So far in this course we have dealt entirely with the evolution of characters that are controlled by simple Mendelian inheritance at a single locus. There are notes on the course website about gametic disequilibrium and how allele frequencies change at two loci simultaneously, but we didn’t discuss them. In every example we’ve considered we’ve imagined that we could understand something about evolution by examining the evolution of a single gene. That’s the domain of classical population genetics. For the next few weeks we’re going to be exploring a field that’s actually older than classical population genetics, although the approach we’ll be taking to it involves the use of population genetic machinery. If you know a little about the history of evolutionary biology, you may know that after the rediscovery of Mendel’s work in 1900 there was a heated debate between the “biometricians” (e.g., Galton and Pearson) and the “Mendelians” (e.g., de Vries, Correns, Bateson, and Morgan). Biometricians asserted that the really important variation in evolution didn’t follow Mendelian rules. Height, weight, skin color, and similar traits seemed to

Human biochemical genetics

Summary

Potatoes destined for making chips, French fries and other fried products, need to have low sugar content to avoid browning of the finished product. The sugar content of potatoes is determined by the genotype and several pre- and post-harvest factors. The major pre-harvest factors affecting sugar content are crop maturity, temperature during growth, mineral nutrition and irrigation, while important post-harvest factors are mechanical stresses and storage conditions. Each genotype requires an ideal pre-and post-harvest treatment to maintain low sugar levels; any kind of stress results in sugar accumulations.

An overview of the factors affecting sugar content of potatoes

Potato (Solanum tuberosum) is the third most important food crop in the world. Potato tubers must be stored at cold temperatures to prevent sprouting, minimize disease losses, and supply consumers and the processing industry with high-quality tubers throughout the year. Unfortunately, cold storage triggers an accumulation of reducing sugars in tubers. High-temperature processing of these tubers results in dark-colored, bitter-tasting products. Such products also have elevated amounts of acrylamide, a neurotoxin and potential carcinogen. We demonstrate that silencing the potato vacuolar acid invertase gene VInv prevents reducing sugar accumulation in cold-stored tubers. Potato chips processed from VInv silencing lines showed a 15-fold acrylamide reduction and were light in color even when tubers were stored at 4°C. Comparable, low levels of VInv gene expression were observed in cold-stored tubers from wild potato germplasm stocks that are resistant to cold-induced sweetening. Thus, both processing quality and acrylamide problems in potato can be controlled effectively by suppression of the VInv gene through biotechnology or targeted breeding.

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Suppression of the Vacuolar Invertase Gene Prevents Cold-Induced Sweetening in Potato

Contributors. Preface. Contents. 1. The Economic Importance of the Potato M.F. Askew. 2. Biology, Physiology of the Potato H.D. Rabinowitch, D. Levy. 3. Important Potato Cultivars D.L. Corsini, C.R. Brown. 4. Historical Perspectives of Potato Virus Research R.H. Lawson, R. Stace-Smith. 5. The Main Viruses Infecting Potato Crops A.A. Brunt, et al. 6. Viroids L.F Salazar, et al. 7. Phytoplasma Diseases M. Klein. 8. Transmission of Viruses G.D. Franc, et al. 9. Epidemiology, Field Control of PVY, PLRV D.W. Ragsdale, et al. 10. Detection, Identification of Viruses in Potatoes A. Gera, S. Marco. 11. Isolation, Purification P.E. Thomas, W.K. Kaniewski. 12. Operation of a Commercial ELISA, Seed Testing Laboratory Y. Alon, I. Ben Zeev. 13. Resistance C.R. Brown, et al. 14. Control of Potato Viruses using Meristem, Stem-cutting Cultures, Thermotherapy, Chemotherapy G. Faccioli. 15. Rapid Propagation of Virus-tested Potatoes A.A. Watad, et al. 16. Seed Certification as a Virus Management Tool G.D. Franc. 17. Common Seed Potato Certification Schemes O.A. Gutbrod, A.R. Mosley. 18. Emerging Potato Viruses that Confront Regulations L.G. Brown. Index.

Virus and virus-like diseases of potatoes and production of seed-potatoes.

Interval mapping of quantitative trait loci (QTL) for 16 yield, agronomic and quality traits in potato was performed on a tetraploid full-sib family comprising 227 clones from a cross between processing clone 12601ab1 and table cultivar Stirling. Thirty-eight AFLP primer combinations and six SSRs provided 514 informative markers which formed a molecular marker map comprising 12 linkage groups (LGs) in 12601ab1 (nine with four homologous chromosomes) which were aligned with 12 in Stirling (11 with four homologous chromosomes), with four partial groups remaining in 12601ab1. Two LGs were identified unequivocally as chromosomes IV and V and eight others were tentatively assigned with chromosomes VII and X unidentified. All of the traits scored had moderately high heritabilities with 54–92% of the variation in clone means over 3 years and two replicates being due to genetic differences. A total of 39 QTLs were identified. A QTL for maturity was identified on chromosome V which explained 56% of the phenotypic variance, whereas the other QTLs individually explained between 5.4 and 16.5%. However, six QTLs were detected for after-cooking blackening and four for each of regularity of tuber shape, fry colour both after storage at 4 and 10°C and sprouting. Just two QTLs were found for each of yield, the two ‘overall’ scores, crop emergence, tuber size and common scab and just one QTL was detected for each of dry matter content, keeping quality, growth cracks and internal condition. The implications for practical potato breeding and for practical linkage and QTL analysis in autotetraploids are discussed.

