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.     

Effectiveness of in vitro selection for agronomic characters in potato

Effectiveness of in vitro selection for agronomic characters was studied by finding correlation coefficients between in vitro and in vivo performance of 22 potato genotypes. Evaluation was performed under eight in vitro and two in vivo conditions. Genotypic differences were highly significant for various characters under all in vitro and in vivo conditions. Error mean squares were much lower in in vitro experiments than in in vivo experiments. In vitro selection was found to be highly effective for tuber colour, stem pigment and number of eyes, and moderately effective for average tuber weight, plant vigour and foliage senescence under specific conditions. The results also indicated the possibility of in vitro selection for heat tolerance. For tuber yield and number of tubers, effectiveness of in vitro selection was low to very low, because differences in phenotypic expressions of genotypes were much enlarged under in vitro conditions compared to under in vivo conditions. Correlation coefficients for certain characters were better when in vitro conditions were closer to in vivo conditions. To improve the efficiency of in vitro selection, there is perhaps a need to simulate and identify the in vitro conditions under which genotypic differences are reflected in phenotypes more realistically as they are expressed under in vivo conditions. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

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.     

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.     

A comparison between single seed descent and early cross selection in wheat breeding

Summary Two selection procedures in wheat breeding were compared on the basis of their ability to supply high yielding inbred lines. The first procedure consists of an early selection between crosses in the F₃ generation, based on predictions of the cross mean and the between line variance. In the second procedure selection is postponed until the F₆, which is derived by single seed descent. The two procedures are evaluated in a two year test, using pseudo-lines of spring wheat. These pseudo-lines consist of mixtures of varieties and enable an estimation of the exact genetic parameters. In this way the accuracy of the predictions can be examined.In case of early selection, it appears that the predictions of the cross mean and especially the between line variance are very inaccurate. This is caused by the effects of plot size, intergenotypic competition and, to a lesser extent, dominance and/or epistasis. It results in an erroneous ranking of the crosses and the discarding of the potentially best cross. The F₆-SSD line estimates are much more accurate and thus the better lines are indeed selected. A first comparison between the two selection procedures therefore indicates a preference to the SSD method.     

A comparison between single seed descent and early cross selection in wheat breeding

Summary Two selection procedures in wheat breeding were compared on the basis of their ability to supply high yielding inbred lines. The first procedure consists of an early selection between crosses in the F₃ generation, based on cross performance. In the second procedure selection is postponed until the F₆, which is derived by single seed descent. The two procedures are evaluated in a two year test, using pseudo-lines of spring wheat. These pseudo-lines consist of mixtures of varieties and enable an estimation of the true genetic parameters. In this way the accuracy of the predictions can be examined.The first year of this comparison comprised the actual selection process and in the second year an F₆ and F₇ generation were mimicked, based on the selected lines of the F₃ and F₆ respectively, and evaluated in a large yield test. This paper gives the final results after the second year. From the first year it was found that the F₃ cross predictions were very inaccurate, whereas selection in the SSD-F₆ appeared to perform well. This resulted in a higher yielding set of lines evolving from the SSD-F₆. Despite a pronounced genotype-year interaction this difference in yield level was also found between the F₇ and the F₆ lines, derived from the selected crosses in the F₃. We conclude that the early selection procedure is not advantageous compared to the fast SSD procedure.     

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.     

Top‐cross vs. poly‐cross as alternative to test‐cross for estimating the general combining ability in potato

Test‐crosses for estimating combining ability in potato although useful are difficult to make being labour intensive and time consuming, and due to the problem of sterility. The objective of the present study was to test methods for estimating general combining ability (GCA) of parents based on fewer matings. The GCA of 12 females estimated based on matings with specific single testers (top‐cross) and three types of bulk pollen (poly‐cross) were compared with those estimated by 72 (12 × 6) test‐crosses, using common testers in three types of matings. The study was conducted for 2 years at each of the two locations representing short‐days subtropical conditions in plains and long‐days temperate conditions in hills of India. Analysis of variance of test‐crosses showed that both general and specific combining ability effects were important for tuber yield, late blight resistance as well as general impression, the characters studied. Correlation coefficients among three types of matings showed that bulk pollen matings had poor effectiveness in predicting the GCA of the females, whereas matings with selected individual testers resulted in GCA estimates similar to those based on six testers. Thus use of top‐cross involving selected tester could substantially reduce the number of matings required for estimating the GCA. Among the six testers used, the tester CFK69.1 could estimate the GCA of the females with reasonably high accuracy both for tuber yield and general impression, and ‘Tobique’ for reaction to late blight.     

