Effectiveness of selection at microtuber crop level in potato

Correlation coefficients were computed between microtuber crops and normal seed crop in order to study the selection efficiency for agronomic characters in potato at microtuber crop level. Two types of microtuber, green and white, were produced by inducing microtuberization under light and dark conditions, respectively. Genotypic differences were significant for various characters in both microtuber and normal seed crops. Green and white microtubers were equally useful for selection, judging by the crops raised from them. Selection at microtuber crop level was found to be highly effective for plant height, stem habit, tuber colour and general impression, and moderately effective for internode length, leaf length, leaflet width, stem pigment, plant vigour and tuber shape. Selection efficiency was low for tuber yield, tuber number, average tuber weight, number of nodes, number of stems, leaf colour and number of eyes. The implications of these results for potato breeding and germplasm evaluation programmes are discussed and a procedure for selecting superior genotypes at microtuber crop level is outlined.     

Genetic divergence for cross prediction in potato

To study the effectiveness of genetic divergence for cross prediction in potato, progeny means, heterosis and specific combining ability effects were correlated with parents’ genetic distances (D values) estimated under six in vitro and four in vivo conditions, for tuber yield in 72 crosses (18 × 4) of 22 parents under autumn crop conditions for three successive generations. Genetic distances under in vitro conditions had no relationship with the progeny means for tuber yield. Whereas, those under in vivo conditions in the autumn seasons were positively associated with the progeny means. Similarly, heterosis for tuber yield had a stronger relationship with genetic distances based on an in vivo crop than those based on an in vitro crop. All correlation coefficients between genetic distances and specific combining ability effects were non-significant. The magnitudes of the significant correlation coefficients showed that genetic divergence can be used as an indirect parameter of moderate effectiveness in selecting parents to produce heterotic high yielding progenies. Such cross predictions, however, would be effective only if parents are evaluated under the conditions similar to those under which crosses are likely to be evaluated. This revised version was published online in July 2006 with corrections to the Cover Date.     

The relations of broad nematode resistance to quality characteristics as a consequence of marker-assisted selection in potato breeding programs

Cultivating resistant varieties of potato is the most effective and environmentally sound method of protecting potato crops against pests and diseases. Potato cyst nematodes (PCN) are major nematode pests causing severe constraints in potato production worldwide. There are five pathotypes of Globodrea rostochiensis (Ro1–Ro5) and three of G. pallida Pa1–Pa3. Cultivation of potato varieties with broad nematode resistance may influence the growth of the wide spectrum of PCN pathotypes, but there is limited availability of such varieties on the market. The use of molecular markers allows for the effective selection of resistant genotypes at early stages of breeding. However, the impact of early selection for nematode resistance on the agronomic value of the final selected clones is a cause of concern for potato breeders. This study investigates the relationships between the presence of the combined resistance genes H1, Gro1-4 and GpaV_vrn, which confer resistance to the nematodes, and certain agricultural traits. Clones with broad nematode resistance conferred by the genes H1, Gro1-4 and GpaV_vrn presented yields and tuber morphology traits similar to those of the clones without identified resistance genes.     

Early selection for extreme resistance to potato virus Y and tobacco etch virus in potato using a β-glucuronidase-tagged virus

The Ry_sto gene from Solanum stoloniferum introduced into potato cultivars (Solanum tuberosum L. ssp. tuberosum) confers resistance to potato virus A, potato virus V and potato virus Y (PVY). In addition to PVY, tobacco etch virus (TEV) and a TEV construct that encodes β‐glucuronidase (TEV‐GUS) were inoculated to determine the inheritance of resistance to these viruses in progenies obtained from potato cultivars containing the Ry_sto gene. While cultivars ‘Karlena’ and ‘Delikat’ were susceptible, ‘Bettina’ and clone 927eY were resistant to PVY, TEV and TEV‐GUS, as determined by enzyme linked immunosorbent assay, biotest and GUS assay, respectively. The segregation ratios obtained from the progenies of ‘Bettina’בDelikat’ and 816eY בKarlena’ indicate that resistances to PVY and TEV are governed by one dominant gene or two genes tightly linked in coupling phase. Evidently, Ry_sto confers broad spectrum resistance to potyviruses. TEV resistance could be reliably detected 4 days after inoculation with the TEV‐GUS construct by GUS assay. Therefore, the GUS‐tagged TEV construct can be used for early selection for resistances based on the gene Ry_sto or closely linked genes.     

