Principles of Plant Genetics and Breeding

fraction of androgenic grains develop into full
sporophytes.
2 High rates of albinos occur in cereal haploids (of no
agronomic value).
3 Chromosomal aberrations often occur, resulting in
plants with higher ploidy levels, requiring several
cycles of screening to identify the haploids.
4 The use of haploids for genetic studies is hampered
by the high incidence of nuclear instability of haploid
cells in culture.
Ovule/ovary culture
Gynogenesis using ovules or ovaries has been achieved
in species such as barley, wheat, rice, maize, tobacco,
sugar beet, and onion. The method is less efficient than
androgenesis because only one embryo sac exists per
ovary as compared to thousands of microspores in each
anther. Ovaries ranging in developmental stages from
uninucleate to mature embryo sac stages are used.
However, it is possible for calli and embryos to develop
simultaneously from gametophytic and sporophytic
cells, making it a challenge to distinguish haploids
from those of somatic origin. Generally, gynogenesis is
selected when androgenesis is problematic (as in sugar
beet and onion).
Haploids from wide crosses
Certain specific crosses between cultivated and wild
species are known to produce haploids. Well-established
systems include the interspecific crosses between Hordeum
vulgare (2n = 2x = 14, VV) and H. bulbosum (2n = 2x =
14, BB), commonly called the bulbosum method, and
also in wheat × maize crosses. The bulbosum method is
illustrated in Figure 11.1. The F1 zygote has 2n = 2x =
14 (7V + 7B). However, during tissue culture of the
embryo, the bulbosum chromosomes are eliminated,
leaving a haploid (2n = x = 7V). This is then doubled by
colchicine treatment to obtain 2n = 2x = 14VV.
Doubled haploids
Researchers exploit haploidy generally by doubling the
chromosome number to create a cell with a double dose
of each allele (homozygous).
Key features
Inbred lines are homozygous genotypes produced by
repeated selfing with selection over several generations.
The technique of doubled haploids may be used to
TISSUE CULTURE AND THE BREEDING OF CLONALLY PROPAGATED PLANTS 187
Chromosome elimination
Preferential loss of
H. bulbosum genome
Parent A × Parent B
F 1 × Hordeum bulbosum
Haploid embryos
Grow embryos into seedlings
Double using colchicine
Homozygous diploid plants
Seed increase
Field evaluation
Figure 11.1 Generating haploids in barley by the
bulbosom method.
produce complete homozygous diploid lines in just 1
year (versus more than 4 years in conventional breeding)
by doubling the chromosome complement of haploid
cells. Such doubling may be accomplished in vivo naturally
or through crossing of appropriate parents, or in
vitro through the use of colchicine. The success of
doubled haploids as a breeding technique depends on
the availability of a reliable and efficient system for
generating haploids and doubling them in the species.
Applications
Doubled haploids have been successfully used in breeding
species in which efficient haploid generation and
doubling systems have been developed. These include
canola, barley, corn, and wheat. Additionally, doubled
haploids are used to generate general genetic information
that can be applied to facilitate the breeding
process. Such information includes gene action and
interaction, estimating the number of genetic variances,
calculating combining abilities, and the detection of
gene linkages, pleiotropy, and chromosome locations.
Haploids are used in mutation studies (recessive mutants
are observable instantly) and in selecting against undesirable
recessive alleles.
Procedure
The first step in using doubled haploids in breeding is
identifying the source of haploids.