Isolation and characterisation of somatic hybrids of diploid Solanum tuberosum and Solanum brevidens and the use of amylose-free starch mutation for detection of introgression

Summary Somatic hybrids of diploid amylose-free (amf) Solanum tuberosum and diploid S. brevidens were made by Poly-Ethylene-Glycol (PEG) or electrofusion methods. For the isolation of interspecific hybrids the use of selection markers (kanamycin and hygromycin resistance) was useful but not essential. In this 2x+2x interspecific combination 4x and 6x somatic hybrids were obtained. Seed set was the best in 4x×4x (S. tuberosum) backcrosses, but seed germination was the best in 6x×4x combinations, using in vitro germination of unripe seeds harvested 25 days after pollination. A high degree of pollen stainability (30–40%) was observed in 7 tetraploid hybrids and very low in all hexaploids. After iodine staining, the recessive amf marker was expressed by a red colour instead of blue, visible in starch-containing cells like columella cells of root tips, (micro)tubers or microspores. As expected, complementation was observed in starch-containing cells of the fusion hybrids. Segregation of the amf marker was clearly observed in microspores of 4x and 6x hybrids. Segregation ratios in the 4x hybrids showed variable recombination frequencies. In the backcross progeny of hexaploid F12-5 with a tetraploid amf mutant one amylose-free recombinant among 67 plants was found, indicating the occurrence of meiotic recombination in the megaspore mother cells.     

Identification of somatic hybrids of dihaploid Solanum tuberosum lines and S. brevidens by species specific RAPD patterns and assessment of disease resistance of the hybrids

Summary Symmetric somatic hybrids were produced by electrofusion of protoplasts of two dihaploid tuber-bearing potato (Solanum tuberosum L.) lines and Solanum brevidens Phil., a diploid non-tuber-bearing wild potato species. A total of 985 plants was obtained. Verification of nuclear hybridity of putative hybrids was based on additive RAPD patterns, general morphological characteristics and chromosome counts. 53 (90%) calli regenerated into plants which were identified as somatic hybrids. Most of the hybrids were aneuploids at the tetraploid (4×) or hexaploid (6×) level. The 20 hybrids tested expressed a high level of resistance to potato virus Y (PVY ^N ) characteristic of the S. brevidens parent. Resistance to late blight (Phytophthora infestans (Mont.) de Bary) varied between hybrids, but was on average better than that of the fusion parents. Resistance of hybrids to bacterial stem rot (Erwinia carotovora subsp. atroseptica (van Hall) Dye) was not superior to that of commercial potato cultivars.     

Transfer of resistance to Verticillium dahliae Kleb. from Solanum torvum S.W. into potato (Solanum tuberosum L.) by protoplast electrofusion

Summary Interspecific somatic hybrid plants were regenerated after electrofusion of mesophyll protoplasts with the objective of transferring resistance to Verticillium dahliae from Solanum torvum into potato. Early selection of the putative hybrids was based on differences in cultural behaviour of the parental and hybrid calli (particularly the ability of the latter to regenerate early) in combination with morphological markers. Four putative hybrids were recovered from hundreds of calli, probably resulting from complementation of the two parental genomes. The regenerates were tetraploids (2n=4×=48 chromosomes) and exhibited intermediate traits including leaf form, plant morphology and the presence of anthocyanin. The hybrid nature of the four selected plants was confirmed by examining isoenzyme patterns for isocitrate dehydrogenase (Idh), malate dehydrogenase (Mdh), phosphoglucoisomerase (Pgi) and 6-phosphogluconate dehydrogenase (6-Pgd). While the hybrid plants rooted readily and grew vigorously under in vitro conditions, in the greenhouse their development and growth were retarded by difficulties in rooting. When grafted on potato or S. torvum rootstocks, the hybrid plants recovered normal development and growth. Again, they exhibited intermediate morphological traits. Tests for resistance realized in vitro with medium containing 50% Verticillium wilt filtrate showed that all the somatic hybrids were resistant to the fungus filtrate.     

