Principles of Plant Genetics and Breeding

BREEDING POTATO 541
Table 1 The Scottish Crop Research Institute (SCRI) strategy for breeding finished cultivars.
Year Strategy
0 Decide objectives and evaluate potential parents
Midparent values used to predict mean performance of crosses for quantitative traits + other genetic information
GLASSHOUSE
1 Choose parents and make 200 crosses
2 Seedling progeny tests on 200 progenies × 2 replicates × 25 seedlings (late blight, potato cyst nematodes, visual
assessment of tubers)
Select best 40 progenies (after first cycle will also have midparent values for fry color and other traits from year 5 of
previous cycle)
SEED SITE
3 Tuber progeny tests on 40 progenies × 2 replicates as 2,000 spaced plants (visual assessment of tubers and fry color).
Select 500 spaced plants at harvest (four tubers of each plant)
Sow more seed of 10 best progenies in the glasshouse to provide a further 10 × 250 = 2,500 (four tubers of each)
clones for year 4
Select clones for use as parents in next cycle of crosses at random from those (500) advancing to year 4
4 3,000 unreplicated four-plant plots (including parents of next cycle of crosses)
Assessment for yield and quality and special disease tests
Seed site Clones Ware site
(number of plants) (number) (number of plots and plants per plot)
5 6 1,000 2 × 5; include parents used in crosses
6 20 360 2 × 10
7 100 120 Two harvests or sites of 2 × 10
8 300 40 Ware trials at a number of sites
9 700 20 Ware trials at a number of sites
10 2,000 2 Official trials
11 2,000 1 Official trials
Multiplication and commercialization from virus-free stock
Intermediate and later generations
The SCRI can again be considered typical in being able to handle 1,000 clones in the first year of replicated yield trials at a ware
site. However, the relatively slow rate of natural vegetative reproduction, together with the complicated logistics of accurately
assessing 1,000 or more clones for a very large number of traits, meant that another 5 years elapsed before one or a few potential
cultivars could be confidently entered into official statutory trials. During this period, decreasing numbers of selected clones were
grown in increasingly sophisticated trials over as wide a geographic range as economics permitted and breeding objectives
demanded. During these intermediate and final stages of selection, the production of seed tubers was separated from the trials
which were grown under ware conditions, designed as far as possible to approximate to those of good commercial practice. In
addition to yield and agronomic performance, clones undergoing selection were assessed for their cooking and processing characteristics
and tested for their resistances to numerous pests and diseases. The selection criteria and testing procedures were
largely governed by practical considerations and experience of the reliability of the various tests used, rather than by genetic
knowledge of, for example, heritabilities or genetic correlations between traits. It would therefore be worthwhile trying to
develop a more robust decision-making process based on multitrait, multistage selection theory and estimated genetic parameters,
or at least to assess the extent to which current practice is suboptimal (Bradshaw & Mackay 1994).
Genetic knowledge and molecular marker-assisted selection
As knowledge increases about the number and chromosomal locations of genes affecting economically important traits, breeders
should be able to design better breeding programs. As well as selecting parents that complement one another genotypically, they
will be able to determine the seedling population size required for certainty of finding the desired genotype, and, more realistically,