Electrofusion of protoplasts from Solanum tuberosum, S. nigrum and S. bulbocastanum

Leaf protoplasts of two wild species, Solanum nigrum var. gigantea (S. ngr gig) and S. bulbocastanum Dun. (S. blb), were electrofused with leaf protoplasts of two diploid potato clones, H-8105 and ZEL-1136, respectively, in order to confer the late blight-resistance from the wild species to the cultivated potato. The S. ngr gig mesophyll (+) H-8105 mesophyll combination resulted in regenerants of mostly normal ngr phenotype. Two regenerants from this combination were proved to be true hybrids by RAPD analysis but they rooted poorely in vitro and did not survive the transfer to soil. The S. ngr gig (+) H-8105 fusion combination was also performed with H-8105 cell suspension derived protoplasts enabling an easy identification of interspecific fusants on basis of their intermediate morphology. From the S. ngr gig mesophyll (+) H-8105 cultured cell combination, many abnormal shoots were regenerated. The two lines which survived had normal ngr phenotype but the presence of tuberosum (tbr) genome in those regenerants was not confirmed by RAPD analysis. No plants with tbr phenotype were obtained from both of S. ngr gig (+) H-8105 combinations. On the contrary, when S. blb mesophyll protoplasts were electrofused with ZEL-1136 mesophyll protoplasts, all regenerated plants had tbr phenotype, indicating much lower morphogenetic potential of S. bulbocastanum in comparison with that of S. nigrum var. gigantea. However, the hybridity of those regenerants has not been confirmed by RAPD analysis with two different primers. The efficiency of the applied fusion procedure and analysis of the regenerants is discussed.     

Plant regeneration from the protoplasts of Solanum tuberosum, S. nigrum and S. bulbocastanum

The mesophyll protoplasts were isolated from the Solanum tuberosum (S. tbr) clones of different ploidy level (4x Bzura cv., 2x H-8105, and 2x ZEL-1136) as well as from the wild species: S. bulbocastanum (S. blb, 2x) and two accessions of S. nigrum (S. ngr, 6x). Additionally, the protoplasts were isolated from the cell suspensions of Bzura cv. and H-8105 clone. The conditions of protoplast isolation as well as the media for their culturing and regeneration, were selected and optimized for the studied genotypes. For mesophyll protoplasts, the shooting calli were produced by all the cultured protoclones except that of S. bulbocastanum. The shoots excised from the protoplast-derived calli developed into whole plants in all the studied potato clones but only in one accession of S. nigrum, i.e. S. ngr var. gigantea. As for suspension-cell-derived protoplasts, only H-8105 clone produced the regenerative type of calli, though normal shoots could not be obtained. The regenerative capacity of the protoplasts isolated from leaves and cell suspensions is compared and discussed. We regret to report the death of M. Sc. Maria Borkowska after the completion of this work.     

Somatic hybrids between Solanum bulbocastanum and potato: a new source of resistance to late blight

 Solanum bulbocastanum, a wild, diploid (2n=2x=24) Mexican species, is highly resistant to Phytophthora infestans, the fungus that causes late blight of potato. However this 1 EBN species is virtually impossible to cross directly with potato. PEG-mediated fusion of leaf cells of S. bulbocastanum PI 245310 and the tetraploid potato line S. tuberosum PI 203900 (2n=4x=48) yielded hexaploid (2n= 6x=72) somatic hybrids that retained the high resistance of the S. bulbocastanum parent. RFLP and RAPD analyses confirmed the hybridity of the materials. Four of the somatic hybrids were crossed with potato cultivars Katahdin or Atlantic. The BC₁ progeny segregated for resistance to the US8 genotype (A-2 mating type) of P. Infestans. Resistant BC₁ lines crossed with susceptible cultivars again yielded populations that segregated for resistance to the fungus. In a 1996 field-plot in Wisconsin, to which no fungicide was applied, two of the BC₁ lines, from two different somatic hybrids, yielded 1.36 and 1.32 kg/plant under a severe late-blight epidemic. In contrast, under these same conditions the cultivar Russet Burbank yielded only 0.86 kg/plant. These results indicate that effective resistance to the late-blight fungus in a sexually incompatible Solanum species can be transferred into potato breeding lines by somatic hybridization and that this resistance can then be further transmitted into potato breeding lines by sexual crossing. Received: 27 October 1997 / Accepted: 11 November 1997     