Assessment of genetic and phenotypic variation among intraspecific somatic hybrids of potato, Solanum tuberosum L.

Intraspecific somatic hybrids have been produced by protoplast fusion in eight combinations involving 10 dihaploids (2n= 2x= 24) in an attempt to provide new material for potato breeding. Cytological analysis revealed extensive variation in chromosome number, such as aneuploid, aneusomatic and mixoploid hybrids. Most of the hybrids represented the expected chromosome number of 48; however, the frequency of aneuploids reached 50% in some combinations. Some hybrids carried structurally rearranged chromosomes and exhibited a high frequency of aberrant anaphases. Isozyme and random amplified polymorphic DNA (RAPD) patterns of the hybrids from the same fusion combination were uniform. In the field, somatic hybrids showed wide phenotypic variation in 20 morphological characters. There was a significant correlation between certain leaf characters and the ploidy level, which may be used to distinguish the tetraploid hybrids from hexaploids and octoploids. Tuber yield and flowering intensity were highest in tetraploid hybrids (2n= 4x= 48). Eighteen of the 73 hybrids reached higher yields than the standard variety ‘Adretta’. Floral development and fertility were restored in hybrids derived from fusions between non-flowering or sterile dihaploids.     

Somaclonal variation for resistance to Verticillium dahliae in potato (Solanum tuberosum L.) plants regenerated from callus

Summary Plant tissue culture is recognized as an important tool to generate useful genetic variability for crop improvement. Regenerated plants from callus induced from stem explants of Solanum tuberosum cv Désirée were assessed by in vitro selection, for resistance to Verticillium dahliae. This fungus is the causal agent of Verticillium wilt, a serious vascular wilt disease both in crops and wild species.The rate of in vitro multiplication by single node cuttings was used as a parameter of screening in two selection cycles with different concentrations of V. dahliae filtrate. One resistant clone was selected and then evaluated by inoculation in the growth chamber. Induced damage, and morphological traits (dry weight, leaf area and tuber production) were estimated. The selected clone was comparable to the resistant control, cv Kondor.The results suggest that genetic variation induced in tissue culture cound be utilized to generate disease resistance.     

Comparison of diploid and tetraploid potato families derived from Solanum phureja x dihaploid S. tuberosum hybrids and their vegetatively doubled counterparts

Summary To determine whether in potatoes the tetraploid level is preferable to the diploid level, especially regarding tuber yield, four diploid (2n=2×=24) Solanum phureja x dihaploid S. tuberosum hybrid parents and their vegetatively doubled, tetraploid (2n=4×=48) counterparts were intermated, which resulted in two F₁ hybrid families at both levels of ploidy. The parents and clones of the F₁ families and their offspring were used in crosses in such a way that in addition Sib₁, Sib₂, F₁×Sib₁, BC₁ and Sib₁×Sib₁ families were produced. Of the first clonal generation of the 12 2 x families and their 12 counterpart 4 x families two tubers per clone were planted in three replications in a field experiment at Sturgeon Bay in 1969; of the parents six tubers were planted in each replication. Data were recorded on 16 characters, including plant height at four different times.The ANOVA's showed significant clone effects within each family for all characters. Computed from all family means as well as from the family means per ploidy level, differences due to family were also significant for all characters except one.As at the 2 x level and at the 4 x level of ploidy the mean phenotypic correlations between characters were of similar magnitude, it is concluded that they are independent of ploidy level.With the exception of eye depth, the mean coefficient of variation was greater at the 2 x than at the 4 x level of ploidy, indicating the greatest response to selection for those characters at the 2 x level. From the differences in family means between the 4 x and the 2 x level of ploidy it was apparent that the 4 x families generally had significantly taller plants, later maturity, fewer tubers, higher mean tuber weight, more tuber yield and more dry matter yield than their counterpart 2 x families. In contrast, the 4 x parents had on the average shorter plants, lower mean tuber weight, much lower tuber yield and lower dry matter yield than their 2 x counterparts.The phenotypic correlation and Spearman's rank correlation between the family means of the 2 x and the 4 x level of ploidy were positive for almost all characters and significant for nearly half of them.From the results it is concluded that 1. in potatoes the 4 x level of ploidy is preferable to the 2 x level, and 2. the performance of 4 x families is predictable from the performance of their counterpart 2 x families.Based on results mentioned in the literature and on the present results, a continued use of S. tuberosum dihaploids in potato breeding needs to be dissuaded.     

