Multitrait selection system using populations with a small number of interploid (4x-2x) hybrid seedlings in potato: Degree of high-parent heterosis for yield and frequency of clones combining quantitative agronomic traits

One of the most attractive features of the 4x-2x breeding scheme via unilateral sexual polyploidization (USP) is the possibility of obtaining heterotic clones with a combination of desirable traits based upon selection in a small number of hybrid seedlings. A set of experiments was carried out to verify this characteristic of the USP scheme using 42 families (with 20 plants each) derived from 2x Phureja-haploid Tuberosum or haploid Tuberosum-Solanum chacoense hybrids as male parents. These clones were 2n-pol-len-producers by either first-division restitution with crossing over (FDR-CO) or without crossing over (FDR-NCO). The 4x parents were eight cultivars of USA and European origin. A total of 168 out 840 clones was initially selected (four clones/family) based on general tuber appearance (GTA). An additional round of selection (not taking into account the parentage of the clone) for total tuber yield (TTY) reduced the sample to 96 clones. These selected clones and the original 4x parents were evaluated in two locations at Wisconsin (Hancock – E?1, and Rhinelander – E?2). The average high-parent heterosis values for TTY ranged from 27.8% (E?2) to 48% (E?1). The population of experimental clones also had a TTY range wider than that of the 4x parents. The best clone gave a yield of 101.6% and 63.2% over the best 4x parent at E?1 and E?2, respectively. For TTY, a significant genotype×environment (G×E) interaction for the experimental clones was found when a combined analysis of variance for both locations was carried out. However, the G×E interaction was not significant when only the 4x-parent group was considered. Clones derived from 4x European cultivars had higher GTA scores than clones derived from 4x USA cultivars. With two culling levels being set on the TTY≥4x parent group mean and the GTA≥three, about 56% (E?1) and 48% (E?2) of the clones would be retained for further evaluation. These percentages of selected clones are much higher than those reported using conventional 4x-4x crosses. Our results indicate that a USP strategy with 4x-2x (FDR) crosses would be more effective than intra-Tuberosum crosses in generating heterotic clones with a combination of desirable quantitative traits using populations with a small number of hybrid seedlings.     

Direct estimation of the effects of meiotic recombination on potato traits via analysis of 4x-2x progenies from synaptic mutants with 2n-pollen formation by FDR without crossing-over

Diploid potato clones with 2n-pollen formation by first-division restitution without crossing-over (FDR-NCO) are ideal testers to estimate the breeding value of elite 4x cultivars by virtue of transmitting their genotypes practically intact to their progenies. This characteristic facilitates genetic analysis, since meiotic recombination would take place only in the 4x parent and not in the diploid parent. We evaluated (under short-day conditions) families from complete factorial crosses between four 4x cultivars and five 2x(FDR-NCO) clones. Families were compared with two standard 4x cultivars (’Bintje’ and ’Delta’) for total tuber yield (TTY), commercial yield (CY), haulm maturity (HM), plant vigor (PV), plant-top uniformity (PU), eye depth (ED), number of tubers per hill (NTH), and the CY/TTY index (CTI). For TTY, the contrasts family group (310 g/ hill) vs ’Delta’ (430 g/hill) and the family group vs ’Bintje’ (210 g/hill) were significant. Only 25% of the families were different from ’Delta’ and 20% of them outyielded ’Bintje’. For CY, differences were observed between families (240 g/hill) vs ’Delta’ (340 g/hill) and families vs ’Bintje’ (150 g/hill). The two best families had 53% CY over ’Bintje’. Surprisingly, only one family had a higher NTH than ’Bintje’. No differences were observed for HM. Seventy five and 30% of the families had an ED similar to ’Delta’ (ED = 2) and ’Bintje’ (ED = 1), respectively. A multivariate analysis indicated that 63% of the data variability could be explained by two factors. TTY, CY, and PV had high loading on the first factor, whereas ED, PU and HM had high loading on the second factor; CTI and NTH had equal sizes on both factors. High TTY and PV were associated with high NTH and CTI. Deep eye, PU, and late maturity were associated with high NTH and reduced CTI. The distributions of factor scores of the entries indicated that some 2x parents had strong influences (irrespective of the direction of their effects) on the crosses. Six crosses due to two 2x males were in the ’Bintje’ quarter with negative scores for both factors (implying low TTY, poor vigor, and low NTH). Also three crosses due to another 2x clone were distributed in the quarter of positive factor 1 and negative factor 2. These crosses plus another one were in the same quarter of ’Delta’ (implying high yields, low ED, low PU, and early maturity). The FDR-NCO clones provide a homogeneous sample of heterozygous 2n-gametes allowing the unique opportunity to estimate the relative contribution of the random meiotic products (from the 4x parents) and the ’somatic’ 2x genome for the phenotypic expression of quantitative traits. The interesting result was that measurable effects (favorable or not) on the data variability were mainly determined by the genomic contribution of the haploid-species hybrids. Three out of five 2x-male parents showed rather strong effects on progenies. No such effects were observed on the four 4x-female parents. Received: 3 September 1999 / Accepted: 24 November 1999     

