Agriculture Reference
In-Depth Information
Figure 7.8
Selecting single potato plants based on visual appearance.
Number of genotypes in initial
populations
Table 7.3
Possible selection intensities used in a wheat
breeding scheme to develop cultivars that have high yield,
good bread making quality, qualitative resistance to stripe
rust, quantitative resistance to mildew, good establish-
ment, cold tolerant, short in stature, early maturity and
resistant to lodging. Also shown is the accumulative fre-
quency of desirable genotypes, given that all characters are
independently inherited.
To many, plant breeding is a '
numbers game
' where the
more genotypes screened results in a greater chance
of
identifying
desirable
recombinants
which
will
eventually become new cultivars.
Why therefore is it necessary to evaluate so many
different genetic lines in a plant breeding programme?
Consider a simple wheat breeding scheme which has
the objective of developing new cultivars that have high
yield, good bread making quality, qualitative resistance
(dominant) to stripe rust, quantitative resistance to
mildew, good establishment, cold tolerant, short in
stature, early maturity and resistant to lodging. Con-
sider that the selection intensities in Table 7.3 are needed
to ensure that at least some individuals will meet the
required standards. Also shown is the accumulative fre-
quency of desirable individuals, given that all characters
are independently inherited.
Obviously, even when this very limited set of criteria
is used as the Objective, and with relatively modest
selection intensities, the number of plants that need
to be screened can be very high. If a breeder wishes
to have some chance of success in having
at least one
Character
Selection
Accumulation
intensity (%)
frequency
High yield
5
1 : 20
Good quality
5
1 : 400
Stripe rust resistance
50
1 : 800
Mildew resistance
20
1 : 4000
Crop establishment
50
1 : 8000
Cold tolerance
20
1 : 40 000
Short stature
10
1 : 400 000
Early maturity
10
1 : 4 000 000
Lodging resistance
10
1 : 40 000 000
individual that meets the criteria the numbers that need
to be screened will be large.
Plant breeding therefore does require evaluation of
many thousands (or millions) of plants to have any
chance of producing a successful new cultivar. So given
