Selection - An Introduction to Plant Breeding - page 144

using the means (and variances) to set target values
which would be needed by a new cultivar

Table 7.9 Progeny means of Breeders' Prefer-
ence ratings and within progeny variances (

σ p ),
estimated on 25 progeny from each of eight
potato crosses (C1 to C8), and the univariate
probability that a genotypes chosen at random
from each cross will exceed a Breeder's Preference
rating greater than 5, on the 1 to 9 scale.

•

To include all parents used in crosses and setting
target values of either P 1 ( the parent with high-
est expression) or (P 1 +

P 2 )/ 2, the average parent

value or m

•

If the F 1 progeny are included in the evaluation trial
then the heterotic F 1 values are often used as target
values in cross prediction

σ p

Cross Mean Predicted

> 5

C1 4.36 1.52 0.337

Predicted from number in sample
When a sample of inbred lines is produced then a sec-
ond option of determining the frequency of particular
recombinants has also shown promise. This method
simply involves counting the number of inbreds within
the sample which exceed the given target value.
Obviously, the accuracy of this method will be
directly proportional to the sample size of inbreds used
on which the counts are made. Several researchers have
shown, however, that even relatively small samples
(around 25 lines) can still provide useful prediction
results.

C2 4.01 1.65 0.274

C3 3.61 1.50 0.176

C4 4.17 1.23 0.251

C5 3.04 0.91 0.015

C6 3.68 1.52 0.192

C7 4.21 1.36 0.281

···

families (C1 C8) were evaluated for breeders' pref-

erence (a visual assessment of commercial worth on a
1 to 9 scale with increasing value attributed to increas-
ing commercial worth) by four breeders independently.
From the average assessment of each genotype, Table 7.9
shows the progeny means and within progeny variances
(

Use of cross prediction in clonal crops
Initially cross prediction was not considered as a tool
in the breeding of clonal crops. This may perhaps have
been due to the fact that heterozygosity is not a problem
in the selection procedure. In other words, although the
initial seedlings in say, a potato breeding programme,
are all genetically unique and highly heterozygous, they
are subsequently multiplied clonally and so are fixed
in the sense that they are the genotypes which can be
commercially exploited. However, it has been shown
in a number of clonal crops that early generation selec-
tion suffers from all the inefficiencies found in inbred
cultivar development.
In clonal crops, there are, just as many difficulties
incurred in trying to identify desirable lines in the early
generation stages (i.e. seedling stage and first clonal year
stage) where only single plants are evaluated.
At the Scottish Crop Research Institute research has
shown severe inefficiencies in the method of selection
used in the early generations. Work resulting from this
prompted an examination of cross prediction meth-
ods which might prove an effective alternative to the
recurrent phenotypic selection used. A sample of 25
seedlings from the 200 grown from each of 8 cross

σ p ) used to determine the frequency of clones which
would exceed a preference score of greater than five on
the one to nine scale.
Seed tubers from a sample of 200 clones from each
cross were increased without selection to a stage where
a large amount of field grown seed tubers were avail-
able and the 1600 genotypes evaluated for three years
in a breeding programme. Selection of the popula-
tions was based on all the characters (yield, quality and
appearance) which are normally assessed in the breed-
ing scheme. Table 7.10 shows the number of selected
clones which survived four, five and six rounds of selec-
tion from each cross. Also shown is the rank of each
cross based on the 25 seedlings and cross prediction of
breeders' preference.
Obviously, there were more highly desirable clones
from cross C1, C2 and C7 which were ranked first,
second and third on the univariate cross prediction of
glasshouse grown seedlings.
In the potato, cross prediction was investigated with
higher numbers of crosses (204) in a similar way as
a result of this first study. Results from the larger
study were in agreement with those shown above.