Effects of 2n-pollen formation by first meiotic division restitution with and without crossover on eight quantitative traits in 4x-2x potato progenies

 The 2n-pollen grains formed by first-division restitution without crossover (FDR-NCO) are unique breeding tools, since they can transmit almost 100% of non-additive genetic effects from the parent to the progeny. FDR-NCO gametes are considered superior to those formed by FDR with crossing over (FDR-CO), which can pass on to the progenies approximately 80% of the heterozygosity and a large fraction of the epistasis. However, 2n-pollen formation by FDR-NCO mechanism requires the incorporation (in homozygous condition) of at least two recessive alleles. In the present work, 40 tetraploid families derived from complete 4x-2x factorial crosses were evaluated under short-day conditions to verify whether or not the postulated genetic superiority of FDR-NCO over FDR-CO gametes holds true for eight quantitative traits in potato. Families were derived from crosses between four 4x commercial cultivars, and a random sample of ten diploid Solanum phureja-haploid S. tuberosum hybrids producing 2n-pollen by either FDR-CO or FDR-NCO. The results indicated no significant superiority of FDR-NCO over FDR-CO families for total tuber yield (TTY) and six other traits (haulm maturity – HM; plant vigor – PV; plant uniformity – PU; eye depth – ED; number of tubers per hill – NTH; and commercial over total yield index – CTI). Based upon cytological observations, the FDR-CO and FDR-NCO gametes are expected to be genetically equivalent for all loci between the centromeres and the chromosomal site of maximum recombination. In our experiment, differences between FDR-CO- and FDR-NCO-derived progenies were not observed for TTY. Therefore, our results can be interpreted as additional evidence for the hypothesis that genes with major effect on TTY expression might have a physical location between centromeres and proximal crossovers in the potato chromosomes. In addition, a similar trend was observed for HM, PV, and ED but apparently not for commercial yield – CY (i.e., tubers with more than 33 mm in diameter). Received: 21 September 1998 / Accepted: 26 October 1998     

Resistance to potato virus Y in somatic hybrids between Solanum etuberosum and S. tuberosum x S. berthaultii hybrid

Somatic hybrids between a potato virus Y (PVY) resistant Solanum etuberosum clone and a susceptible diploid potato clone derived from a cross between S. tuberosum Gp. Tuberosum haploid US-W 730 and S. berthaultii were evaluated for resistance to PVY. All but one of the tested somatic hybrids were significantly more resistant than cultivars Atlantic and Katahdin. However, none was as resistant as the S. etuberosum parent. One hexaploid somatic hybrid, possibly the product of a triple-cell fusion involving one S. etuberosum protoplast and two haploid x S. berthaultii protoplasts, was as susceptible to PVY infection as the cultivars. Tetraploid progeny of the somatic hybrids, obtained from crosses with Gp. Tuberosum cultivars, were neither as resistant as the maternal somatic hybrid parent, nor as susceptible as the paternal cultivar parent. It appears that the introgression of PVY resistance from (1EBN) S. etuberosum into (4EBN) S. tuberosum (EBN-endosperm balance number) will be successful through the use of somatic hybridization and subsequent crosses of the somatic hybrids back to S. tuberosum.     

