Principles of Plant Genetics and Breeding

474 CHAPTER 27
and Southern Europe, Australia, South Africa, South
America, and Asia.
Durum wheat
Durum wheat is grown mainly in North Dakota,
Minnesota, and South Dakota. Other smaller production
states are California, Arizona, Oregon, and Texas.
Elsewhere, it is grown in north Africa, Southern Europe,
and the former Soviet Union. Durum wheat is used in
making semolina, which is used for producing products
such as macaroni and spaghetti.
Germplasm resources
Plant breeders have access to over 400,000 accessions
in natural and international germplasm banks. These
banks include the USDA National Seed Storage Lab at
Fort Collins, Colorado, the Centro Internationale de
Mejoramiento de Maiz y Trigo (CIMMYT) in Mexico,
and the N. I. Vavilov All-Union Institute of Plant Industry,
St Petersburg, Russia. Over 40,000 accessions are held
at Aberdeen, Idaho, as a working collection and parts of
the United States National Small Grains Collection.
Cytogenetics
The species of Triticum are grouped into three ploidy
classes: diploid (2n = 2x = 14), tetraploid (2n = 2x = 28),
and hexaploid (2n = 6x = 42). The cytoplasmic malesterility
(CMS) gene used in modern wheat breeding is
derived from T. timopheevii, a wild tetraploid variety.
Three genomes (A, B, D) comprise the polyploid series
of wheat. The A genome comes from T. monococcum,
while the D comes from Aegilops squarrosa (or T.
tauschii). The origin of the B genome is debatable.
The genomic formula of the ploidy classes are AA or BB
for diploids and AABB for tetraploid or emmer wheat.
Common wheat (T. aestivum) is an allohexaploid of
genomic formula AABBDD. In hexaploid wheat, the
21 chromosomes are divided into seven homeologous
groups (partially homologous chromosomes) identified
with numbers from 1 to 7. The three chromosomes within
the ABD homeologous group usually share some loci in
common for a specific trait. An example of this is that
there are two genes for rust resistance on chromosome
2A, three genes on 2B, and three genes on 2D.
Tetraploid and hexaploid wheat reproduce naturally
as diploids (2n = 28 or 2n = 42). This reproductive
mechanism is made possible by the presence of a gene
on chromosome 5B, Ph1, which enables diploid pairing
to occur. The Ph1 gene causes truly homologous paring
within the same genome. When absent, paring between
one chromosome and a homeologous chromosome
from another genome is possible.
The homeology that exists in its three component
genomes allows the species to tolerate a range of aneuploidy.
T. aestivum exhibits vigor and morphology
similar to disomic wheat. Among other applications,
aneuploidy has been used to locate genes that confer
agronomically important traits (e.g., the mlo locus for
resistance to powdery mildew). Classic wheat genetics
was advanced through the work of E. R. Sears of the
University of Missouri. He developed a compatible set
of the possible 21 monosomics (2n − 1) of wheat, and
sets of related aneuploid forms in the hexaploid wheat
cultivar, “Chinese Spring”.
Introgression of alien genes is problematic because of
the lack of crossability between hexaploid and diploid
species, as well as the numerous problems that manifest
at various stages in the ontogeny of the hybrid.
Crossability genes (kr1kr1, kr2kr2) located on chromosomes
5B, 5A, and 5D, respectively) have been
identified in the “Chinese Spring” wheat, which facilitates
a wheat × rye cross. Some breeders also use genetic
bridges and chromosome number doubling to overcome
problems with ploidy differences. In particular,
alien autotetraploids of Agropyron cristatum and
Psathyrostachys juncea have been used to overcome
hexaploid × diploid alien species crossability barriers.
Generally, in practice, the parent with the higher ploidy
is used as the female in crosses. However, successes with
the reserve have also been recorded. Widening the
genetic base of T. aestivum through intergeneric crosses
often involves complex wheat and alien chromosome
combinations. Research has shown that alien genes
must be epistatic to those of wheat or interact with
them to produce the desired effect. Modifications of
the expression of disease- and pest-resistance genes
usually occur when they are introduced into a new
genetic background. Nonetheless, successes with spontaneous
translocations have been reported in triticale ×
wheat crosses. One of the notable induced translocations
was conducted by Sears and involved chromosome
6B and an Aegilops umbelluta chromosome,
resulting in leaf rust resistance in the release cultivar,
“Transfer”.
Fertile wheat × alien amphiploids can result from
chromosome doubling, the most successful so far being
triticale (wheat × rye). Other wheat × alien amphiploids