Evaluation of seedling selection for salinity tolerance in potato (Solanum tuberosum L.)

Summary Salt-tolerance in potato (Solanum tuberosum L.) was selected on the basis of germination and survival of seed sown in trays of perlite suspended in either 75 or 150 mM NaCl. Salinity reduced the germination of seed. Genetic differences in salt-tolerance were apparent with salt reducing germination more in seed collected from cv. Cara than in that collected from cv. Maris Piper. Progeny from the seedling selection were then grown to maturity to produce tubers.The relative tolerance of the parental cultivars and of unselected and selected progeny to long-term exposure to salinity was examined in a pot experiment in which plants were irrigated with either fresh water or 50 mM NaCl solution from one week after plant emergence. In this experiment, salinity significantly reduced leaf conductance, total dry matter production and partitioning of assimilate to tubers. Salinity reduced dry matter production and assimilate partitioning to tubers to a greater extent in Cara than in M. Piper. Progeny selected for short-term salt-tolerance did not exhibit greater long-term salt-tolerance than unselected progeny, and both were more sensitive than M. Piper.These results demonstrate genetic variation in salt-tolerance in potato. However, although there was a correlation between the performance of the parent to long-term salinity and survival of progeny in the seedling selection, there was no correlation between short- and long-term salt-tolerance. This suggests that characters underlying short-term tolerance may contribute to long-term tolerance but do not of themselves confer long-term tolerance. Future progress in selecting for improved salt-tolerance depends on understanding the effects of salinity on the physiological processes underlying growth and carbon partitioning.     

Introgression of the low browning trait from the wild Mexican species Solanum hjertingii into cultivated potato (S. tuberosum L.)

Internal discoloration of tubers resulting from impact damage (blackspot bruise) is a serious quality problem in potato production and utilization, reducing profits to growers and increasing costs for processors. Resistance to blackspot bruise has been identified in the wild species Solanum hjertingii and is therefore a potential germplasm resource for genetic resistance to this problem. A bridging cross between S. hjertingii and a cultivated diploid clone was used to produce a triploid hybrid population that exhibited very low tuber browning potential, indicating a dominant pattern of inheritance for this trait. The triploid progeny were subjected to in vitrochromosome doubling and the resulting hexaploid clones were screened for browning potential. A hexaploid clone selected for low browning was reciprocally crossed with cultivated S. tuberosum cultivars exhibiting high susceptibility to blackspot bruise. Tubers obtained from the seed progeny of these 4x-6x crosses (hereafter referred to as the BC₁ populations) were evaluated for browning potential and polyphenol oxidase (PPO) activity. Tubers from the BC₁ populations displayed a very low potential for melanin production, while PPO activity was quite variable. The low Pearson correlation coefficient (r² = 0.45), between browning potential and PPO activity suggests that the mechanism of blackspot bruise resistance derived from S. hjertingii cannot be explained simply as a reduction in the initial PPO activity. The expression of substantial resistance to browning and dominant expression pattern in these BC₁ progeny indicate that utilizing genetic elements derived from S. hjertingii provides a robust approach for developing blackspot bruise resistant potato varieties. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fertile somatic hybrids of Solanum etuberosum (+) dihaploid Solanum tuberosum and their backcrossing progenies: relationships of genome dosage with tuber development and resistance to potato virus Y