Genetic-cytoplasmic male sterility in progeny of 4x-2x crosses in cultivated potatoes

Summary Forty-three percent of the progeny from 4x × 2x crosses [Group Tuberosum cultivar x diploid hybrid (Groups Phureja or Stenotomum x Group Tuberosum haploid)] were male sterile. In contrast only four percent of the progeny from the reciprocal crosses were male sterile. Male sterility among the former progeny is presumed to result from the interaction of Tuberosum cytoplasm with dominant genes from the cultivated diploids, Groups Phureja and Stenotomum, an interaction known to occur in crosses of Tuberosum haploid x cultivated diploid species (2n = 2x = 24). The frequency of fertile progeny from the 4x × 2x crosses (57%) was significantly higher than that from the 2x × 2x crosses (Tuberosum haploid x cultivated diploid), (28%). The frequency of male fertility among progeny from different cultivars in 4x × 2x crosses varied from 31–82 percent. The difference between cultivars strongly suggests that some cultivars may have dominant male fertility restorer genes.     

The significance of 2N gametes in potato breeding

Summary Phureja-haploid Tuberosum hybrids, which produce 2n gametes in addition to n gametes, were used to obtain diploid progenies in 2x – 2x matings, and tetraploid progeny in 4x – 2x matings. Seven of these diploid clones were intermated in a modified diallel design, and also crossed to seven cultivars, to obtain 21, 16-clone diploid; and 49 (35, 2x X 4x and 14, 4x X 2x) 24-clone tetraploid families, respectively. These progenies were included together with the 14 parental clones, in a tuber yield trial conducted in two locations in Wisconsin. Heterotic responses were striking in 4x X 2x progenies. Three of these entries had mean yields of 5.0, 5.0 and 5.2 lbs/hill, for 24 unselected clones, thus outyielding not only the mid-parent but also the best tetraploid cultivars in the experiment, which are highly selected; Wis 643 (4.8 lbs/hill), and Kennebec (4.8 lbs/hill). As a group, the 14, 4x X 2x families averaged 4.4 lbs/hill, while the mean of all seven cultivars was 4.0 lbs/hill, and the mid-parent was at 3.4 lbs/hill. This unusually high mean performance of 336 unselected clones represented in the 14, 4x X 2x progenies, is interpreted as a manifestation of the capacity of 2n pollen, formed by first meiotic division restitution (FDR), to pass onto the progeny the already heterotic diploid genotype in a largely intact array. Beneficial intra and inter-locus interactions are presumably compounded upon syngamy with an unrelated n egg from the tetraploid parent. -The performance of 2x X 4x progenies was at or below that of the midparent. The failure of these families to perform as well as 4x X 2x families may be a reflection of the incapacity of 2n megasporogenesis to avoid meiotic reassortment as efficiently as FDR does, which would result in inbred 2n gametes. However, the method of 2n megasporogenesis is not known.Hereditary variances were large, both within and among families, and indicated considerable nonadditivity in the action of genes affecting tuber yield at both levels of ploidy. Inbreeding was strongly depressing at both ploidy levels.     

Somatic hybrids between Solanum etuberosum and diploid,tuber bearing Solanum clones

Electrofusion was used to obtain somatic hybrids between Solanum etuberosum (2n=2x=24) and two diploid potato lines. These hybridizations were conducted to determine if haploidxwild species hybrids are better fusion partners than conventional S. tuberosumGp. Tuberosum haploids. Restriction fragment length polymerase (RFLP) analyses of the putative somatic hybrids confirmed that each parental genome was present. The somatic hybrids between S. etuberosum and a haploid S. tuberosum clone, US-W730, were stunted and had curled, purple leaves. In contrast, somatic hybrids between S. etuberosum and a haploidxwild species hybrid (US-W 730 haploidx S. berthaultii), were vigorous and generally tuberized under field conditions. These hybrids were designated as E+BT somatic hybrids. Analyses of 23 E+BT somatic hybrids revealed a statistically significant bias towards the retention of S. etuberosum chloroplasts. Stylar incompatibilities were observed when the E+BT somatic hybrids were used as pollen donors in crosses with S. tuberosum cultivars. Reciprocal crosses did not show this incompatibility. The progeny were vigorous and had improved tuber traits when compared to the maternal E+BT parent. RFLP analyses of three sexual progeny lines confirmed the presence of all 12 S. etuberosum chromosomes. In two of these lines, RFLPs that marked each of the 24 chromosome arms of S. etuberosum were present. However, RFLP markers specific for regions on chromosomes 2, 7, and 11 were missing from the third clone. Because other markers for these chromosomes were present in the progeny line, these results indicated the likelihood of pairing and recombination between S. etuberosum and S. tuberosum chromosomes.     

