Principles of Plant Genetics and Breeding

Triticum monococcum
(2n = 2x = 14)
(AA)
Figure 13.1 The proposed origin of common wheat
Triticum aestivum.
×
2x = 14
Chromosome doubling
T. turgidum
2n = 4x = 28
(AABB)
Unknown
(Aegilops speltoides?)
2n = 2x = 14
(BB)
In some species of higher plants, a pattern of ploidy
emerges whereby the gametic (haploid) and somatic
(diploid) chromosome numbers increase in an arithmetic
progression, as illustrated by oat and wheat
×
3x = 21
(ABD)
Chromosome doubling
T. aestivum
2n = 6x = 42
(AABBDD)
Table 13.1 Polyploid series of selected species.
POLYPLOIDY IN PLANT BREEDING 215
T. tauschii
2n = 2x = 14
(DD)
(Table 13.1). The set of species displaying this pattern
constitute a polyploid series.
Variation in chromosome number
In nature, there exist two types of variation in chromosome
number. In one type, called euploidy, the
individuals contain multiples of the complete set of
chromosomes that is characteristic of the species (the
basic number, x). In another, called aneuploidy, individuals
contain incomplete sets of chromosomes that
may be equivalent to the euploid number plus or minus
one or more specific chromosomes (Table 13.2). The
state of having multiples of the basic set in the somatic
cell in excess of the diploid number is called polyploidy,
and the individuals with such cells, polyploids.
Polyploids are euploids. When euploids comprise
multiples of the genome (i.e., duplicates of the genome
from the same species) they are called autoploids
and the condition autoploidy (or autopolyploidy).
However, when a combination of genomes from
different species are involved, the term alloploid or
allopolyploid (and similarly, alloploidy or allopolyploidy)
is used. Alternatively, the term amphiploid
or amphidiploid (and similarly, amphiploidy or
amphidiploidy) is also used to describe polyploids with
Ploidy Oat (Avena spp.) Wheat (Triticum spp.)
Diploid (2n = 2x = 14) A. brevis (short oat) T. monococcum (einkorn)
A. strgoga (sand oat) T. tauschii (wild oat)
Tetraploid (2n = 4x = 28) A. barbata (slender oat) T. timopheevii (wild)
A. abyssinica (Abyssinia oat) T. turgidum (emmer)
Hexaploid (2n = 4x = 42) A. sativa (common oat) T. aestivum (common bread wheat)
A. byzantina (red oat)
Table 13.2 Classification of polyploidy.
Ploidy Genome Description
Diploidy AA BB Contains two of a basic chromosome set
Euploidy
Autoploidy AAA BBBB Multiples of a basic set (n) of one specific genome
Alloploidy AAB AABB Multiples of the basic number but of different genomes
Segmental alloploidy AA′B AB′B′ Multiples of the basic number but the genomes have similar parts
Aneuploidy AA 2n ± 1,2, . . . , k