Mutagenesis—A Potential Approach for Crop Improvement - Crop Improvement: New Approaches and Modern Techniques - page 163

temperature, can enhance somatic effects of the mutagen. Surface-dry seeds are
ready for sowing and are termed the M 1 generation. In a well-organized labora-
tory, pre-soaking is done overnight and mutagenic treatment in the early morning.
This allows the M 1 seeds to be sown the same day. Should this be impossible, due
to prolonged pre-soaking or mutagenic treatment, the mutagen treated seeds, after
brief drying, can be kept in a refrigerator at a temperature of around 6-8 °C. Some
mutagens are active in a particular acidity of a treatment solution. This is the case
for sodium azide, which is a very efficient mutagen in several species if applied at
low pH. For this reason, sodium azide is dissolved in a phosphate buffer at pH 3 and
this solution is used for treatment.
Mutation Breeding Strategy
A sequential strategy is essential for any mutation breeding steps where mutagen-
ic induction and its mutagenesis are much helpful for autogamous crops than the
cross-pollinating one. This is due to several problems regarding the incorporation,
selection and maintenance of recessive mutations in crop plant, many plant breed-
ing problems in the cross-pollinating species, sometime many handling based prob-
lem for the existing variability. Where the lack of variability exists for specific
and simply inherited traits, the basis of choosing between induced mutations and
hybridization is essentially the same in self- and cross-fertilizing species. However,
the genetic consequences of the failure of recessive imitations to express in cross-
fertilizing systems without forced selling or sib-mating must be taken into consid-
eration in assessing the cost of such ventures. The efficiency of mutation breeding,
more than any other breeding method is dependent on the effectiveness with which
useful variants can be recognized in M 2 or M 3 generation. The first step in the muta-
tion breeding selection process is to reduce the population of potential variants to a
sufficiently small fraction to permit more detailed analysis and evaluation.
ProbabilityofObtainingMutants
Mutations occur more or less at random and for mutagenesis-derived populations,
unlike segregating populations derived from cross-breeding, there is no clue as to
the kind or magnitude of genetic change. A particular gene can be expected to mu-
tate once in about 10,000 mutagen treated cells, provided that an effective treatment
was given. On an average, it appears that tail about five cells of the embryonic
shoot apex may become part of the germ line and thus relate to the next generation.
This would mean that for seed propagated species the M 2 generation of 2,000 mu-
tagen treated (M 1 ) plants has to be examined in order to have reasonable chance of
finding mutation in a particular gene. To extend the simplified calculation further, one
might assume that plant genome possesses 10-100 × 10 3 relevant genes. Based