Fertility of somatic hybrids from protoplast fusions of Solanum brevidens and S. tuberosum

Summary Two sets of somatic hybrids between Solanum brevidens (2x) and S. tuberosum (2x and 4x) were evaluated for male fertility, meiotic regularity and female fertility. The somatic hybrids were tetraploids from 2x + 2x fusions and hexaploids from 2x + 4x fusions. Pollen stainability ranged from 0 to 83% in tetraploids and from 0 to 23% in hexaploids. The tetraploids had more regular meiosis, lower levels of micropollen and fewer unassociated chromosomes than hexaploids. However, except for a low level of selfing, the pollen of both sets of hybrids was ineffective in pollinations. The tetraploids, as females, crossed poorly with 2x and 4x tester species and selfed only at low levels. The hexaploid fusion hybrids also crossed poorly with the 2x tester species and selfed only to a limited degree; however, they crossed well with 4x testers. Seed set in crosses with S. tuberosum Group Andigena, and S. tuberosum Group Tuberosum cultivars Kathadin and Norland averaged 16.7, 15.6 and 28.6 seeds per fruit, respectively. Progeny from these crosses had 5x or nearly 5x ploidy levels. The results indicate that reasonable levels of female fertility can be obtained in somatic fusion hybrids of S. brevidens and S. tuberosum.Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that may also be suitable     

A possible heterotic threshold in the potato and its implications for breeding

Summary Complex hybrids containing genomes from three different Solanum tuberosum Groups were synthesized (3-way hybrids), utilizing 2n gametes in 4x–2x crosses. Ten such families were compared to nine analogous two-Group (2-way) hybrid families and nine (1-way) families representing conventional Gp. Tuberosum breeding materials. The three types of crosses, representing three descending levels of heterozygosity, were placed in four field trials.The 3-way hybrids were never significantly superior to the 2-way hybrids for vigor, yield, or tuber type. When yields were adjusted for maturity differences, the 3-way hybrids tended to be inferior to the 2-way hybrids for yield. This suggests that there may be a heterotic threshold in the cultivated potato, beyond which point more heterozygosity does not result in greater vigor or more yield.While the 2-way and 3-way hybrids did not significantly differ from each other, they both dramatically surpassed the conventional 1-way crosses for vigor and yield (42%). The evidence of a possible heterotic threshold indicates that more sophisticated methods such as cell fusion and bilateral sexual polyploidization may not be necessary to exploit the full potential of the hybrid approach in the potato. A simple and direct 2-way hybridization approach may be optimal, or at least would seem comparable with other hybrid approaches, and is a technology ready for immediate and widespread implementation.     

Rescuing abortive inter-EBN potato hybrids through double pollination and embryo culture

Summary Regeneration of inter-EBN hybrids among potato species was achieved using embryo rescue techniques. Tetraploid hybrids between 4x(2EBN) Solanum stoloniferum x 4x(4EBN) S. tuberosum Gp. Andigena as well as diploid hybrids between 2x(1EBN) S. chancayense x 2x(2EBN) S. chacoense were obtained by culturing immature hybrid embryos in nutrient medium. Identification of appropriate embryo developmental stages was critical in developing a suitable protocol for rescuing viable hybrid embryos. The use of IVP clones as the second pollinator in 4x(2EBN) x 4x(4EBN) crosses reduced premature fruit drop and helped to identify triploid hybrids. Morphological and cytological examination confirmed true hybridity for a few of the regenerated plants. Male sterility and meiotic abnormalities were characteristic of the hybrids. Several S. stoloniferum-Andigena hybrids were successfully backcrossed to Gp. Andigena.Cooperative investigation of Vegetable Crops Research Unit, USDA, Agricultural Research Service, and the Wisconsin Agricultural Experiment Station Note: Reference to a specific brand or firm name does not constitute endorsement by the U.S. Department of Agriculture over others of similar nature not mentioned     