The wild non-tuberous species Solanumetuberosum is resistant to biotic andabiotic stresses, but is very difficult tocross with cultivated potato. Therefore,interspecific somatic hybrids between adihaploid clone of potato S.tuberosum (2n=2x=24, AA genome) and thediploid species S. etuberosum(2n=2x=24, EE genome) were produced byprotoplast fusion. Among the 7 fertilefusion hybrids analysed by genomic insitu hybridisation (GISH), three groups ofplants were found with the genomicconstitution of AAEE, AAEEEE and AAAAEE.Four fusion hybrids had exactly theexpected chromosome composition, while eachof the three aneuploid hybrids had lost twochromosomes of S. etuberosum. Twobackcross progenies were developed, andGISH analysis was applied to analysetransmission of the parental chromosomesinto the sexual generations. BC₁hybrids derived from the crosses of thehexaploid somatic hybrids with tetraploidpotato were pentaploid with thetheoretically expected genomic compositionor with slight deviation from thisexpectation. In the three BC₂ hybridsanalysed by GISH seven to 12 chromosomes ofS. etuberosum were detected in thepredominant S. tuberosum background.No recombinant chromosomes in the hybridswere detected. Genome dosage affects tuberformation in hybrids and their progenies,but has less effect on resistance to potatovirus Y (PVY) in fusion hybrids. Severalgenotypes of the fusion hybrids andBC₁ progeny did not show viralinfection even in the graftingexperiments.     

Failure in an interspecific cross between Solanum khasianum Clarke and Solanum mammosum L.

Summary Attempts to obtain a successful interspecific cross between Solanum khasianum Clarke and Solanum mammosum L. were unsuccessful, and the probable reasons for this failure were investigated. A reduced number of viable pollen grains and low receptivity of the stigmas of S. mammosum L. and heterostyly in both species are the probable reasons for the failure of the attempted interspecific crosses between S. khasianum Clarke and S. mammosum L.     

Female fertility of potato (Solanum tuberosum ssp. tuberosum) dihaploids

Summary Results of testcrosses and seed set data of the dihaploid breeding program at the Potato Research Institute in Gross Lüsewitz, Germany, indicate that the low level of female fertility could hinder success in crossing work. With the objective to reveal major components of female fertility, the behavior of 47 outstanding potato dihaploids (S. tuberosum ssp. tuberosum, 2n=2x=24) was assessed in testcrosses over a period of 3 years. Analysis of variance was carried out for the traits: number of seed per berry (S/B), number of intact seed per berry (IS/B), and number of non-germinable seed per berry (NGS/B). Significant sources of variation included cross combinations, years, and cross combination-year interactions. More than 20 S/B were obtained in 25.4% of all cross combinations performed over 3 years using up to three pollinators. Berry set was not correlated to seed set, but was in general determined by the pollinator used. The number of seedless berries (SLB/F) was related to neither total berry set nor seed production. The trait S/B remained stable over different years, and thus it deemed to be most useful as a measure of the level of female fertility in practical breeding work. Selection for female fertility in early generations of a breeding program is recommended to avoid limitations in the production of variability via sexual recombination.     

Reciprocal differences between hybrids of Solanum tuberosum Groups Tuberosum (haploid) and Phureja