Introgression of resistance to root-knot nematodes from wild Central American Solanum species into S. tuberosum ssp. tuberosum

 Crossing experiments were conducted to introduce resistance to the root-knot nematodes, Meloidogyne chitwoodi and M. fallax, from various polyploid Central American Solanum spp. into the cultivated potato, S. tuberosum ssp. tuberosum. The most effort was put into producing tetraploid hybrids through inter-EBN (Endosperm Balance Number) crosses. From the crosses of tetraploid S. tuberosum (4 EBN) with tetraploid S. stoloniferum and S. fendleri (both 2 EBN), few seeds were derived that led to viable plants. In vitro culture of immature seeds also yielded several hybrid plants. From crosses of diploid S. tuberosum (2 EBN) with hexaploid S. hougasii (4 EBN) four hybrids were obtained through in vitro culture. Backcrosses were made with selected hybrids and a variable number of seeds was produced depending on the hybrid genotype. The successful introgression of resistance into backcross populations is shown. A scheme is presented for the introgression of traits at a tetraploid level from allotetraploid Solanum species into autotetraploid S. tuberosum through sexual crosses. The relevance of EBN for potato breeding is discussed. Received: 25 November 1996 / Accepted: 14 February 1997     

Biological containment of potato (Solanum tuberosum): outcrossing to the related wild species black nightshade (Solanum nigrum) and bittersweet (Solanum dulcamara)

The biological containment of the potato (Solanum tuberosum) was assessed by establishing the crossability of this tuberous crop with the related wild non-tuberous species in The Netherlands, black nightshade (S. nigrum) and bittersweet (S. dulcamara). To circumvent crossability barriers, genotypes with different ploidy number were employed and crosses were performed under different environmental conditions. S. dulcamara was shown to be incongruent with potato at all ploidy levels, while S. nigrum displayed unilateral incompatibility. If S. nigrum was emasculated and used as female, fertilization by potato pollen resulted in berry set and seed development. Emasculation of S. nigrum was essential in this cross, because analysis of the fertilization process demonstrated that this species is highly self-compatible and potato pollen was outcompeted by pollen of S. nigrum. The hybrid seeds derived from this cross did not mature and appeared not to be viable. By application of the technique of embryo rescue of immature embryos, hybrid plants could be obtained. However, these hybrid plants proved to be sterile. These data demonstrate that gene flow by pollen dispersal from potato to its most common wild relatives in Western Europe is highly unlikely. The potato is thus a naturally contained species in this part of the world.     

Growth and development of plants with potential for use as trap crops for potato cyst nematodes and their effects on the numbers of juveniles in cysts

Growth and development of three plant accessions with potential for use as trap crops for potato cyst nematodes (PCN), Solanum sisymbriifolium and two varieties of S. nigrum, were studied under 12 h and 17 h photoperiods. In pot experiments, rate of plant emergence, plant height, and shoot and root mass were greater for the S. nigrum varieties ‘90‐4750‐188’ and ‘88‐4750‐061’ than for S. sisymbriifolium and markedly greater than for a S. nigrum variety found as a weed of arable fields in The Netherlands. However, the last mentioned S. nigrum variety produced the most berries. Plant height and shoot weight of all the S. nigrum varieties were greater under the longer photoperiod, whereas the root mass was hardly affected. Plant height and shoot weight of S. sisymbriifolium also were greater under the longer photoperiod but the root weight was less. Under field conditions, with sowing dates from the end of March to mid August, S. sisymbriifolium and S. nigrum‘90‐4750‐188’ grew better than S. nigrum‘88‐4750‐061’. In contrast to S. nigrum, S. sisymbriifolium appeared resistant to night frosts in autumn. The stubbles of both S. sisymbriifolium and S. nigrum showed good regrowth after cutting the plants 5 or 10 cm above the soil surface 11 wk after sowing. In a pot experiment, all the plant accessions strongly reduced the numbers of juveniles in cysts compared with flax. Tolerance to Globodera rostochiensis of S. sisymbriifolium and S. nigrum‘90‐4750‐188’ was investigated in pot experiments under glasshouse conditions in sandy soil at pH 4.8 and 6.0. At soil infestation levels ranging from 0 to 56 juveniles ml^?1 soil, S. sisymbriifolium appeared much more tolerant than S. nigrum‘90‐4750‐188’. Shoot yield of S. nigrum decreased markedly with increasing soil infestation and root weight also decreased, except at pH 4.8 and light infestation levels. Both S. sisymbriifolium and S. nigrum grew better at soil pH 4.8 than 6.0. The proportion of lateral roots in the total root mass increased in both species with increasing PCN infestation and soil pH. However, although the proportion of lateral roots in plants grown at soil pH 6.0 was greater at PCN infestations up to 14 juveniles ml^?1 soil, the proportion of laterals in S. nigrum was considerably less at PCN infestations of 56 juveniles ml^?1 soil. The proportion of PCN juveniles hatching was similar for the two species and decreased slightly with increasing initial nematode population densities.     