The use of Endosperm Balance Number and 2n gametes to transfer exotic germplasm in potato

Summary A triploid hybrid (2n=3x=36) between a colchicine-induced 4x(2EBN) Solanum brevidens (a non-tuber-bearing species) and 2x(2EBN) S. chacoense (a tuber-bearing species) was used as a vehicle for germplasm transfer to S. tuberosum Group Tuberosum. The use of 2n gametes from the triploid allowed the unique opportunity for transferring exotic germplasm from Series Etuberosa to Gp. Tuberosum material. The triploid hybrid used had a pollen stainability of less than 0.1%. Observations of microsporogenesis revealed that metaphase I pairing configurations were primarily 12 bivalents and 12 univalents with occasional trivalents. Anaphase I separations were irregular, often with lagging univalents. Meiotic observations and pollen morphology suggest that the stainable pollen produced by the hybrid was 2n=3x=36. A single pentaploid hybrid (2n=5x=60) was produced by the fertilization of a rare 2n egg from the triploid with a normal male gamete from the clone Wis AG 231 (2n=4x=48). Limited crosses to other 1, 2 and 4EBN species and cultivars were unsuccessful. The pentaploid hybrid had a more regular meiosis than the triploid and dramatically improved pollen stainability (37% stainable pollen). Stylar blocks prevented estimates of male fertility in crosses. Female fertility in 47 crosses with nine cultivars averaged 19 seeds per fruit. Although S. brevidens is non-tuber-bearing, and the triploid produced only stolons, the pentaploid hybrid tuberized well under field conditions, despite being very late. Results suggest that the tuberization response is a dosage and/or threshold effect. This approach to the incorporation of 1EBN germplasm indicates the utility of the EBN concept coupled with 2n gametes. Further, it demonstrates a means for the introgression of 1EBN species genes into Gp. Tuberosum material.     

A cytogenetic and phenotypic characterization of somatic hybrid plants obtained after fusion of two different dihaploid clones of potato (Solatium tuberosum L.)

Summary Somatic hybrid plants of various ploidy levels obtained after chemical fusion between two dihaploid clones of potato Solanum tuberosum L. have been analysed by cytological, morphological and molecular methods. The hybrid nature of tetraploid and hexaploid plants and the genome dosage in hexaploid hybrids were confirmed by Giemsa C-banding. Tetraploid and hexaploid hybrids showed numerical as well as structural chromosome mutations. The latter occurred mainly in the nuclear organizing chromosome. The tetraploid hybrids were more vigorous than the dihaploid parents as demonstrated by an increase in height, enlargement of leaves, increase in the number of internodes, restored potential for flowering and increased tuber yield. The grouping of tetraploid somatic hybrids into various classes on the basis of leaf morphology revealed that plants with a full chromosome complement were more uniform than aneuploids. Many hexaploid somatic hybrids were also more vigorous than the dihaploid parents and could be grouped into two different classes on the basis of floral colour and tuber characteristics, the differences being due to their different dosage of parental genomes. Most of the tetraploid somatic hybrids showed pollen development halted at the tetrad stage as one of the parental clones contained a S. Stoloniferum cytoplasm. However, one tetraploid plant produced pollen grains with high viability. The chloroplast genome in the hybrid plants was determined by RFLP analysis. All of the hybrids had a cpDNA pattern identical to one parent, which contained either S. Tuberosum or S. Stoloniferum cpDNA. A slight preference for S. Tuberosum plastids were observed in hybrid plants. No correlation between pollen development and plastid type could be detected.     