Summary Yield components including total tuber yield, total tuber number, vine maturity, vine vigor and average tuber weight as well as seed germination and pollen stainability were studied in 28 exact reciprocal hybrid families, between 12 Solanum tuberosum Group Tuberosum (haploid) (TH) and 12 S. tuberosum Group Phureja (PH) clones in a NC Mating Design II in 4 sets of 3 by 3 reciprocal hybrids at three locations, one in Minnesota (USA) and two in Morocco. TH parents were insensitive to daylength whereas most PH clones required critical photoperiods equal to or less than 14 hours for tuberization. Mean reciprocal differences were significant for most traits at all 3 locations. Hybrids with TH cytoplasm were superior to those with PH cytoplasm by 18% for tuber yield, 21% for tuber number, and 9% for vine maturity, but inferior to hybrids with PH cytoplasm by 11% for vine vigor and 19% for time to reach 80% seed germination. Reciprocal differences were not detected for average tuber weight. Six reciprocal crosses revealed significant differences for pollen stainability but the overall mean difference (one location) was not significant. Possible reasons for the observed reciprocal differences included gamete selection, pseudo-self-compatibility and/or interaction of cytoplasm with both nuclear genes and environment. Differences for critical photoperiod of TH and PH parents were also demonstrated as a probable source of reciprocal differences.Scientific Journal Series Article 14896 of the Minnesota Agricultural Experiment Station supported in part by the U.S. Agency for International Development, contract no. 608-0160, IAV Morocco Project.     

Resistance to potato late blight (Phytophthora infestans (Mont.) de Bary) in Solanum nigrum,S. villosum and their sexual hybrids with S. tuberosum and S. demissum

Summary Solanum nigrum and S. villosum, and their sexual hybrids with S. tuberosum and S. demissum respectively, were inoculated with a complex race of Phytophthora infestans. No visible reaction was seen on S. villosum and one genotype of S. nigrum. Another genotype of S. nigrum and the hybrids showed a hypersensitive response on most inoculated leaves. In one experiment, some sporulation was observed on detached leaves of a hybrid derived from S. nigrum. Microscopical examination of infections in S. nigrum and in a hybrid from S. nigrum, showed that penetration of epidermal cells and subsequent intercellular growth of the pathogen into the spongy mesophyll occurred, but without the formation of haustoria, and that invaded and neighbouring cells became necrotic. Callose appositions were found in epidermis and mesophyll cells of all inoculated genotypes, and also in epidermal cells of the unrelated nonhost species Brassica campestris.     

Interspecific hybrids between three eggplant (Solanum melongena L.) cultivars and two wild species (Solanum torvum Sw. and Solanum sisymbriifolium Lam.)

Three Greek eggplant cultivars, ‘Langada’, ‘Tsakoniki’ and ‘Emi’ (2n= 24), were crossed with two wild species (Solanum torvum Sw., 2n= 24 and Solanum sisymbriifolium Lam., 2n= 24). Ovules isolated 15-27 days after pollination were cultured in a modified MS medium at 24°C and a 16h photoperiod. Fifty days later, the ovules were dissected and the interspecific embryos were cultured in the same medium. Interspecific hybrids were achieved only from crosses between the eggplant cultivars and S. torvum. The hybridity of the putative interspecific F¹ hybrid (Solanum melongena×S. torvum) was confirmed by using morphological and biochemical (isozyme isocitrate dehydrogenase A, phosphoglucomutase A, phosphoglucose isomerase B, 6-phosphogluconate dehydrogenase A, 6-phosphogluconate dehydrogenase B) markers. The F¹ plants (‘Langada’×S. torvum) were selfpollinated and backcrossed to both parents. Fruits, however, were produced only when the F¹ hybrid was backcrossed as female with the eggplant cultivar ‘Langada’.     

Multivariate analysis of variation among Solanum commersonii (+) S. tuberosum somatic hybrids with different ploidy levels

In order to integrate the outcomes of interspecific somatic fusion experiments in breeding schemes, it is important to understand factors involved in the variability observed among regenerated plants. With such a purpose in mind, a population of Solanum commersonii (+) S. tuberosum somatic hybrids was examined by means of discriminant and cluster analyses. Data were collected on 56 hybrids with different ploidies and on the two diploid parents grown in a greenhouse. The ploidy group was used as discrimination criterion. Three significant canonical variables were extracted by discriminant analysis; they were mainly correlated with the number of leaflets and stems, degree of flowering, plant height, and leaf length. After cluster analysis carried out with the significant canonical variables, parental and hybrid clones were grouped in 7 clusters. In the canonical space of reduced dimensions, patterns of morphological variation depended mainly on ploidy level and non-additive gene interactions. Hybrids were in general more similar to the cultivated than to the wild species, suggesting a good chance of fast introgression of useful traits from S. commersonii into the S. tuberosum genetic background.     