Factors affecting minimal growth conservation of potato microplants in vitro

Combined effects of sucrose, mannitol and photoperiod on microplant conservation were studied in four potato genotypes belonging to two different groups viz., Tuberosum and Andigena. Minimal growth medium was based on Murashige and Skoog (MS) supplemented with 6 different concentrations of sucrose (30, 40, 50, 60, 70 and 80 gl^-1) with 4 different concentrations of mannitol (0, 20, 40 and 60 gl^-1). The cultures were conserved under two photoperiod conditions i.e. continuous illumination and 16-h photoperiod at 6 ± 1 °C. There were significant interactions between photoperiod and sucrose, and between photoperiod and mannitol. Maximum microplant survival and desirable microplant growth were observed under 16-h photoperiod. Sucrose alone did not improve culture viability over 30 months of storage. Inclusion of mannitol in the conservation medium increased microplant survival. Sucrose x mannitol interaction showed that sucrose was effective in enhancing microplant survival in combination with 20 or 40 gl^-1 mannitol, but not with 60 gl^-1 mannitol. Combined effect of sucrose, mannitol and photoperiod showed that optimum microplant growth and maximum culture viability were obtained when the cultures were grown in MS medium containing 40 gl^-1 sucrose and 20 gl^-1 mannitol under 16-h photoperiod. Potato microplants can be conserved in this medium and cultural conditions up to 30 months without subculturing. This revised version was published online in July 2006 with corrections to the Cover Date.     

A comparison between single plant plots and five plant plots for the initial selection stage of a potato breeding programme

Summary The efficiency of single plant selection in the initial selection stage of a potato breeding programme was examined. A random sample of potato clones was grown in single plant plots and also in tworeplicates of five plant plots. After harvest, each plot was visually assessed by three potato breeders; total tuber weight and number of tubers per plant were also recorded, from which mean tuber weight was calculated. The error variances of the single plant plots were found to be significantly greater than from the five plant plots for total tuber weight, mean tuber weight and number of tubers per plant, but not significantly different for breeders' preference. Coefficients of correlation between single and five plants plots were significantly greater than zero for all traits examined, but they were lower than the corresponding correlations between the two replicates of five plant plots. Although a random sample of clones was examined, each clone had been assessed for breeders' preference the previous year. When the clones were grouped according to the previous years preference scores, it was found that the correlation coefficients between single and five plant plots for breeders' preference were inversely related, in magnitude, to the mean preference score of each group. It was concluded that single plant selection was generally ineffective, particularly when only the better clones, from the previous years assessment, are examined.     

Identifying superior parents in a potato breeding program using cross prediction techniques

Cross prediction techniques were applied to data collected from over 600 hybrid combinations of potato (Solanum tuberosum) and analyzed to determine the potential of using early generation cross prediction techniques to identify superior parental clones. Performance of parental lines based on parameters collected in early generations were compared with the observed frequencies of desirable recombinants with a common parent in the latter stages of a breeding program. Results showed that value of parents, in their ability to produce desirable recombinants in breeding programs, can be predicted using univariate cross prediction techniques. This type of information can be available from early generation progeny trials and could easily be incorporated into a practical potato breeding scheme. This revised version was published online in August 2006 with corrections to the Cover Date.     

Correlation between Potato Clones Grown in vivo and in vitro under Sodium Chloride Stress Conditions

Salinity is one of the most serious and widespread agricultural problems resulting in losses of yield and arable land. One strategy available to cope with saline soil is to choose salt-tolerant crops or to select salt-tolerant cultivars within a crop. Difficulties in breeding for salt tolerance arise from its polygenic control and from the variable salt composition and distribution in the soil. An in vitro approach for screening large amounts of parental material is suggested and the present results confirm previous work conducted on in vitro response of the potato to a high level of sodium chloride. Furthermore, a highly significant correlation was found between in vitro growth parameters and field performance of ten potato clones.     