An assessment of gene transfer by pollen from field-grown transgenic potatoes to non-transgenic potatoes and related species

Information on the extent of transgene dispersal by pollen to adjacent potato plots and to related weed species is an important requisite for risk assessment; a procedure followed before novel transgenic plants are evaluated under field conditions. The purpose of the investigation was to determine the frequency of cross-pollination between potato (Solanum tuberosum) plants at different distances, using a kanamycin resistnace transgene (nptII) as a selectable marker. All potato plants were from the variety Désirée. Non-transgenic potato plants, used as potential recipients of transgene-containing pollen, were planted in 12 sub-plots, at distances of 0–20 m from the nearest transgenic potato plants. Seeds harvested from the non-transgenic plants were screened for resistance to kanamycin, and molecular methods were used to confirm that resistant progeny contained thenptII gene. Where transgenic and non-transgenic potato plants were in alternate rows (leaves touching), 24% of seedlings from the non-transgenic parent plants were kanamycin-resistant. Comparable seedlings from plants at up to 3 m distance had a resistance frequency of 2%, at 10 m the frequency was 0.017% and at 20 m no resistant progeny were observed. Plants of the weed speciesS. dulcamara andS. nigrum were also planted close to the transgenic potatoes to test for evidence of hybridization, and no kanamycin-resistant seedlings were observed among progeny fromS. dulcamara andS. nigrum. This investigation provided evidence that the extent of gene dispersal from transgenic potatoes to non-transgenic potatoes falls markedly with increasing distance, and is negligible at 10 m. There was, also, no evidence of transgene movement from potato toS. dulcamara andS. nigrum under field conditions. These data will be valuable in defining genetic isolation procedures for the early field evaluation and the use of novel transgenic potato genotypes.     

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.     

Reduced recombination and distorted segregation in a Solanum tuberosum (2x) x S. spegazzinii (2x) hybrid.

In this paper we describe the reduced recombination and distorted segregation in an interspecific hybrid between Solanum tuberosum and Solanum spegazzinii. To study these phenomena, a cross was made between a (di)haploid S. tuberosum, used as a female parent, and a diploid wild potato species, S. spegazzinii, used as a male parent. Next, a backcross (BC) population was made with F1 genotype 38 that was backcrossed to S. tuberosum. In the backcross, S. tuberosum was used as the male parent. RFLP linkage maps were made using the F1 and the BC populations, yielding linkage maps of the interspecific hybrid, S. spegazzinii, and S. tuberosum from which male and female linkage maps could be constructed. The computer program JOINMAP was used to construct and combine the separate linkage maps. Subsequently, the separate linkage maps were compared with each other, and reduced recombination was observed in the linkage maps of the male S. tuberosum and the interspecific hybrid. The reason for this reduced recombination is discussed. Another common feature in linkage maps is the observation of distorted segregation. The distorted segregation of alleles from the interspecific hybrid was studied in more detail in the BC population. Most of the distortion was probably caused by gamete selection, but for 3 loci, on chromosomes 2, 3, and 4, we found evidence for the presence of a strong selection force acting at the zygote level against homozygous genotypes.     