Use of 2n pollen-producing triploid hybrids to introduce tetraploid Mexican wild species germ plasm to cultivated tetraploid potato gene pool

Summary Tetraploid (2n=4x=48) 2EBN Mexican wild species in the series Longipedicellata, which consists of Solanum fendleri, S. hjertingii, S. papita, S. polytrichon, and S. stoloniferum, were crossed with two 2EBN cultivated diploid (2n=2x=24) clones. The resulting triploid hybrids (2n=3x=36) produced 2n pollen (triplandroids) by the mechanism of parallel orientation of anaphase II spindles. The percentage of stainable pollen in 520 triploids ranged between 0 and 23.5%, with a mean of 2.7%. Triploids producing between 13.0 and 23.5% stainable pollen were crossed as staminate parents to the tetraploid cultivars, resulting in abundant pentaploid (2n=5x=60) and near-pentaploid hybrid progeny. Crosses of triploids with lower percentage of stainable pollen as pollen parent to the tetraploid cultivars did not yield fruit, unless rescue pollen from a tetraploid cultivar was added 2 days later. Pentaploid hybrids were selected among selfed tetraploid progenies using morphological and isoxyme markers transmitted from their cultivated diploid parents. These pentaploid hybrids were vigorous and had uniformly sterile pollen. They were female fertile and were crossed with tetraploid cultivars, yielding an average of 19 seeds per fruit. Triplandroids provide the opportunity of transferring 2EBN tetraploid Mexican wild species in the series Longipedicellata germ plasm into the 4EBN cultivated potatoes.Cooperative investigations of the ARS, USDA, and the Washington State University Agricultural Research Center, Prosser, WA 99350, USA. H/LA Paper No. 90-03, College of Agriculture and Home Economies Research Center, Washington State University, Pullman, WA 99164, USA     

Strategy differences of two potato species in response to nitrogen starvation. Do plants have a genetic switch for nitrogen signalling?

Survival responses to nitrogen starvation are well known in micro‐organisms but little studied in plants. To construct a framework for study of the plant responses, we investigated the strategy differences of tubers from two closely related potato species. Solanum tuberosum conserves tuber nitrogen by inhibiting shoot growth, but S. phureja mobilizes tuber nitrogen to grow shoots, flowers and seeds. Genetic analysis of progeny from S. phureja–haploid S. tuberosum crosses uncovered segregation of a single dominant gene for the S. tuberosum inhibition strategy. Within S. tuberosum, haploid progeny closely resembled their tetraploid parents, suggesting strong genetic control of the inhibition. Growth of the inhibited shoots was proportional to sub‐optimal levels of added nitrate, and was triggered by exogenous gibberellic acid (GA₃). These observations support the notion that potato plants can closely tie shoot growth to ambient nitrogen levels – probably by a root–shoot nitrogen signal transduction pathway, and that this can be overridden by emergency mobilization of nitrogen reserves, perhaps by GA signalling from the tuber. Furthermore, genes for such developmental switches can be identified by classical genetic analysis of closely related species, such as S. tuberosum and S. phureja, that exhibit opposite survival strategies.     

Short‐term hybridisation activates Tnt1 and Tto1 Copia retrotransposons in wild tuber‐bearing Solanum species

Interspecific hybridisation in tuber‐bearing species of Solanum is a common phenomenon and represents an important source of variability, crucial for adaptation and speciation of potato species. In this regard, the effects of interspecific hybridisation on retrotransposon families present in the genomes, and their consequent effects on generation of genetic variability in wild tuber‐bearing Solanum species, are poorly characterised. The aim of this study was to analyse the activity of retrotransposons in inter‐ and intraspecific hybrids between S. kurtzianum and S. microdontum, obtained by controlled crosses, and the effects on morphological, genetic and epigenetic variability. For genetic and epigenetic analysis, S‐SAP (sequence‐specific amplification polymorphism) and TMD (transposon methylation display) techniques were used, respectively, with specific primers for Tnt1 and Tto1 retrotransposon families (Order LTR, Superfamily Copia). The results indicate that at morphological level, interspecific hybrid genotypes differ from their parental species, whereas derived intraspecific hybrids do not. In both cases, we observed significant reductions in pollen grain viability, and a negative correlation with Tnt1 mobility. Both retrotransposons, Tto1 and Tnt1, were mobilised in the genotypes analysed, with mobility ranging from 0 to 7.8%. Furthermore, at the epigenetic level, demethylation was detected in the vicinity of Tnt1 and Tto1 in the hybrids compared with the parental genotypes. These patterns were positively correlated with the activity of the retrotransposons. The results suggest a possible mechanism through which hybridisation events generate genetic variability in tuber‐bearing species of Solanum through retrotranposon activation.     