Effects of plot size and selection intensity on efficiency of selection in the first clonal generation of potato

To improve efficiency of selection in first year clones, investigations were carried out on the use of two-plant plots instead of one-plant ones. Extended plot size did reduce the environmental variance of characters connected with tuber yield, but had only a slightly positive effect on the selection result, as was concluded from a comparison between the first and the second clonal generation. Only lowering selection pressure in the first year clones seems to be a solution for optimizing selection efficiency, as could be concluded by comparing simulated selection percentages of about 5%, 20% and 50%.Owing to genotype × harvest time-interaction for the majority of traits important to either seed or ware potatoes, assessments at both harvest times are necessary for proper evaluation.     

Genetic gain from early generation selection for cold chipping genotypes in potato

Early generation selection for chip colour may accelerate potato chipping cultivar development. The research objective was to measure the response from early generation selection for light potato chip colour. Progeny from 175 4×× 4× families were propagated to the field using greenhouse grown tubers or seedling transplants. Tubers from progeny were made into potato chips after 3 and 6 months’ storage at 4° C. Chip colour was rated as 1 = light to 10 = dark. Genotypes were categorized as retained (≥ 4) or discarded (> 4) based on the 6‐month chip colour, then field grown a second year at two locations and re‐evaluated for chip colour. Positive selection responses resulted, but were lower in the seedling transplants. Interaction between genotype and environment, and genotype and storage duration reduced responses. Selected genotypes should be evaluated over multiple environments and storage conditions to insure reliable performance. Expanding variation by introgressing favourable alleles from wild species may further enhance breeding success.     

Combining ability analysis and correlation between breeding values in true potato seed

True potato seed (TPS) requires the selection of appropriate parents for developing hybrid offspring. Parents for routine crossing schemes need to be selected according to their combining ability. Hierarchical and factorial mating designs provide a mean values to assess the general combining ability (GCA) of clones included in crossing schemes. Furthermore, specific combining ability (SCA) may be investigated using the factorial mating design. The aim of this research was to determine the combining ability of clones included in early and intermediate TPS breeding populations developed by the Centro Internacional de la Papa (CIP). Likewise, correlations between breeding values (or additive genetic correlation, p_A) were calculated. Two hierarchical mating designs (in both the early and intermediate populations) and one factorial mating design (in the intermediate population) were evaluated in two contrasting Peruvian locations (La Molina ‐ coastal desert, and San Ramon, warm humid tropics). Plant and tuber characteristics were recorded in these experiments. Significant GCA was observed for tuber yield in all experiments. The male's p_A between plant vigour after transplanting and tuber yield was significant, thereby suggesting that offspring with early vigorous growth are high yielding. Plant vigour was also correlated with tuber set, but only among the breeding value of female parents, which suggests that high tuber yield could be achieved because offspring with early, vigorous growth have many tubers. High tuber yielding hybrids can be obtained by choosing parents with significant GCA, whose tuber yield can also be further enhanced if the crosses have a significant SCA.     

Effectiveness of selection for quality traits during the early stage in the potato breeding population

Potato cultivars resistant to cold‐temperature sweetening are of major importance to the processing industry producing both chips (crisps) and French fries. When most modern potato cultivars are maintained in cold storage to retard sprouting, the tubers accumulate reducing sugars, and the products become an unacceptable brown colour when fried. Selection for better processing quality during the early generations of a breeding programme could be of considerable advantage. Using a portable ‘sugarmeter’, which requires only a drop of sap from the tuber on a test strip, many samples can be efficiently surveyed for low sugar as early as the F₁ generation. Using seedlings of three test crosses, glucose and specific gravity of field‐grown tubers, minitubers from greenhouses and microtubers from in vitro culture were compared after cold treatment. Although the mean glucose levels of minitubers and microtubers were higher than field‐grown tubers, the correlation between the glucose contents of the three types of tubers was fairly high. A considerable genetic improvement was noted when progenies were grown as minitubers or microtubers, even though the response to selection for low glucose levels in minitubers and microtubers was lower than from direct selection from field‐grown tubers. The specific gravity of field‐grown tubers showed a significant association with freshly harvested minitubers and microtubers. Selection for low glucose content in minitubers can therefore save considerable resources in a breeding programme.     

