The efficiency of selection in the early generations of a potato breeding programme
Abstract
The efficiency of selection in the early generations of a potato
breeding programme is examined. A number of unselected potato
(Solanum tuberosum L) clones were grown in a glasshouse, from true
botanical seed, and thereafter in the field at two locations (a seed
site and a ware site) for three consecutive years. At each stage, a
number of tuber characteristics were visually assessed and yield plus
the yield components were recorded. Four potato breeders visually
assessed the produce from each clone after harvesting each years
trials and were in good agreement as to which clones would be selected
in each environment. Selection for visually assessed characters in
both the glasshouse and first clonal year produced a desirable
response. However, such selection carried a high cost in terms of
losing clones with commercial potential. Selecting clones for yield
in the seedling and first clonal year was only marginally more
effective than a random reduction in number of genotypes, while
selection in the second clonal year appeared to be somewhat more
effective as judged by performance in later generations. Comparison
of a random sample of clones with ones from the same crosses which had
been selected at the seedling and first clonal year stage was at best
random, with some suggestion, however, of a negative effect.
The causes behind the inefficiency of selection were found to be
complex. The inefficiency was ascribed, at least in part, to (i) the
inaccuracy of assessment on single plant plots; (ii) the "carry-over"
effect of the mother tubers and (iii) selection under a short growing
season.
Although there was a formally significant interaction between
progenies and environments, the rank of the mean of a cross remained
relatively consistent over different growing conditions. It was found
that the mean and square root of the variance obtained from breeders'
preference in any of the environments provided a good basis for
prediction of the number of clones in each cross which would exceed
(or equal) a given target value. The square root of the variance
added increasingly to the accuracy of the prediction as the target
value increased but was never a major component in such predictions.
When the predictions were used to provide ranking of the crosses, the
rank correlations showed good agreement between the different
environments and between observed and expected ranks. There was no
evidence to suggest that univariate cross prediction for any of the
other characters under study would not be effective.
A new cultivar is unlikely to be successful simply because of high
expression for a single character, but will rather be an all round
improvement over cultivars already available. Three methods of
multivariate cross prediction were therefore examined namely
multivariate probabilities, sum of ranks and the frequency of
genotypes in a sample that transgress set target values. The
characters total tuber weigh t, mean tuber weight, number of tubers and
regularity of tuber shape were examined. It was found that a sample
as small as 25 clones provided good predictions (as judged by the
observed frequencies in a larger progeny sample examined in various
environments). The best estimates were obtained using multivariate
probabilities based on the means, within progeny variances,. and the
phenotypic correlations between variates. The ranking of crosses
according to these multivariate probabilities provided good
indications of the number of clones which survived selection in an
actual breeding scheme.
Thus it is suggested that an empirical examination of a
sub-sample of the progeny from a cross could be used to determine the
crosses which would have the highest probability of producing
improved, potato cultivars. Selection of crosses rather than
individual clones has several advantages which would favour such
techniques being used in the early generations of a potato breeding
programme.
Cross prediction based on parental performance also provided an
indication of the crosses, .and parents, .which would give the highest
frequency of desirable recombinants. These predictions were not as
accurate as those derived from examination of a sub-sample of progeny
from each cross. But they would allow an earlier, and hence powerful,
method of prediction.
Type
Thesis, PhD Doctor of Philosophy
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