Effects of potnetial trap crops and planting date on soil infestation with potato cyst nematodes and root-knot nematodes

In 1997 and 1998 the stimulation of hatch of potato cyst nematodes (PCN) by a trap crop was studied at various times during the growing season in a container and a field experiment. Solanum nigrum‘90‐4750‐188’was used as the trap crop in both experiments and was sown on 1 May, 16 June or 1 August in two successive years on different plots. Neither experiment revealed much seasonal variation in hatchability of PCN juveniles under a trap crop. In the container experiment, the hatch of the Globodera pallida Pa3 population was equally and strongly stimulated (89%) at all sowing dates in both years, except for the 1 August sowing in 1998 (when the hatch was 77% under extremely wet soil conditions). In the control treatment with non‐hosts (flax followed by barley) the total spontaneous hatch was 50% over 2 yr. In the field experiment, the hatch of PCN, averaged over the four populations, was also equally stimulated (71%) at all sowing dates in both years. In the control treatment with non‐hosts (flax‐barley) the total spontaneous hatch was 36% over 2 yr. Total hatch under the trap crop over 2 yr varied between the four PCN populations from 63% to 80%. In 1998 and 1999, control of potato cyst nematodes (PCN) by the potential trap crops Solanum sisymbriifolium and S. nigrum‘90‐4750‐188’was studied in the field. Potato was also included as a trap crop. In the 1998 experiment, potato, S. sisymbriifolium and S. nigrum strongly stimulated the hatch of PCN compared with the non‐host white mustard (Sinapis alba). Roots of potato and white mustard were mainly found in the top 10 cm of soil, whereas roots of S. sisymbriifolium and S. nigrum were also abundant at depths of 10–20 cm and 20–30 cm. In the 1999 experiment, soil infestation with PCN decreased markedly with potato and S. sisymbriifolium as trap crops. In plots moderately to severely infested with 2‐yr old cysts (2–29 juveniles ml^?1 air dried soil), potato reduced soil infestation by 87% and S. sisymbriifolium by 77%. In plots moderately to severely infested with 1‐yr old cysts the reductions were 74% and 60%, respectively. The reduction was least on plots very severely infested with PCN (110–242 juveniles ml^?1 soil): 69% and 52% for potato and S. sisymbriifolium, respectively. Soil infestations of plots that were initially slightly to severely infested with the root‐knot nematode Meloidogyne hapla were greatly reduced under fallow and S. sisymbriifolium but increased under potato. From these and previous experiments it was concluded that, for several reasons, S. sisymbriifolium is a promising trap crop.     

R6 and R7 alleles of potato conferring race-specific resistance to Phytophthora infestans (Mont.) de Bary identified genetic loci clustering with the R3 locus on chromosome XI

In potato, 11 resistance alleles (R1–R11) are known which confer race-specific resistance to the fungus Phytophthora infestans. R1 has been mapped previously to potato chromosome V and R3 to chromosome XI. Here we report on the localization of the R6 and R7 alleles on the genetic map of potato. Differential resistant strains of tetraploid Solanum tuberosum, clones MaR6 and MaR7, were used as parental plants for the parthenogenetic induction and selection of diploid genotypes containing the R6 or the R7 resistance allele to P. infestans. One resistant dihaploid from MaR7 could be used directly as a parent to produce diploid F₁ progeny suitable for phenotypic and RFLP analysis. MaR6 did not produce useful dihaploids directly. After crossing MaR6 with a tetraploid susceptible genotype, resistant F₁ clones were selected. The resistant genotypes were then used as parents for the induction of dihaploids. Six dihaploids bearing R6 were identified that could be crossed with a diploid susceptible genotype. Two diploid F₁ populations, segregating for R6 and R7, respectively, were analysed with RFLP markers known to be linked with previously identified R genes. Markers linked with R3 were found also to be linked with R6 and R7. The resistance alleles R6 and R7 mapped to a similar distal position on chromosome XI as the R3 allele.     

Freezing tolerance and tuber production in selfed and backcross progenies derived from somatic hybrids between Solanum tuberosum L. and S. commersonii Dun.