In vitro adventitious bud techniques for vegetative propagation and mutation breeding of potato (Solanum tuberosum L.). II. Significance for mutation breeding

Summary It was investigated what potentialities for mutation breeding of potato are offered by using adventitious sprouts that arise in vitro from leaf explants (rachis, petiole, leaflet-disc) after X-irradiation. Mutation frequency and chimerism were studied in subterranean and aerial parts in three vegetative generations (vM₁, vM₂, vM₃). Plants obtained from irradiated series produced a very high mutation frequency, a wide mutation spectrum and a very low rate of chimerism. Mutations were observed also in control series, especially in plants derived from the rachis and petiole explant group.     

Use of isozyme patterns in characterization of potato mutant lines selected for agronomic quantitative traits

Summary Seven varieties and 57 spontaneous or induced in vitro mutant lines (20 macromutant and 37 micromutant events) of potato were tested by starch gel electrophoresis for ADH, GOT, PGI, PGM, ACO, IDH, MDH and 6PGDH isozymes in tuber extracts. The data showed that in contrast to variety comparisons, the isozyme patterns rarely differentiate mutant lines which have altered morphological traits. But trying to identify isozyme differences in mutants can still be useful for a chimeric structure for GOT-2 alleles in a mutant from Atlantic and a new tuber specific locus for 6PGDH in mutants from Russet Burbank were found.Abbreviations ACO aconitase - ADH alcohol dehydrogenase - GOT glutamate oxaloacetate transaminase - IDH isocitric acid dehydrogenase - MDH malate dehydrogenase - 6PGDH 6-phosphogluconate dehydrogenase - PGI phosphoglucoisomerase - PGM phosphoglucomutase - SGE starch gel electrophoresis - EMS ethyl metanesulfonate     

Correlations within and between characters between and within generations as a measure for the early generation selection in potato breeding

Summary The main objective was to determine the reliability of selection in early generations of a potato breeding programme. To that end data were recorded in the seedling generation, an early and a late harvested experiment of the first clonal generation and the late harvested second clonal generation on up to 30 genotypes of 55 populations from crosses of three Solanum tuberosum ssp. tuberosum cultivars and seven selected S. tuberosum ssp. andigena clones, adapted to the northern temperate zone. Phenotypic correlations were computed per population in almost all possible combinations within and between characters between and within generations. The mean across 55 comparable correlation coefficients was considered to be the best estimate of the relationship concerned.In all four experiments data were collected on number of tubers, tuber yield and mean tuber weight; in the three experiments of the two clonal generations in addition on date of emergence, number of stems, plant height, haulm type, and general impression of the underground plant parts and in the two late harvested clonal generations on date of maturity and under water weight.The results confirmed that general impression which is largely decisive for the selection in early generations, is a little reliable measure. However, from the magnitude of the mean correlations between experiments within characters it must be concluded that selection for most of these separate characters is hardly more reliable. An exception was date of maturity for which a mean correlation was determined of r=0.78 between the two clonal generations. The second strongest mean correlations, perhaps surprisingly, were for tuber yield, viz. r=0.52 between the seedling generation and the late harvested first as well as the second clonal generation and r=0.63 between the late harvested clonal generations. From the correlations between characters it appeared that for a high tuber yield, besides its components, in particular an early emergence, a large plant height, an attractive haulm type and a late maturity are of importance.The main conclusion is that, as long as not enough seed tubers are available for the layout of statistically sound experiments, a negative instead of a positive selection should be applied to avoid the rejection of most of the valuable genotypes in an early stage. On the basis of the results directives for the application of such a negative selection are given.     

