Principles of Plant Genetics and Breeding

of sterility because of the odd chromosome number
that results in irregular meiosis. The sterility favors
species that are grown for vegetative commercial parts
(e.g., grasses) and ornamentals and fruits (seedless). In
sugar beet, triploid cultivars of monogerm types have
significantly impacted the sugar industry.
Triploid hybrids are produced by crossing diploids
with tetraploids. Breeders use three kinds of genotypes.
The diploid is male-sterile (female, cytoplasmic malesterile),
while the tetraploid is the pollinator. The third
component is a male-sterility maintainer (a diploid, N).
The tetraploid is derived from a diploid by colchicine
treatment of the seed (soak in 0.2% for 15 hours at
30°C). Seedless watermelon (3x = 33) is also produced
by crossing a diploid (2n = 2x = 22) with a tetraploid
(2n = 4x = 44).
Natural alloploids
A number of economically important crops are alloploids.
These include food crops (e.g., wheat, oat),
industrial crops (e.g., tobacco, cotton, sugarcane), and
fruits crops (e.g., strawberry, blueberry). These crops,
by definition, contain a combination of different
genomes. Researchers over the years have attempted to
elucidate the ancesteral origin of some alloploids. One
of the most widely known successes was the work of
B. nigra
(2n = 16)
(black mustard)
B. carinata
(2n = 34)
(e.g., wild mustard)
Figure 13.5 The triangle of U showing the origins of various alloploids in Brassica.
POLYPLOIDY IN PLANT BREEDING 221
B. oleracea
(2n = 18)
(e.g., cabbage,
cauliflower)
n = 9 n = 9
B. juncea
(2n = 36)
(e.g., brown
mustang)
Nagaharu U, the Japanese scientist who described the
genomic relationships among naturally occurring mustard
(Brassica) species (Figure 13.5). Dubbed the triangle
of U, it describes the origins of three Brassica species
by alloploidy. The diploid species involved are turnip or
Chinese cabbage (B. campestris, n = 10), cabbage or kale
(B. oleracea, n = 9), and black mustard (B. nigra, n = 8).
For example, rutabaga (B. napus) has 2n = 38, being a
natural amphiploid of B. oleracea and B. campestris.
In cereal crops, wheat is a widely studied alloploid
that comprises genomes from three species. Cultivated
common wheat (Triticum aestivum) is a hexaploid with
21 pairs of chromosomes and is designated AABBDD.
The AA genome comes from einkorn (T. monococcum).
Tetraploid wheats have the genomic formula AABB.
Emmer wheat (T. dicoccum) crossed naturally with
Aegilops squarrosa (DD) to form common wheat.
Genetics of alloploids
As previously indicated, alloploids arise from the combination
and subsequent doubling of different genomes, a
cytological event called alloploidy. The genomes that
are combined differ in degrees of homology, some being
close enough to pair with each other, whereas others are
too divergent to pair. Sometimes, only segments of the
chromosomes of the component genomes are different,
a condition that is called segmental alloploidy. Some of
B. napus
(2n = 38)
(e.g., rutabaga, rape)
n = 8 n = 10
n = 8
n = 10
B. campestris
(2n = 20)
(e.g., Chinese
cabbage, turnip)