A comparison of performance of tetraploid progenies produced by diploid and their vegetatively doubled (tetraploid) counterpart parents

 Two diploid potato parents, W5295.7 and W5337.3, and their colchicine-doubled tetraploids were used as male to cross with three cultivars: ‘Raritan’, ‘Shepody’ and ‘Superior’. Both diploids produced 2n gametes via first division restitution (FDR) during meiosis. Field experiments were carried out on seedlings of the 12 hybrid progenies in 1991 and 1992. Six of the progenies represented the tetraploid-diploid (4x-2x) hybrids and the other six tetraploid-tetraploid (4x-4x) progenies. The genetic consequence of FDR gametes was examined by comparing the mean and genotypic variance of six tuber traits of 4x-2x and 4x-4x progenies. Tuber appearance, eye depth, specific gravity and tuber size showed significant differences between the means of 4x-2x and 4x-4x progenies from either both or one of the diploid parents. The genotypic variances of these traits in 4x-2x progenies were reduced in various degrees from those observed in 4x-4x progenies. Major genes responsible for the inheritance of these traits are probably located close to the centromere. The two diploid parents may also carry alleles of different size and direction of non-additive genic effects in their 2n gametes. The 4x-2x progenies also outperformed the 4x-4x progenies with respect to the means of total and marketable yield. There were thus heterotic gene loci for high yield located close to the centromere. No marked reduction of genotypic variance, however, was observed. The results suggest that genes for yield would be scattered between the centromere and the site for maximum recombination. Received: 24 April 1996 / Accepted: 26 July 1996     

Interspecific hybridization between the cultivated potato Solanum tuberosum subspecies tuberosum L. and the wild species S. circaeifolium subsp. circaeifolium Bitter exhibiting resistance to Phytophthora infestans (Mont.) de Bary and Globodera pallida (Stone) Behrens

Summary Somatic fusions between the cultivated potato Solanum tuberosum and the wild species S. circaeifolium subsp. circaeifolium Bitter were produced in order to incorporate desirable traits into the potato gene pool. Selection of the putative hybrids was based on a difference in callus morphology between the hybrids and their parents, with the hybrids showing typical purple-colored cells in otherwise green calli. In all, 17 individual calli regenerated to plants. Of the nine plants that could be transferred to the greenhouse, eight showed a hybrid and one a parental morphology. Restriction fragment length polymorphism (RFLP) analysis confirmed the hybrid character in the former group. Chloroplast counts in stomatal guard cells and flow cytometric determination of nuclear DNA content showed that four hybrid plants were tetraploid (4x), one was mixoploid (5x–8x), and the others were polyploid (6x; 8x). Three out of four tetraploid hybrids were found to be fully resistant to Phytophthora infestans, and all four hybrids were resistant to Globodera pallida pathotypes Pa2 and Pa3. It was further observed that the type and amount of steroidal glycoalkaloids varied among the tubers of the parents and the hybrids. Using the hybrids as female parents in crosses with S. tuberosum, viable seeds could be obtained. This demonstrates the potential of these hybrids in practical plant breeding.     

Allozyme variability and phylogenetic relationships in the cultivated potato (Solanum tuberosum) and related species

