Principles of Plant Genetics and Breeding

226 CHAPTER 13
Bread wheat
(Triticum aestivum)
2n = 42
Genome = AABBDD
Intercross
and select
AABBDDRR
2n = 56
Primary octoploid
Method 1
F 1
ABDR
n = 28
Sterile haploid
Figure 13.6 Steps in the development of triticale.
Breeding alloploids
Alloploids may be induced by crossing two species with
different genomes, followed by chromosome doubling of
the hybrid. Compared to autoploids, inducing alloploids
is not commonly done by plant breeders. If successful,
the newly induced amphiploid instantly becomes a new
species (unable to cross to either parent). It also becomes
reproductively isolated from its parents. Success of
induced alloploids is enhanced by the proper choice
of parents. In particular, using parents with low ploidy
levels increases the chance of high fertility and seed set
in the amphiploid. Commercially successful induced
alloploids are few. The most noted success with induced
alloploidy is the commercially grown amphiploid, triticale
(× Triticosecale), derived from a cross between wheat
(Triticum) and rye (Secale) (Figure 13.6). The objective
of developing triticale is to obtain a product that com-
×
Rye
(Secale cereale)
2n = 14
Genome = RR
×
AABBDDRR
2n = 56
Fertile
Embryo
culture
Double
×
AABB (RR/DD)
2n = 42
Secondary hexaploid
Method 2
F 1
ABR
n = 21
Sterile haploid
AABBRR
2n = 42
Fertile
Durum wheat
(Triticum turgidum)
2n = 28
Genome = AABB
Intercross
and select
AABBRR
2n = 42
Primary hexaploid
bines the qualities of wheat with the hardiness of rye. In
lieu of doubling the F 1 to produce the desired synthetic
product, a wheat × rye cross may be undertaken. The F 1
plant possesses 28 chromosomes and exhibits intermediate
traits that favor rye (hairy neck, spike length). All
F 1 s are sterile because of the formation of univalents
and irregular gametogenesis. F 1 s are backcrossed to
wheat to produce progenies containing 42 chromosomes
(seven from rye and the rest from wheat). The
wheat chromosomes form bivalents at meiosis, while
the rye chromosomes form univalents. The bivalent
wheat chromosomes are irregularly arranged. Fertilization
of an ovule with 21 + 7 chromosomes by pollen
with the same genomic constitution will contain the full
complement of chromosomes for wheat and rye (56
chromosomes). This product is the synthetic alloploid
called triticale. Hexaploid triticale (AABBRR, 2n = 6x
= 42) is superior agronomically to octoploid triticale