Agriculture Reference
In-Depth Information
example that corn could be easily reproduced asexually
(i.e. say through apomixis), then there would be no need
to develop hybrid corn cultivars because the highly het-
erozygous nature of a hybrid line could be “genetically
fixed” and exploited through asexual reproduction.
The process of developing a clonal cultivar is, in
principle, very simple. Breeders generate segregating
progenies of seedlings, select the most productive geno-
typic combination and simply multiply asexually, this
also stabilizes the genetic make-up (i.e. avoids problems
relating to genetic segregation arising from meiosis).
Despite the apparent simplicity of clonal breeding it
should be noted that while clonal breeders have shared
in some outstanding successes, it has rarely been due to
such a simple process, as will be noted from the example
below.
250 -300 crosses
140 000 seedlings
140 000 single plants
4 000
×
3 plant plots
1 000
×
12 plants
1 000
×
6 plants
500
×
12 plants
500
×
20 plants
Outline of a potato breeding scheme
200
×
12 plants
200
×
100 plants
The breeding scheme that was in use at the Scottish
Crop Research Institute prior to 1987 is illustrated in
Figure 4.8. It should be noted that the programme used
two contrasting growing environments. The
seed site
(indicated by grey boxes) was located at high altitude
and was always planted later and harvested earlier than
would be considered normal for a typical
ware crop
(indicated by black boxes), in order to minimize prob-
lems of insect borne virus disease infection. A ware crop
is the crop that is produced for consumption rather than
for re-planting.
Each year between 250-300 cross pollinations were
carried out between chosen parents. From each cross
combination the aim was to produce around 500 seeds,
leading to 140 000 seedlings being raised in small pots
grown in a greenhouse (two greenhouse seasons were
needed to accommodate the 140 000 total). At harvest,
the soil from each pot was removed and the tubers pro-
duced by each seedling were placed into the now empty
pots. At this stage a breeder would visually inspect
the small tubers in each pot (each seedling being a
unique genotype) and either select or reject each one.
One tuber was taken from amongst the tubers pro-
duced by the selected seedlings, while all tubers from
rejected clones were discarded. The seedling genera-
tion, as in most clonal crops, in the greenhouse was
the one and only generation that derived directly from
true botanical seeds
(i.e. from sexual reproduction). All
Figure 4.8
Potato breeding scheme used at the Scottish
Crop Research Institute prior to 1987.
other generations in the programme were by vegetative
(clonal) reproduction, in other words from tubers.
The following year the single selected tubers (approx-
imately 40 000) were planted in the field at the '
seed
site
' as single plants within progeny blocks. This stage
was referred to as the first clonal year. At harvest each
plant was harvested, by hand digging, and the tubers
exposed on the soil surface in a separate group for
each individual plant. A breeder would then visually
inspect the produce from each plant and decide, on that
basis, to reject or select each group of tubers (i.e. each
clone). Three tubers are retained from '
the most desir-
able
' plants and planted in the field at the same seed
site in the following year (the second clonal year) as a
three plant, un-replicated plots. First and second clonal
year evaluations were therefore carried out under seed
site conditions to reduce as far as possible the chances
of contamination, especially by virus diseases.
Second clonal year plots were harvested mechanically
and, again, tubers from each plot exposed on the soil
surface. A breeder examined the tubers produced and
decided to select or reject each clone, again on the basis
of visual inspection. Tubers from selected clones were