Selection for cold chipping genotypes from three early generations in a potato breeding program

Chipping potatoes are an important market for the processing industry. Of the traits influencing chip quality,chip color is market limiting since other traits have diminished value without light chip color. Cold (4 ^°C) storage can reduce storage losses and cultivars producing light colored chips directly from cold storage are needed for this reason. Selection for cold chipping cultivars is traditionally practiced by visual evaluation of progeny the first few years in the field, followed by selection for cold chipping in years 4–5. Early generation selection for cold chipping may reduce cultivar development time by identifying good chipping progeny, families and parents earlier in the breeding cycle. Progeny from 175 4x × 4x families were evaluated from three early generations. They were greenhouse grown tubers (GGT) evaluated after6 mo. storage using randomly selected progeny, and field grown single hills propagated from either seedling transplants (FTR), or from greenhouse grown tubers (FGT). Progeny from field grown generations were selected first by visual evaluation, and then by random selection and chipped after6 mo. (visual selections) or 3 and 6 mo. storage (random selections). Cold chipping progenies were found in all three early generations. The earliest generations for selection are GGT or FTR, but may be less efficient than FGT. Correlations suggest that reliable parent and family selection is possible using GGT data. Visual selection eliminated 4/5 of the good chipping progeny, while identifying 8 of 11,714 genotypes combining visual merit and cold chipping potential. Increased genetic variation and sexual polyploidization are possible strategies for increasing the number of desirable genotypes. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Combining ability and correlations for yield components in early generations of potato breeding

A partial diallel set of crosses was made between 14 potato cultivars chosen for their fertility, from those included in a potato breeding programme at the NEIKER – Basque Institute for Agricultural Research. The progeny were grown in completely randomized trials from 1997 to 1999. Performance for yield, tuber number and average tuber weight was analysed in seedling and two clonal generations. Variance estimates due to both general combining ability (GCA) and specific combining ability (SCA) were significant in all generations for all traits under study. However, SCA was more important than GCA in almost all cases. Correlation coefficients among characters, generations, GCA and SCA effects were examined. For tuber yield no relation was obtained between generations; however, average tuber weight and yield were positively associated in all generations. The results indicate that appropriate selection criteria depend strongly on the particular cross. The implication for a breeding strategy are discussed.     

Reciprocal effects in true potato seed breeding in short-day length environments

Reciprocal crosses may have significantly distinct performances regarding tuber characteristics in potato, suggesting the importance of cytoplasmic effects in this crop. The selection of parents for true potato seed breeding therefore needs to consider this potential effect when determining the direction of a cross. The aim of this research was to determine whether a broad‐based true potato seed breeding population, developed at the Centro Internacional de la Papa, could be affected by cytoplasmic effects in the short‐day length environments of the tropics. Two random sets of reciprocal biparental crosses were included in the first set of experiments, which were grown in two contrasting Peruvian locations. Only one out of 14 reciprocal crosses showed significantly distinct performance for tuber yield and tuber set. In the second set, only one of each of the 12 reciprocal crosses had distinct performance for vine earliness, days to flowering and flowering intensity, but four of the reciprocal crosses in the second set showed distinct pollen production. The results suggest that cytoplasmic effects in this breeding population are more important for reproductive characteristics such as pollen production than for tuber yield. This finding is not surprising because male sterility in potato results from the interaction between sensitive cytoplasm and dominant nuclear genes.     

A breeding method for accelerated development of cold chipping clones in potato

Summary The development of potato cultivars having acceptable chipping quality after cold storage is important because it reduces costs to growers while ensuring continuity of supply to processors throughout the year. Over 5100 single-hill progenies were planted to select for cold chippers. A breeding procedure for selecting and evaluating these progenies for cold chipping ability was used, and 38 desirable genotypes were identified. These selections were increased in the second clonal generation and evaluated for yield, specific gravity, and seven chipping treatments of varying storage duration/temperature/reconditioning duration and were compared to standard chipping cultivars Atlantic, Monona, Norchip, and Snowden. Twenty-two selections yielded Norchip the highest yielding cultivar, while 15 selections had a specific gravity Atlantic, the highest specific gravity cultivar. Snowden was the best chipping cultivar overall and some selections were not significantly different than Snowden. Overall, nine selections combined high yield and specific gravity with the ability to produce attractive chips from 4° C. Single-hill selection for cold chipping could potentially save four years in the breeding process by the identification of good parents, the recycling of good parental cross combinations, and the evaluation of progeny for chipping earlier in the breeding program.     

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.     

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.     

On the number and size of cross combinations in a breeding programme of self-fertilizing crops

Summary The number and size of crosses in a breeding programme were discussed to the conclusion that the number of crosses rather than the size of a cross should be increased with a given total population size. The advantage due to this manipulation, however, is relatively small when such genotypes as improved for many (twenty-five or more) loci are to be obtained, or when a high proportion (around 0.1 or more) of available combinations have the potentiality of releasing the objective genotypes. In such a case, the ease with which crossing and cultivation can be carried out becomes the decisive factor.     

Genomic prediction for yields,processing and nutritional quality traits in cultivated potato (Solanum tuberosum L.)

Current potato breeding approaches are hampered by several factors including costly seed tubers, tetrasomic inheritance and inbreeding depression. Genomic selection (GS) demonstrated interesting results regardless of the ploidy level, and can be harnessed to circumvent these problems. In this work, three GS models were evaluated using 50,107 informative SilicoDArT markers and 11 traits in two values for cultivation and use (VCU) potato trials. Two key breeding problems modelled included predicting the performance of (i) new and unphenotyped clones (cross‐validation) and (ii) a VCU using another as training set (TS). GS models performed comparably. Cross‐validation accuracy was high for D35, D45, DMW and BVAL, in ascending order. Prediction accuracies of the VCUs were highly correlated, but the best prediction was obtained for the smaller VCU using the bigger as TS. Cross‐validation and VCU prediction accuracies were higher when bigger TSs were used. The findings herein indicate that GS can be attractively integrated in potato breeding, particularly in early clonal generations to predict and select for traits with low heritability which would otherwise require more testing years, environments and resources.     

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.