Production of somatic hybrids between S. tuberosum L. and late blight resistant Mexican wild potato species

Interspecific somatic hybrids between dihaploid breeding clones of potato, S. tuberosum and two accessions of wild Mexican species S. pinnatisectum and the hybrid line S. pinnatisectum × S. bulbocastanum were regenerated following electrofusion of mesophyll protoplasts to combine important agricultural traits of S. tuberosum and a high level of late blight resistance from selected wild accessions. In two fusion combinations 239 calli were regenerated; 162 from 195 calli analysed were identified as hybrids by means of isozyme analysis of peroxidases and, for some hybrid clones, by RAPD analysis. Depending on the fusion combination, 47–89 percent of the somatic hybrids had the expected ploidy level and 7–16 percent were mixoploids. Somatic hybrids were phenotypically intermediate as compared to their parents and some of them were able to be backcrossed sexually with potato. Fertility and crossability depended on combination and ploidy level of the somatic hybrids. In tests with detached leaves the wild partner clones had a high late blight resistance score of 8,6 and 8,9; the susceptible tuberosum-partners of 2,8 and 3,5, respectively. Nearly 25 percent of somatic hybrids had a resistance level of 6 or higher in the first year of assessment. The average resistance value of most somatic hybrids was lower than the average parental level. The reasons for variation in resistance values are discussed in connection with the practical application of fusion hybrids. This revised version was published online in July 2006 with corrections to the Cover Date.     

The partial stability of additional chromosomes in Solanum tuberosum cv. Torridon

Summary Commercial cultivars of the potato (Solanum tuberosum L.) are widely believed to be cytologically invariable (2n=48), although there are several reports of anomalous counts in experimental material. The presence of two additional chromosomes is reported for the first time in cv. Torridon and their stability in the growing plant examined. The additional chromosomes were found to be completely stable in the shoots but were progressively eliminated from the root tissue. Laggards and bridges were commonly observed during somatic division in the roots. Laggards were also present in the shoots and did not change significantly in frequency in the roots during chromosomal loss. Consequently, they were considered as unlikely to have a central involvement in the elimination process. Somatic bridges on the other hand were only found in the roots and increased in occurrence as elimination increased. A link between the appearance of somatic bridges and the elimination of additional chromosomes in the roots is suggested. The possible origins and mechanism of elimination of the additional chromosomes are briefly discussed.     

Induction of novel organelle DNA variation and transfer of resistance to frost and Verticillium wilt in Solanum tuberosum through somatic hybridization with 1EBN S. commersonii

Somatic hybridization can be used to induce genetic variability in plastidial and mitochondrial genomes, and transfer multiple uncloned genes across sexual barriers. Somatic hybrids were produced between a dihaploid clone of the common potato, S. tuberosum subsp. tuberosum, and the wild sexually incongruent diploid species S. commersonii. Fusion products were selected on the basis of callus growth and regeneration in vitro. Genome composition of putative somatic hybrids was determined by flow cytometric analysis of nuclear DNA content, RAPD analysis, and Southern analysis with probes specific to organellar DNA. All regenerated fusion products proved to be hybrids based on RAPD analysis. Seventy per cent of somatic hybrids were (near) tetraploids, 22% (near) hexaploids and 8%(near) octoploids. A high correlation was found between the nuclear DNA content determined by flow cytometry and the number of chloroplasts in stomata guard cell pairs. Somatic hybrids inherited the parental plastids in a random manner. On the contrary, they preferentially inherited the mitochondrial DNA fragments of S. tuberosum. The majority of them had a rearranged mitochondrial genome with fragments from both parents. Hybrids were highly vigorous and morphologically more similar to the cultivated than to the wild parent, produced tubers on long stolons under long photoperiod conditions, showed a high degree of flowering, but did not produce pollen. In addition, somatic hybrids were generally more resistant to frost and Verticillium wilt than the cultivated parent, indicating the introgression of relevant resistance genes from the wild species into the genetic background of S. tuberosum. This revised version was published online in July 2006 with corrections to the Cover Date.     

