Principles of Plant Genetics and Breeding

196 CHAPTER 11
clones with other superior traits and harvest
individually.
Year 2 Grow progenies of selected clones and evaluate
as in year 1. Select superior clones.
Year 3 Conduct preliminary yield trials. Select
superior clones.
Years 4–6 Conduct advanced yield trials at multilocations
for cultivar release.
Hybridization with clonal selection
This procedure is applicable to species that are capable
of producing seed in appreciable quantities. Because
heterosis can be fixed in clonal populations, the breeder
may conduct a combining ability analysis to determine
the best combiners to be used in hybridization.
Year 1 Cross selected parents. Harvest F 1 seed.
Year 2 Plant and evaluate the F 1 s. Select vigorous
and healthy plants.
Year 3 Space plant clonal progeny rows of selected
plants. Select about 100–200 superior plant
progenies.
Year 4 Conduct preliminary yield trials.
Years 5–7 Conduct advanced yield trials for cultivar
release.
Other techniques that are applicable include backcrossing
to transfer specific traits and wide crossing. The
challenges with backcrossing are several. As previously
indicated, clonal species are very heterozygous and
prone to inbreeding depression. Backcrossing to one
parent (the recurrent parent) provides an opportunity
for homozygosity and consequently inbreeding depression.
To prevent this, breeders may cross the backcross
to another clone instead of the recurrent parent, followed
by selection to identify superior plants. The process
is repeated as needed.
Mutation breeding
The subject is discussed in detail in Chapter 12.
Inducing variability via mutagenesis is challenging for
two key reasons. Being rare events, a large population of
M1V2 is needed to have a good chance of observing
desired mutants. Obtaining a large number of vegetative
propagules is difficult. Also, mutations occur in individual
cells. Without the benefit of meiosis, the mutated
clonal material develops chimeras. Using adventitious
buds as starting material reduces the chance of chimeras.
A mutation in the epidermal cell (usually there is one)
would result in an adventitious shoot that originated
from a single mutant cell. This technique is not universally
applicable.
Breeding implications, advantages, and limitations of
clonal propagation
The breeding implications of vegetative propagation
were discussed in Chapter 4. There are several
advantages and limitations of breeding vegetatively
propagated species.
Advantages
1 Sterility is not a factor in clonal propagation because
seed is not involved.
2 Because clonal plants are homogeneous, the commercial
product is uniform.
3 Micropropagation can be used to rapidly multiply
planting material.
4 Heterozygosity and heterosis are fixed in clonal
populations.
Disadvantages
1 Clonal propagules are often bulky to handle (e.g.,
stems, bulbs).
2 Clones are susceptible to devastation by an epidemic.
Because all plants in the clonal population are
identical, they are susceptible to the same strain of
pathogen.
3 Clonal propagules are difficult to store for a long
time because they are generally fresh and succulent
materials.
Breeding apomictic cultivars
Apomixis, the phenomenon of seed development without
fertilization was discussed in Chapter 4, including its
occurrence in nature, mechanisms, and benefits to the
farmer and breeder.
Genetic control of apomixis has been demonstrated,
implicating a few genes. Efforts using modern molecular
genetic tools continue to be made to isolate these
apomictic genes. Apomixis can be a two-edge sword – it
can hinder breeding progress or it can be an effective
breeding tool. To improve an apomict, there should be
suitable materials, i.e., sexual or partially sexual plants
for use as female plants for crossing. Generally, an
obligate apomict cannot be used as a female parent in
a hybridization program. However, most apomictic