPTER 11
the natural reproductive systems of species to develop
plants that have an atypical genetic constitution. The
terms pure line, inbred line, and clone are applied to
materials developed by plant breeders to connote sameness
of genetic constitution in some fashion. However,
there are some significant distinctions among them.
1 Pure lines. These genotypes are developed as cultivars
of self-pollinated crops for direct use by farmers.
As products of repeated selfing of single plants, pure
lines are homogeneous and homozygous and can be
naturally maintained by selfing.
2 Inbred lines. These are genotypes that are developed
to be used as parents in the production of hybrid
cultivars and synthetic cultivars in the breeding of
cross-pollinated species. They are not meant for direct
release for use by farmers. They are homogenous and
homozygous, just like pure lines. However, unlike
pure lines, they need to be artificially maintained
because they are produced by forced selfing (not
natural selfing) of naturally cross-pollinated species.
3 Clones. Clones are identical copies of a genotype.
Together, they are phenotypically homogeneous.
However, individually, they are highly heterozygous.
Asexually or clonally propagated plants produce
genetically identical progeny.
Categories of asexually propagated species
In Chapter 4, asexually propagated species were grouped
into two according to economic use as those cultivated
for vegetative products and those cultivated for their
fruits. For breeding purposes, vegetatively propagated
crops may be grouped into four, based on flowering
behavior.
1 Species with normal flowering and seed set. Species
in this category produce normal flowers and are capable
of sexual reproduction (to varying extents) without
artificial intervention (e.g., sugarcane). However,
in crop production, the preference is to propagate
them sexually. Such species enjoy the advantages of
both sexual and asexual reproduction. Hybridization
is used to generate recombinants (through meiosis)
and introduce new genes into the adapted cultivar,
while vegetative propagation is used to maintain,
indefinitely, the advantages of the heterozygosity
arising from hybridization.
2 Species with normal flowers and poor seed set.
Some plant species produce normal-looking flowers
that have poor seed set, or set seed only under certain
conditions but not under others. These restrictions
on reproduction make it unattractive to use seed as a
means of propagation. However, the opportunity for
hybridization may be exploited to transfer genes into
adapted cultivars.
3 Species producing seed by apomixis. The phenomenon
of apomixis results in the production of
seed without fertilization, as was first discussed in
Chapter 4. Over 100 species of perennial grasses have
this reproductive mechanism.
4 Non-flowering species. These species may be
described as “obligate asexually propagated species”
because they have no other choice. Without flowers
(or with sterile flowers) those species can only be
improved by asexual means. Genetic diversity is not
obtained via recombination but by other sources
(e.g., mutation).
Genetic issues in asexual breeding
Genetic makeup
All the progeny from an individual propagated asexually
are genetically identical (clones) and uniform. Clones
are products of mitosis. Any variation occurring among
them is environmental in origin.
Heterozygosity and heterosis
Many species that are asexually propagated are highly
heterozygous; they are highly heterotic. Consequently,
they are susceptible to inbreeding depression. For those
species that can be hybridized without problems, an
advantage of asexual propagation is that heterosis, where
it occurs, is fixed for as long as the cultivar is propagated
asexually.
Ploidy
Many known species that are asexually propagated are
interspecific hybrids or have high ploidy.
Chimerism
Clones are stable over many generations of multiplication.
The only source of natural variation, albeit rare, is
somatic mutation in the bud. Plant breeders may generate
variability by the method of mutagenesis. Whether
natural or artificial, somatic mutations are characterized
by tissue mosaicism, a phenomenon called chimerism.
A chimera or chimeric change occurs when an individual
consists of two or more genetically different types
of cells. Though heritable changes, these mosaics can