Principles of Plant Genetics and Breeding

344 CHAPTER 18
storage or in nurseries from which potential parents
could be selected for future programs. These materials
should be evaluated for performance capabilities and
especially for traits of interest in the proposed breeding
program. Germplasm may be introduced from germplasm
banks and other sources. Such material should
also be evaluated as is done with local materials.
Development and maintenance of inbred lines
An inbred line is a breeding material that is homozygous.
It is developed and maintained by repeated selfing
of selected plants. In principle, developing inbred lines
from cross-pollinated species is not different from developing
pure lines in self-pollinated species. About 5–7
generations of selfing and pedigree selection are required
for developing an inbred line. As previously indicated,
inbreeders tolerate inbreeding, whereas outbreeders
experience varying degrees of inbreeding depression.
Consequently, the extent of inbreeding in developing
inbred lines varies with the species. Species such as alfalfa
and red clover that are more intolerant of inbreeding
may be selfed only a few times. Alternatively, sib mating
may be used to maintain some level of heterozygosity in
these sensitive species.
Hybrid breeding as previously stated exploits the
phenomenon of heterosis. Heterosis will be highest
when one allele is fixed in one parent to be used in a
cross and the other allele fixed in the other parent.
Inbred lines of inbreeding species
Inbred lines in self-pollinated species were previously
discussed. They are relatively easy to maintain. The
breeder should be familiar with the material to be able
to spot off-types that may arise from admixtures or
outcrossing in the field. Off-types should be rogued out
and discarded, unless they are interesting and warrant
additional observation and evaluation. Physical mixtures
occur at harvesting (e.g., due to equipment not cleaned
properly before switching to another line), threshing,
processing and handling, storage, and at planting. When
maintaining certain lines, especially those developed from
wild species, it may be necessary to be more vigilant and
harvest promptly, or bag the inflorescence before complete
maturity occurs to avoid losing seed to shattering.
Inbred lines of cross-pollinated species
Because of the mode of reproduction, breeding lines
from cross-pollinated species are more challenging to
develop and maintain. Inbred lines may be developed
from heterozygous materials obtained from a natural
population, or from F 2 selected genotypes. Depending
on the breeding procedure, parents for hybrid production
may be developed in the conventional fashion, or
non-conventional fashion.
1 Conventional or normal inbreds. Normal inbreds
are developed by repeatedly self-pollinating selected
plants, from S 0 –S n (for materials drawn from natural
populations) or from F 1 –F n (for materials obtained
from crossing), the latter being akin to the pedigree
breeding method previously described for selfpollinated
species. The S 1 or F 2 populations are
heterogeneous, as are results of segregation of traits.
Superior plants are selected and progeny-rowed to
expose inferior genotypes. Superior individuals are
selected for the next cycle of selfing. By S 3 , the plants
in the progeny should be fairly uniform. After about
6–8 generations of selfing, the negative effects of
inbreeding ceases. The next step then is to compare
different lines. The value of n, the number of generations
of self-pollination, varies from about 5 to 8. The
goal is to attain a level of homozygosity at which the
inbred lines are uniform in characteristics and will
remain so under continued selfing, with no further
loss of vigor. At this stage, the inbred line may be
maintained by self-pollination.
Inbred lines should be evaluated for performance
and other general agronomic qualities (e.g., drought
resistance, lodging resistance, disease resistance),
especially those that are basic to the specific crop
industry and the production region. This way, the
final lines developed should have high desirability and
productive potential. These materials are maintained
by conventional selfing or sibbing procedures.
2 Non-conventional inbred lines. To facilitate hybrid
production, cytoplasmic male sterility may be incorporated
into lines to eliminate the need for mechanical
emasculation. Three different inbred lines are required
to implement a CMS breeding project. Different
kinds of parent materials need to be developed and
maintained when making use of a cytoplasmic–
genetic male-sterility system in breeding. Two kinds
of female parents are needed (Figure 18.1): an A-line
(male-sterile, sterile cytoplasm (S), with non-restorer
genes (rfrf ) in the nucleus) and a B-line (male-fertile,
fertile cytoplasm (N), with non-restorer genes (rfrf )
in the nucleus). The A-line is the seed-producing
parent. To develop an A-line, cross a B-line as male
to a male-sterile female with sterile cytoplasm and
fertility-restorer genes, followed by repeated backcrosses
(5–7) to the B-line. The A-line and B-line are