Agriculture Reference
In-Depth Information
8
Alternative Techniques in Plant Breeding
very low frequency in nature. However, if the mutation
results in an advantageous effect, the genotype possess-
ing the mutation may thus be more adapted to the
environment compared to the non-mutant types and
hence will tend to leave more offspring. Over genera-
tions this therefore leads to an increase in the frequency
of the new allele within the population.
Obviously, the extremely low rate of natural mutation
and even lower frequency of
desirable mutation
events
are such that natural mutation has had little impact on
modern plant breeding. However, in the mid 1920's
it was discovered that X-rays could be used to induce
high mutation rates first in fruit fly and later in bar-
ley. Plant breeders were quick to realize the potential
of induced mutation and mutation breeding became a
common practice in almost all crop species and in many
ornamental flower breeding programmes.
The aim of mutation breeding is to stimulate an
increase in the frequency of mutation events within
crop species and then to select desirable new alleles
from amongst the mutants produced. More basically,
mutation breeding has been utilized to make minor
advantageous genetic changes in already established
and adapted cultivars through induced mutation treat-
ments. For example, by inducing mutation in a highly
adapted crop cultivar, and screen the resulting mutated
lines for a specific character of interest. In doing this it is
hoped to retain the existing cultivar adaptability while
adding the mutated
advantageous
trait.
Mutagenesis derived lines in a plant breeding scheme
are labelled according to the number of generations after
mutation has taken place. For example, the generation
immediately after mutation is termed the M
1
. These
plants can be self pollinated to produce an M
2
genera-
tion, and so on (compared with F
1
,F
2
,F
3
, etc. for the
more usual sexual generations).
INTRODUCTION
In this topic so far it has been assumed that plant
breeding involves artificial hybridization between cho-
sen parents and selection (using visual assessment or
by means of recording data) of desirable recombinants,
over several generations. However, there is a range of
techniques available to plant breeders that have made,
or are starting to make, contributions to the produc-
tion of new cultivars. These include: induced mutation;
interspecific species hybridization;
in vitro
propagation;
and plant transformation, all of which have been used
to increase the genetic variability available to breeders.
In addition, markers (mainly molecular based markers)
are being increasingly used in breeding program, for
example to aid selection for characters that are difficult
to evaluate in the usual way.
INDUCED MUTATION
The variation that exists within all living plants and ani-
mals, including all crop species, is the result of natural
mutations at the DNA level, with subsequent recombi-
nation and selection occurring, much of it over millions
of years. But this has also been accompanied by changes
at a structural level, such as rearrangement within and
between chromosomes.
Mutations result in the generation of additional
genetic variation within plant species. It has been esti-
mated that mutations occur naturally with a frequency
of 1 in 1 000 000 (one in a million) individuals. Most of
these mutations are recessive and deleterious and these
new alleles usually do not survive at anything other than