Pear mutagenesis: In vitro treatment with gamma-rays and field selection for vegetative form traits

In vitro shoots of four pear (Pyrus communis L.) cultivars, 'Conference', 'Doyenné d'Hiver', 'Passe Crassane' and 'Bartlett', were irradiated with gamma rays (3.5 Gy). Around 1000 microcuttings of each cultivar were treated. After three subcultures, microcuttings from both the irradiated shoots and additional non-irradiated shoots were rooted; plants obtained were used to establish a survey orchard in Ostellato (Ferrara), Italy. For the first 2 years in the field, growth and survival were observed, but no data were collected. Subsequently, field surveys were conducted to characterize the population and identify mutants for vegetative traits. Trees that showed phenotypic evidence of vegetative growth characteristics more desirable than those shown by the overall population and by control plants were selected and measured for three years. Traits used for selection were small tree size, wide branch angle and short internodes. Frequencies of variants with compact habit varied with cultivars from 0.5% to 2.7% of irradiated trees. Effect of mutagenic treatment and efficacy of the selection methods are discussed.     

The reaction of potato differentials to Phytophthora infestans isolates collected in nature

Available data have been evaluated on the reaction of potato differentials to over 5000 Phytophthora infestans isolates, collected in various parts of the world. The differentials were able to identify up to 11 virulence factors in P. infestans. The isolates differed in virulence expression, depending on the isolate and testing conditions. All 11 virulence factors were found in both ‘old’ and ‘new’P. infestans populations. The resistance of individual differentials was not overcome with equal frequency. The resistance of differentials R5, R8 and R9 was overcome least frequently. This may be due to instability of expression of the respective virulence factors in P. infestans and/or the kind of resistance present in the differentials. Whatever the reason, such resistance may possibly be used in breeding potato cultivars with durable resistance to P. infestans.     

Breeding for impact resistance in potato: use of a pendulum in progeny testing

Breeding for impact resistance in potato is common at the end of a breeding scheme, where there is no more variability for it. The purpose of this study was to propose an objective progeny test using a pendulum and single-year data. Data were recorded over a 5-year period using 275 progenies, organized in small factorial and hierarchical designs. The variability of response of the additive variance made it impossible to generalize parental choice. However, once the crosses were made, the high degree of variability among the progenies and the high level of broad-sense heritability in every case suggest that selection can be carried out efficiently using single-year data.     

Selection for verticillium wilt resistance in potato breeding populations derived from four cross types

Verticillium wilt (VW) is one of the important yield-limiting diseases for potato production. To develop resistant clones, the potential for early generation selection was studied using three basic selection methods, individual, family, and within family selection, for two clonal generations. A total of 152 clones were derived from four cross types (2x × 2x, 2x × 4x, 4x × 2x and 4x × 4x). Clones were evaluated for maturity, symptom expression, yield and stem colonization in replicated trials. Heritability and selection response for the traits were estimated for each selection method. Direct selection in the second clonal generation and individual selection showed more gain than that from other methods. Both 2x × 2x and 4x × 2x families were higher yielding and had lower stem colonization scores than 2x × 4x and 4x × 4x crosses. Therefore, 2x × 2x or 4x × 2x crosses between carefully chosen parents with high yield and VW resistance may produce offspring with superior performance.     

Behaviour of genetically modified amylose free potato clones as progenitors in a breeding program

Three amylose-free genetically modified potato clones were used both as male and female parents in a breeding program with non-GMO potato clones. Segregation data on the expression of the inserted antisense gene construct in tubers of progeny plants were in agreement with previous molecular analysis of the transgenic clones. The inheritance of the inserted genes was according to Mendelian segregation. Therefore, these clones can be very useful in a breeding program for large scale introduction of amylose free potato cultivars into agriculture. Because of varying number and expression levels of inserts in the GMO-clones, but also because of the varying strength of the endogenous GBSS-alleles of the non-GMO-clones, a segregation into a range of amylose contents occurred. The segregation of the starch colour after iodine staining of pollen of transgenic clones did not follow the obtained segregation in the progeny and was, therefore, not useful in predicting the breeding result. This revised version was published online in July 2006 with corrections to the Cover Date.