Principles of Plant Genetics and Breeding

206 CHAPTER 12
cytological effects (chromosomal aberrations), and
sterility (evaluated by counting the number of inflorescences
per plant, etc.).
Field planting and evaluation
Treated seed (M1 ) may be planted in a small plot to
produce M2 seed that is harvested for planting an M2 spaced population. It is advisable to plant plots of the
untreated genotype (M0 ) used in the project for comparison,
to aid in readily identifying mutants. Dominant
mutants are identifiable in the M1 generation; recessive
mutants are observable after selfing to produce the M2 generation.
Furthermore, in species that produce tillers, it is
possible that only one of several tillers produced will
carry an induced mutation. To produce the mutation
throughout the plant, gametes (pollen grain) should be
treated. Using seed also frequently produces chimeras,
since mutations are induced in single cells that divide
and differentiate into parts of the plant. Consequently,
the stem may be a mutant tissue while the leaf is normal.
It should be pointed out that M1 and F1 generation
plants could be genetically different. F1 plants produced
from inbred lines are genetically identical. However M1 plants in a population may have different mutations.
Hence, it is logical and proper to handle an M1 population
as a segregating population such as an F2 .
To use mutagenesis as a breeding method for producing
new cultivars, various cultivar selection strategies
may be used, including the following.
1 Bulk selection. First, the breeder grows M 1 plants,
and then harvests and bulks all the seed. A sample of
seed is planted in the next season, harvested, and the
seed bulked. Alternatively, individual M 2 plants may
be harvested and bulked for progeny testing in the
next season. Seed from progeny rows showing the
desired mutant phenotype are identified and harvested.
If rows are segregating, the M 3 plants may
be harvested and advanced individually. The breeder
conducts replicated tests, evaluating on the basis of
the desired mutant and other desirable agronomic
traits. A weakness in the bulk method is that mutant
plants often have low productivity. Consequently, by
planting only a sample of the M 2 , it is likely to exclude
the desired mutant.
2 Single-seed descent. In this method, one or a few
M 2 seeds are selected from each plant and bulked
for planting M 2 plants. Desirable M 2 plants are harvested
and progeny rowed; alternatively, seeds from
desirable plants may be harvested and bulked. This
method also has the potential of excluding desirable
mutants, even though it allows a larger number of M 1
plants to be sampled.
3 Pedigree method. Each M 1 plant is harvested separately.
M 2 progenies are grown from M 1 plants.
Desirable M 2 plants are harvested and progeny
rowed. Desirable rows are harvested and bulked separately.
The M 2:4 lines are evaluated in replicated tests.
Mutation breeding of clonally
propagated species
Species that reproduce vegetatively or by apomixis
may also be improved through mutation breeding.
Vegetatively propagated species tend to be highly
heterozygous. Consequently, selfing is accompanied
by inbreeding depression. Mutation breeding offers a
method of crop improvement whereby the genetic
structure is largely unperturbed. Physical mutagens are
more frequently used in such species than chemical
mutagens. The plant parts targeted for mutagen treatment
are those that can produce a bud from which a
plant can develop. These parts include modified parts
(e.g., tubers, rhizomes, shoot apex, cuttings, bulbs).
The exposure to the mutagen must occur as early as possible
in the development of the bud and target the
meristematic cell. A mutation in the meristematic cell is
critical in avoiding chimeras, which can delay selection
because the breeder may not be able to identify the
mutant without propagating all the material a number
of times.
Chimeras are desirable in the breeding of certain
ornamental species such as African violet (Saintpaulia
ionantha). Induced mutations in clones are of necessity
dominant mutations, unless the starting material was
heterozygous and hence could yield recessive mutants.
Furthermore, clonally induced mutants are primarily
chimerical and start as sectorial mutants, later becoming
periclinal. The growing point commonly has two
layers (some have three), the outer layer generating the
epidermis as well as some leaf mesophyll, while the inner
layer generates the remaining parts of the plant. The
innermost layer (LI) has one cell per layer, while LIII
has multiple layers. Mutations, being one-celled events,
result in part of whole layers exhibiting chimeras. A periclinal
chimera is one that involves the entire layer (the
whole layer is mutated), whereas a mericlinal chimera
involves only part of one layer. Periclinal mutations are
stable. The number of initial cells in the layers in the