Principles of Plant Genetics and Breeding

(AABBDDRR, 2n = 8x = 56), but it requires embryo
culturing to obtain F 1 s between durum wheat and rye.
All amphiploid breeding is a long-term project
because it takes several cycles of crossing and selection
to obtain a genotype with acceptable yield and product
quality. Common undesirable features encountered in
triticale breeding include low fertility, shriveled seeds,
and weak straw. Even though tetraploid (2n = 2x = 28),
hexaploid (2n = 6x = 42), and octoploid (2n = 8x = 56)
forms of triticale have been developed, the hexaploid
forms have more desirable agronomic traits and hence
are preferred. Alloploids have been used to study the
genetic origins of species. Sometimes, amphiploidy is
used by breeders as bridge crosses in wide crosses.
Aneuploidy
Whereas polyploidy entails a change in ploidy number,
aneuploidy involves a gain or a loss of one or a few
chromosomes that make up the ploidy of the species
(i.e., one of a few chromosomes less or more than the
complete euploid complement of chromosomes). Just
like polyploidy, aneuploidy has its own nomenclature
(Table 13.7).
Cytogenetics of autoploids
The diploid complement of chromosomes is designated
2n. A nullisomic, for example, is an individual with a
Table 13.7 Aneuploidy nomenclature.
Chromosome
number Term Nature of chromosomal change
2n
Aneuploidy
Diploid Normal
2n – 1 Monosomy One of a pair of chromosomes
missing
2n – 2 Nullisomy Two chromosomes missing
2n + 1 Trisomy Three copies of one
chromosome (i.e., an extra copy)
2n + 2 Tetrasomy Four copies of one chromosome
(i.e., two extra copies)
2n + 3 Pentasomy Five copies of one chromosome
(i.e., three extra copies)
The individual with the condition, e.g., trisomy, is called a
trisomic
POLYPLOIDY IN PLANT BREEDING 227
missing pair of chromosomes (2n − 2), while a tetrasomic
has gained a pair of chromosomes (2n + 2).
Similarly, a monosomic has lost one chromosome from
a homologous pair (2n − 1), while a trisomic has gained
an extra chromosome (2n + 1).
Aneuploidy commonly arises as a result of irregular
meiotic mechanisms such as non-disjunction (failure of
homologous chromosomes to separate) leading to an
unequal distribution of chromosomes to opposite poles
(Figure 13.7). Consequently, gametes resulting from
such aberrant meiosis may have a loss or gain of chromosomes.
Furthermore, chromosome additions often cause
chromosome imbalance and reduced plant vigor.
Applications of aneuploidy
Aneuploidy is used in various genetic analyses as
described next.
Chromosome additions
Chromosome addition lines are developed by backcrossing
the synthetic alloploid (F1 ) as seed parent to
a cultivated species as pollen parent. This strategy is
preferred because male gametogenesis is more readily
perturbed by chromosomal or genic disharmonies than
is the case in the female gametophyte. For example, E.
R. Sears transferred the resistance to leaf rust of Aegilops
umbellulata to Triticum aestivum (bread wheat) via
bridge crossing with T. dicocoides as follows:
T. dicocoides (AABB) × A. umbellulata (UU)
(female) ↓ (male)
F 1 × T. aestivum (AABBDD)
↓
BC 1 F 1 × T. aestivum
↓
BC 2 F 3
(one plant contained
21″ wheat + 1′ Aegilops)
However, it had drawbacks (sterile pollen, brittle spike
axis, etc.). Subjecting this chromosome addition line to
irradiation successfully translocated the segment of the
Aegilops chromosome with the desired resistance genes
to chromosome 6B of wheat, effectively removing the
negative effects. The new genotype has been used in
breeding as a source of resistance to leaf and stem rust.
Trisomics are important in genetic analysis. There are
several types of trisomics. The term primary trisomic is
used to refer to a case in which the euploid complement