Agriculture Reference
In-Depth Information
choice of the breeder, seed regulatory agencies, farmers
and end-users. Almost all pure-line breeding schemes
involve selection of individual plants at one or more
stages in the breeding scheme. The stages of single plant
selection will have a large impact regarding the degree
of heterogeneity in the end cultivar. If single plant selec-
tion is carried out at an early generation, say F
2
, there
may be greater heterogeneity within the resulting culti-
var compared to a situation where single plant selection
was delayed until a later generation, say F
8
where indi-
vidual plants would be more homozygous. Breeders
must ensure that a level of uniformity and stability exists
throughout multiplication and into commercialization.
Farmers will have preferences for cultivars which are
homozygous, and hence homogeneous, for particular
characters. These characters may be related to uniform
maturity, plant height or other traits related to ease of
harvest. Many believe that farmers are not concerned
with uniformity of characters that do not interfere with
end-use performance (e.g. flower color segregation).
However, farmers take a natural pride in their farms
and, therefore, like to grow '
nice looking
' crops and
these are ones which are uniform for almost all visible
characters. For most end-users there will be an obvious
preference for cultivars which have high uniformity of
desirable quality characters. For example, there may be
a premium for uniform germination in malting barley
or more uniform characters relating to bread making in
wheat.
In contrast, some breeders of inbred cultivars like
to maintain a relatively high degree of heterogeneity in
their developed cultivars. They believe that this hetero-
geneity can help to '
buffer
' the cultivar against changes
in environment and hence make the cultivar more
stable
over different environments. Often statutory authorities
determine the degree of variability that is allowed in a
cultivar. For example, all inbred cultivars released in the
European Community countries, Canada or Australia
must comply to set standards set for
distinctness unifor-
mity
and
stability
(DUS) in Statutory National Variety
Trials. In these cases it is common to have almost
total homogeneity and homozygosity in released inbred
cultivars.
With most breeders time is at a premium. Therefore,
some methods are commonly used to reduce the time
taken to achieve homozygosity and these include
single
seed descent
and the use of
off-season sites
(this excludes
the production of homozygous lines through doubled
haploids, which are relevant here but will be discussed
separately in a later chapter).
Single seed descent
Single seed descent involves repeatedly growing a num-
ber of individuals from a segregating population, usually
under high density, low fertility situations to acceler-
ate seed-to-seed time. At maturity, a single seed from
the self of every plant is replanted. This operation
is repeated a number of times to obtain homozygous
plants. Single seed descent is very well suited for rapid
generation increase in a greenhouse where a number of
growth cycles may be possible each year. Single seed
descent in wheat and barley can be further accelerated
by growing plants under stress conditions of high den-
sity, high light, restricted root growth, and low nutrient
levels which result in stunted plants with only one or
two seeds each but in a shortened growing period com-
pared to growth under normal conditions (upto three
or four generations in a year are possible in barley or
canola).
It is very important, however, when using single
seed descent, that unintentional selection is not being
carried out for adverse characters. For example, in a sin-
gle seed descent scheme in winter wheat (where plants
will require a vernalization period prior to initiating
a reproductive phase) vernalization requirements may
be overcome artificially in a cold room. If this is done
then care should be taken so that all seedlings do indeed
receive sufficient cold treatment to overcome the vernal-
ization requirement, otherwise the system will indeed
be selecting these plant types with lower vernalization
requirements. In addition, some genetic characteristics
are not fully expressed when plants are grown under
high competition stress conditions used for single seed
descent. For example, the erectoides dwarfing gene
(
ert
) is not expressed under single seed descent in the
glasshouse, and therefore genotypes cannot be selected
for this character under these conditions. In any case it
is strongly advised that
no
selection be practiced during
this phase.
Off-season sites
Off-season growing sites can also reduce the time for
achieving a desired level of homozygosity. This is pos-
sible by having more than one growing season per year.
Dual locations at similar latitudes in the Northern and
