Agriculture Reference
In-Depth Information
traits or novel substances. Mutations are the orig-
inal source of variation for all genes. Mutagenesis
is the process of generating mutations. If a muta-
tion cannot be passed on through the germ line,
hence is not heritable, it has little value in cultivar
improvement in seed-propagated crops like wheat.
A mutation may be used directly for cultivar
release or used as new trait for incorporation
into another background for release as a cultivar.
As of 2007, 200 cultivars of
T. aestivum
L. and
30 cultivars of
T. turgidum
ssp.
durum
Desf.
feature a mutation (http://www-infocris.iaea.
org/MVD/).
Japanese wheat breeders were the fi rst to
domesticate spontaneous mutations for reduced
height (Kihara 1984; Nonaka 1984). Two spon-
taneous mutations,
Rht8
(on chromosome 2D)
and
Rht9
(7BS), occurred in Japanese landrace
'Akakomugi' (Gale and Youssefi an 1985). In 1913
Strampelli recombined the short straw attributes
and early maturity of Akakomugi with the pro-
ductivity of 'Wilhelmina Tarwe' and the local
adaptation of 'Rieti'. The cultivars Villa Gloria,
Ardito, Mentana, and Damiano were derived
from this combination of three parents, resulting
in widespread use of
Rht8
in southern and central
Europe (Borojevic and Borojevic 2005). Two
other spontaneous reduced-height mutations in
landrace Daruma were the source of
Rht-B1b
(formerly
Rht1
) and
Rht-D1b
(
Rht2
) in cultivar
Norin 10, which was brought to North America
by S.C. Salmon, deployed in the cultivar Gaines
by O. Vogel, and introduced into the Centro
Internacional de Mejoramiento de Maíz y Trigo
(CIMMYT) gene pool by N. Borlaug (Gale et al.,
1981). These spontaneous mutations contributed
to the Green Revolution by expressing shorter
and stronger straw that would not lodge under
high inputs of nutrients and water.
A current example of novel trait introduction
via mutation breeding is resistance to the imidaz-
olinone (IMI) class of herbicides that inhibits
acetohydroxyacid synthase (ALS), based on
the induced semidominant nuclear gene coding
for IMI-resistant ALS. Imidazolinone-resistant
wheat was originally developed through seed
mutagenesis of 'Fidel' winter wheat followed by
screening with the herbicide (Newhouse et al.,
1992). In further studies, Pozniak and Hucl (2004)
mutated wheat and identifi ed two additional IMI-
tolerance genes and showed that having multiple
genes for IMI tolerance was superior to having a
single gene.
Their work illustrates a number of important
concepts. The fi rst is that common wheat is a
hexaploid, often displaying gene redundancy (one
copy of the homoeologous gene in each genome);
so the three mutations for IMI-tolerance reside at
homoeologous loci, in this case on chromosome
group 6, but in different genomes (Pozniak et al.,
2004). The second important point is that common
wheat shares many of its genomes with its wild or
weedy relatives. The original IMI-mutation was
in the D genome, which caused concern because
one of the weeds that the IMI-herbicides control
is jointed goatgrass (
Aegilops cylindrica
Host, 2
n
=
4
x
= 28, genomes CCDD). Hence the IMI toler-
ance on the D-genome of wheat could potentially
be transferred to the D genome of jointed goat-
grass via pollen fl ow, thus making jointed goat-
grass tolerant to the herbicide (Zemetra et al.,
1998; Tan et al., 2004). Imidazolinone-tolerant
mutations on chromosomes 6A or 6B would have
far less potential to be transferred to jointed goat-
grass, because the A and B genomes are not
present in the weed. Third, if a successful muta-
tion can be induced in wheat, most likely it can
and will occur naturally in nature. Spontaneous
mutations for IMI tolerance in weeds are common.
Hence, while gene fl ow is a concern, equally so
should be spontaneous mutations in weedy
species, whether or not they are related to the
herbicide-tolerant crop. Imidazolinone-tolerant
wheat cultivars have been released in Australia,
Canada, and the US (Tan et al., 2004; Baenziger
et al., 2006a).
Limitations of mutagenesis as a plant breeding
technique include mutation drag (potentially del-
eterious or undesirable genes linked to the muta-
tion that when carried with the mutation during
crossing may hinder genetic improvement), low
rate of production of viable mutants, higher fre-
quency of recessive mutations than dominant
mutations, and the random genetic site at which
a mutation occurs. As one advantage, induced
mutations can be used to understand gene
