Principles of Plant Genetics and Breeding

188 CHAPTER 11
Procedure using natural sources Haploids originate
in nature through the phenomenon of parthenogenesis
(gamete formation without fertilization). The haploids
may be maternal or paternal in origin. It is estimated
that haploids occur in corn at the rate of one in 1,000
diploids, 99% of which arise from parthenogenesis of
maternal origin. Spontaneous doubling occurs in corn
at the rate of 10% of haploids developed. The key is
distinguishing between haploid and diploid plants.
A marker system for this purpose was first developed
by S. S. Chase based on seedling color, purple plants
being encoded by the dominant gene (P) while normal
green plants are recessive (p). A cross of F 1 pp × PP
would yield 999 Pp (purple diploids) and one pp (green
haploid). Another marker used is the purple aleurone
color.
To use this marker system, the breeder should cross a
heterozygous female to a male with marker genes. The
seed from those with dominant endosperm marker of
the male are saved and planted, discarding seedlings
with the dominant male marker. Next, cytological evaluation
of plants with the recessive female marker (by
root tip squash) is conducted. The haploid plants are
retained and grown in the greenhouse or field, and selfpollinated
to produce diploids.
Procedure using artificial sources Haploid production
through interspecific and intergeneric crosses is in
use, one of the most well known being the barley system
(discussed above). After doubling the chromosome, the
diploid plants are grown to maturity. Seeds are harvested
for planting in plant rows. Because diploids produced
by this method are normally completely homozygous,
there is no need to grow segregating generations as in
conventional programs.
Advantages
1 Complete homozygosity is attainable in a shorter
period.
2 Duration of the breeding program can be reduced by
several (two or three) generations.
3 It is easier and more efficient to select among homogeneous
progeny (versus heterogeneous progeny in
conventional breeding).
4 The cultivar released is homogeneous.
Disadvantages
1 The procedure requires special skills and equipment
in some cases.
2 Additional technology for doubling may increase the
cost of a breeding program.
3 The frequency of haploids generated is not predictable.
4 There is a lack of opportunity to observe line performance
in early generations prior to homozygosity.
Genetic issues
Unlike conventional methods of inbreeding, it is possible
to achieve completely homozygous individuals.
Using an F1 hybrid or a segregating population as the
female parent in the production of maternally derived
haploids increases genetic diversity in the doubled
haploid line. It is advantageous if the female also has
agronomically desirable traits. F1 hybrids are suitable
because their female gametes will be segregating.
In vitro selection
In vitro selection is essentially selection of desirable
genotypes under controlled environments in the Petri
dish. Reports on the use of this technology have
dwindled over the years, after an explosion of reports
following the awareness of its potential as a source of
biological variation. As a tool, it is applicable to both
the sporophyte and the gametophyte.
Using whole plants or organs
Whole plants, seedlings, or embryos may be the units of
in vitro selection. As previously indicated, an appropriate
tissue culture system is needed for in vitro selection.
The method has been used for screening for resistance
to diseases such as Fusarium culmorum in wheat, and
susceptibility to fungal spores in disease breeding.
Tolerance (or resistance) to inorganic salts (e.g., salt
tolerance in sugar beet) has been reported.
Using undifferentiated tissue
The capacity for regeneration by the callus makes the
material attractive for use in selection. Plants can be
multiplied using a callus system as well. Sometimes,
spontaneous variability arises in callus or suspension
culture, some of which may be heritable, while some is
epigenetic, disappearing when plants are regenerated or
reproduced sexually. Variability originating during tissue
culture is called somaclonal variation.