QTL mapping of yield, agronomic and quality traits in tetraploid potato (Solanum tuberosum subsp. tuberosum).

SUMMARY

The efficiency of visual selection in the early generations of a potato breeding programme is examined. Tubers from 571 potato genotypes were scored by four breeders after being grown, from true seed, in a glasshouse and in the field for three consecutive years. The four breeders were in good agreement as to which clones would be selected in any environment. The association between breeders was greatest when the clones were grown in plots in the field. All correlations of breeders' preferences between different environments were significantly greater than zero, but only accounted for a small percentage of the total variation. Selection 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. Comparison of a random sample of clones with ones from the same crosses which had been selected, indicated that selection in the glasshouse and first clonal year was at best random with some suggestion, however, of a negative effect.

The efficiency of visual selection in early generations of a potato breeding programme.

The efficiency of visual selection in a glasshouse of 1600 seedlings compared with visual selection in the field of the same clones is examined. Also the amount of agreement in assessment between four different potato breeders screening the same clones is investigated. Seedlings were grown from true seed in four-inch pots in a glasshouse and all the clones which produced tubers were grown in the field the following year. Clones which produced more than one tuber from a seedling were grown at two locations in the first clonal year. One of the sites used in the first clonal year is normally used for potato yield trials (i.e. a ware site) and the other is normally used for the production of healthy seed tubers (i.e. a seed site). In all three environments, the tubers produced from each plant were assessed by four breeders independently on a 1–9 scale of increasing attractiveness. From the data it was found that the repeatability of assessment between the glasshouse and the first clonal year was low in that the correlation between the average preference score of the four breeders in the glasshouse and each of the two first clonal year sites accounted for only 8·24 and 6·84% of the total variation. Many clones which had low scores in the glasshouse were subsequently given high scores in the first clonal year. It was therefore concluded that selection of seedlings was not very efficient. Although, in general, the weight of the tuber that was planted greatly influenced whether a clone was selected in the first clonal year, a large number of clones which produced only small tubers in the glasshouse were subsequently selected in the first clonal year. Within each environment the four breeders were either all selecting, or all rejecting, a much higher proportion of clones than would be expected if selection had been made completely at random. The breeders were in most agreement when assessing clones at the ‘ware’ site, and in most disagreement when assessing them grown in the glasshouse from true seed. Therefore the poor efficiency of selection of seedlings grown in the glasshouse was not, in the main, a result of a high error variance in visual assessment but rather due to poor association between the performance of seedlings and first clonal year plants.

The efficiency of seedling selection by visual preference in a potato breeding programme.

Several clones from a potato breeding programme at the Scottish Crop Research Institute (SCRI) produced acceptable (pale) coloured fry products after five months' storage at 4°C Chemical analysis of tuber samples taken at five-week intervals during storage at 4°C and 10°C gave a substantial variation in glucose, fructose and sucrose concentrations among the 22 clones examined Several unnamed SCRI clones showed little accumulation of reducing sugars when stored at 4°C In marked contrast, the cultivars Record and Pentland Dell, currently the most widely used cultivars for fry processing in the UK, accumulated far greater levels of sugar during low temperature storage Glucose concentration proved more important than fructose concentration in determining fry colour Clones with the lowest concentrations of glucose after storage at 4°C also showed lowest concentrations when stored at 10°C These results are in agreement with previous reports on the predictive value of glucose levels at harvest, but this is the first identification of such low temperature, low sweetening variants in agronomically adapted clones of the cultivated tetraploid potato

The processing potential of tubers of the cultivated potato, Solanum tuberosum L., after storage at low temperatures. 2. Sugar concentration

Routine fry tests of material from the potato breeding programme at the Scottish Crop Research Institute (SCRI) detected clones which gave superior (paler coloured) crisps than the control cultivar Record, after storage at low temperatures (4°C). Thirteen of these clones and nine cultivars were fry tested at intervals after storage at 4°C and 10°C. The ability of these clones to produce an acceptable fry product direct from storage at low temperature and their superiority in this respect over existing cultivars was confirmed. Some clones examined produced an acceptable fry colour even after 30 weeks' storage at 4°C.

The processing potential of tubers of the cultivated potato, Solanum tuberosum L., after storage at low temperature. 1. Fry colour.

Most previous studies on cross prediction methods have examined relatively few crosses, particularly in relation to the numbers involved in most breeding programmes. In this paper the feasibility of using cross prediction methods was examined in a practical potato (Solanum tuberosum) breeding scheme by the analyses of progeny from 52 crosses. The variate considered was breeder's preference, a visual assessment made of the harvested tubers to estimate their commercial potential. The results showed that it was possible to identify the superior crosses. Cross prediction based simply on the mean preference scores, averaged over scorers and clones within progenies, estimated on seedlings or first clonal year plants, provided the best estimate of a progeny's performance in the third clonal generation. Predictions based on the expected proportion of clones that would transgress a given target value also provided a good indication of a progeny's potential. The poorest prediction was obtained by using the observed frequency of desirable clones in a progeny sample. The implications for potato breeding are discussed.

G. R. Mackay

Papers

The efficiency of visual selection in early generations of a potato breeding programme.

The efficiency of seedling selection by visual preference in a potato breeding programme.

The processing potential of tubers of the cultivated potato, Solanum tuberosum L., after storage at low temperatures. 2. Sugar concentration

The processing potential of tubers of the cultivated potato, Solanum tuberosum L., after storage at low temperature. 1. Fry colour.

The use of cross prediction methods in a practical potato breeding programme.