An assessment of morphogenic and transformation efficiency in a range of varieties of potato (Solanum tuberosum L.)

Summary To provide a truly genotype-independent transformation system, it is necessary to be able to transform a wide range of potato genotypes. The ability to regenerate shoots in vitro was determined for 34 potato varieties using tuber disc explants. Following a culture regime used extensively in previous studies with the variety Desiree, half of the varieties could be regenerated from tuber discs and half could not. From a sample of varieties that could be regenerated from tuber discs, all but one variety gave transgenic plants. Twelve varieties were evaluated for the capacity to regenerate shoots from leaf and internode explants excised from in vitro grown plants. All of the varieties tested regenerated adventitious shoots. Leaf and internode explants from 5 varieties were subsequently used for transformation, and transgenic plants were produced from two potato varieties that did not give transgenic plants from tuber disc explants. Some varieties could not be transformed by either method, and will require modification of the in vitro regeneration and transformation system to be successful.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - GA₃ gibberellic acid - IAA indole-3-acetic acid     

Analysis of an incomplete diallel cross among three ssp. tuberosum varieties and seven long-day adapted ssp. andigena clones of the potato (Solanum tuberosum L.)

Summary The main aim of the experiment was to study the behaviour of seven long-day adapted clones of Solanum tuberosum ssp. andigena (A) and three varieties of S. tuberosum ssp. tuberosum (T) in an incomplete diallel cross. The four groups of crosses, TxT, TxA, AxT and AxA, held 2, 12, 20 and 21 populations, respectively. Eleven of the intersubspecific crosses were exact reciprocals. Thirty random clones per population were grown as second clonal generation in an incomplete block design with four replications in plots of two plants per clone in 1980, the parents included in many plots. Data were recorded on ten characters.The results showed that group TxA gave the highest tuber yield and AxT the second highest. The F-values indicated highly significant genotypic effects for all characters in all populations. The mean genotypic coefficients of variation (GCV), were the highest for group AxA for all characters. The ANOVA, based on population means showed significant mean squares for general combining abilities (GCA) and for differences of reciprocalls for all characters; the specific combining ability (SCA) mean squares were significant only for date of emergence, plant height and haulm type. Almost all GCA effects of the parents were significant positive or negative. Several populations showed also significant SCA effects. The correlations between the GCA effects and the parental values were of about the same magnitude as the heritabilities; those between the population means and the midparental values were slightly lower.Compared to the midparental values, most populations showed significant positive heterosis for date of emergence, haulm type, number of tubers and under water weight; all populations had significant negative heterosis for mean tuber weight and nearly all for plant height and general impression. All populations from groups TxT and AxA had significant negative heterosis for tuber yield and so had 13 out of the 20 populations from group AxT, while seven of the 12 from group TxA showed significant positive and four significant negative heterosis. The average heterosis for tuber yield of group TxT was –11%, of TxA 3.4%, of AxT–9% and of AxA–18%. The highest mean tuber yields were obtained from crosses with a very high yielding A-parent as female, while the heterosis was negative. Though significant differences between the 11 reciprocals occurred for all characters, consistent differences occurred only for tuber yield. Averaged over the 11 crosses the TxA populations outyielded their exact reciprocals by 10.7%. For the highest tuber yields in A-T crosses, therefore, the tuberosum cytoplasm seems to be preferable. By contrast for a high male fertility the andigena cytoplasm is preferable.