Gene frequencies at 13 isozyme loci were determined in three South American taxa of cultivated potatoes [the diploid group (gp.) Stenotomum, the diploid subgroups (subgp.) Goniocalyx, and the tetraploid gp. Andigena ofS. tuberosum], in the diploid weed speciesS. sparsipilum, and in most of the main cultivars now raised in the Northern Hemisphere (the tetraploid gp. Tuberosum ofS. tuberosum). High levels of genetic variability (mean number of alleles per locus, percentage of polymorphic loci, and mean heterozygosity) were detected, being higher in tetraploid potatoes. An equilibrium among the evolutionary factors which increase genetic variability and artificial selection for maximum yield would explain the high uniformity of heterozygosity values we observed in both Andigena (0.36 ± 0.02) and Tuberosum (0.38 ± 0.01) cultivars.—The low value of genetic distance (D = 0.044) between Stenotomum and Goniocalyx does not support the status of species forS. goniocalyx.—In most isozyme loci, the electromorphs of gp. Andigena were a combination of those found in both gp. Stenotomum andS. sparsipilum, suggesting an amphidiploid origin of gp. Andigena from that two diploid taxa. The presence in Andigena of unique electromorphs, which were lacking in both gp. Stenotomum andS. sparsipilum, suggests that other diploid species could be also implied in the origin of tetraploid Andean potatoes. Furthermore, since Andigena were more related to Stenotomum (D = 0.052) than toS. sparsipilum (D = 0.241), the autopolyploidization of Stenotomum individuals and the subsequent hybridization with gp. Andigena may also have occurred. Thus, our study suggests a multiple origin (amphidiploidy, autoploidy, and hybridization at tetraploid level) of gp. Andigena.—Most of the electromorphs of gp. Tuberosum were also found in gp. Andigena; both the direct derivation of that group from the Andean tetraploid potatoes and the repeated introgression provided by breeding programmes could explain this result. However, the allele c of Pgm-B, present in 30 out of 76 Tuberosum cultivars from Northern Hemisphere as well as in 3 Chilean Tuberosum cultivars, lacks in the 258 Andigena genotypes sampled, suggesting that Chilean germplasm could have taken part in the origin of at least the 39% of the potato cultivars from Europe and North America analyzed here.—The distanceWagner procedure provides an estimate of a 30% of heterogeneity in the evolutionary divergence shown by different groups of cultivated potatoes. Diploid groups show a higher (22.5%) evolutionary rate than tetraploids, which can be attributed to both tetrasomic inheritance and facultative autofecundation that exists in Andigena and Tuberosum groups. Thus, artificial selection acting since 10000 years has not resulted in a higher rate of molecular evolution at the isozyme level in the tetraploids.     

Efficiency of potato breeding using FDR 2n gametes for multitrait selection and progeny testing

Summary The objective of this research was to compare the efficiency of the 4x×2x breeding scheme with the traditional 4x×4x method with respect to potato improvement. The basis for such a comparison was the parental value of four 2x and four 4x male parents from the International Potato Center (CIP) as measured by multitrait selection and progeny testing. The 2x parents produced 2n pollen by parallel spindles at anaphase II, which is genetically equivalent to a first division restitution (FDR) mechanism. Both 2x and 4x parents were crossed with four common 4x female parents. Thus, 32 families were evaluated over 2 years at four Peruvian locations. A selection index which considered tuber yield, tuber number, average tuber weight and specific gravity was used for multitrait selection. Three FDR 2x parents had better selection index scores than the 4x parents over the four locations. Estimates of broad-sense heritability for total yield using different number of replications and locations were calculated by using the variance components. The 4x × 2x breeding scheme was found to be better than the traditional 4x × 4x method since fewer replications and locations are required to evaluate tuber yield in 4x × 2x progenies than in 4x × 4x progenies. The FDR 2x parents were also better material than the 4x parents for testing combining ability for tuber yield of the 4x progenitors. This could be the result of the mode of FDR 2n pollen formation. The pollen of FDR 2x parents is more heterozygous, but more homogenous than n pollen from 4x parents.Paper from the Laboratory of Genetics. Research supported by the College of Agricultural and Life Sciences; International Potato Center; USDA-CRGO-88-37234 3619, and Frito-Lay, Inc., USA     

Effects of kinetin,paclobutrazol and their interactions on the microtuberization of potato stem segments cultured in vitro in the light

Single-nodal cuttings of Solanum tuberosum (four cultivars) and Solanum chacoense were induced to produce in vitro microtubers on Murashige & Skoog (MS) medium supplemented with 8 g l^–1 sucrose and various concentrations of kinetin and paclobutrazol. The cultures were kept 10 days in darkness and then transferred to a 14 h daylength with 100 µE m^–2 sec^–1 light intensity at 21 °C. Kinetin (2.5 mg l^–1) had no significant influence on tuber formation. However, its addition together with paclobutrazol (0.001 mg l^–1) significantly enhanced tuberization. Paclobutrazol alone stimulated early tuber initiation and inhibited stem growth. Despite some genotype × treatment interactions, all genotypes (from very early to late and wild type) formed the maximum proportion of explants bearing microtubers on the media containing both plant growth regulators.