Agriculture Reference
In-Depth Information
Table 7.4
Selection scheme used to develop new potato cultivars at the Scottish Crop Research Institute.
Year
Number of
Number of
Plot
Characters assessed
genotypes
replicates
Size
1
140 000
1
1
Visual assessment of commercial worth
2
40 000
1
1
Visual assessment of commercial worth
3
4000
1
3
Visual assessment of tuber size, shape, tuber number, yield
and defects
4
1000
2
5
Actual assessment of yield, initial quality tests, visual
assessment of appearance and defects
5
500
2
10
Actual assessment of yield, fry quality, boil quality, initial
disease testing for late blight and common scab, visual
assessment of appearance and defects
6
100
2
40
Yield, fry and boil quality assessment from early and late
harvest, initial taste testing, multiple disease testing, initial
virus testing, visual assessment of appearance and defects
7
50
4
40
Yield, quality and disease testing at seven locations throughout
the target region
8
10
4
40
Repeat multiple locations testing, initial on-farm testing (large
field scale trials)
that the initial population needs to be large and that
selection of the better lines should be carried out as
efficiently as possible, how can the best genotype be
identified?
To further examine the different stages of selection
consider the two examples. The selection scheme used
to develop new potato cultivars at the Scottish Crop
Research Institute is shown in Table 7.4.
In this scheme years 1, 2 and 3 would be considered
to be early generation selection, years 4, 5 and 6 would
be intermediate generation selection and years 7 and 8
would be advanced selection.
The wheat breeding programme at the University of
Idaho has the selection scheme shown in Table 7.5.
In this scheme years 1 and 2 would be considered as
early generation selections, years 3 and 4 would be inter-
mediate selection and years 5 and 6 would be advanced
generation selection. In year 7, and subsequent years,
remaining selections (2 to 3 lines) would be entered for
regional testing where they would be evaluated at many
western USA locations.
Therefore early generation selection should eliminate
the very worst genotypes, intermediate selection would
identify the very best genotypes and advanced selection
would confirm genotypic performance over differing
locations, assess environmental stability and identify the
superior (cultivar quality) genotypes from those which
may just fail to become cultivars.
After each stage of selection fewer genotypes will
remain for further testing. Increased rounds of selection
will also be associated with decreased genetic variation
between selected lines. It will therefore require more
detailed evaluation studies to differentiate between
remaining selections.
Early generation selection
Selection in the early generation stages differs from later
selection because:
•
Many thousands of lines are to be screened
•
Only small amounts of planting material are available
from each genotype and so sophisticated experimen-
tal designs with large numbers of plots and high
replication are not possible
Selection is often carried out on highly heterozygous
populations where dominance effects can be large and
can mask the true genotype being selected
•
The first two points can be considered as a single
problem because, even in cases where large quantities