 Selfed and backcross progenies developed from tetraploid somatic hybrids between Solanum tuberosum (tbr) and S. commersonii (cmm) were characterized for nonacclimated freezing tolerance (NA) and acclimation capacity (ACC) (two independent genetic components of freezing tolerance) under controlled environments. The segregation covered 28% and 71% of the parental range for NA and ACC, respectively, with the distribution skewed toward the tbr parent. Therefore, ACC appeared to be relatively easier to recover in the segregating generation. Some first backcross progeny had greater freezing tolerance than the cultivated parent primarily through the increase in ACC. When grown in the field, the improved freezing tolerance observed in the selfed progeny under controlled conditions was confirmed. Among NA, ACC, and freezing tolerance after acclimation (AA, which is the cumulative performance of NA and ACC), AA exhibited the highest correlation coefficient with field frost tolerance. In addition to freezing tolerance, vine maturity and tuber traits including tuber yield, tuber number per plant, mean tuber weight, and specific gravity were also segregating. No significant correlation between undesirable tuber traits and freezing tolerance was detected. Vine maturity and freezing tolerance were significantly correlated, so more careful selection for earliness was necessary in incorporating freezing tolerance. Yield comparable or superior to the backcross parent Wis AG 231 and an early Canadian cultivar, ‘Sable’, was found in many backcross progeny and some selfed progeny. The observed high yield can be attributed to the increase in mean tuber weight as well as tuber number. Moreover, a high portion of progeny had a specific gravity higher than 1.085, and some greater than 1.1. The implications derived from this study in breeding for freezing tolerance and further use of these materials are discussed. Received: 22 August 1998 / Accepted: 4 January 1999     

Screening of non-tuber bearing Solanaceae for resistance to and induction of juvenile hatch of potato cyst nematodes and their potential for trap cropping

Ninety accessions of non‐tuber bearing Solanaceae were screened for (i) resistance to and (ii) stimulatory effect on juvenile hatch of potato cyst nematodes, and (iii) their growth under temperate climatic conditions. All plant species belonging to the genus Solanum tested induced hatching but this effect was most pronounced for plant species of the Solanum nigrum complex. Hatching of juveniles was hardly or not stimulated by other plant genera of the Solanaceae. Solanum sisymbriifolium combined a high hatching effect with complete resistance to both Globodera rosiochiensis and G pallida. Two S. nigrum varieties showed full resistance to G rostochiensis and a high level of resistance to G pallida. Moreover, S. sisymbriifolium and the two varieties of S. nigrum performed very well under Dutch field conditions and, therefore, they are suggested as candidate trap crops for the control of potato cyst nematodes.     

Comparative responses of tetraploid wheats pollinated with Zea mays L. and Hordeum bulbosum L.

Ten different tetraploid wheat (Triticum turgidum) genotypes were pollinated with maize (Zea mays). Fertilization was achieved in all ten genotypes and no significant difference in fertilization frequency between the tetraploid wheat genotypes was detected. A mean of 41.1% of pollinated ovaries contained an embryo. All these crosses were characterized by the elimination of the maize chromosomes, and the resulting embryos were haploids. Six of the tetraploid wheat genotypes were also pollinated with Hordeum bulbosum. Fertilization frequencies with H. bulbosum were much lower (mean=13.4%), and significant differences between the tetraploid wheat genotypes were detected. Observation of pollen tube growth revealed that part of the incompatibility reaction between tetraploid wheats and H. bulbosum was due to an effect similar to that of the Kr genes, namely pollen tube growth inhibition. These results indicate that pollinations with maize may have potential as a broad spectrum haploid production system for tetraploid wheats. Present address: Agriculture Canada, Research Branch, Central Experimental Farm, Bldg 50, Ohawa, Ontario, Canada K1A OC6     

Isolation and characterization of potato-tomato somatic hybrids using an amylose-free potato mutant as parental genotype

Summary Using different genotypes of tomato and diploid potato, possessing alien selectable markers as well as endogenous markers, very high frequencies of protoplast fusion hybrids were obtained. One endogenous genetic marker, the amylose-free (amf) mutant of potato, was helpful not only for the confirmation of fusion products but also for the study of genetic complementation and the segregation of amylose-free starch in microspores. Cytological analysis of the fusion hybrids indicated that except for one which was hexaploid, all of them had a perfectly balanced chromosome number of allotetraploid constitution (2n = 4x = 48). Despite normal chromosome pairing and a diploid behaviour, the microspores in some of the fusion hybrids segregated for the recessive amf-locus. This anomalous segregation of a recessive character in these hybrids was shown not to be due to chromosome elimination or to the absence of the wild-type tomato Amf gene. Although all fusion hybrids were totally sterile, the hexaploid produced stainable pollen and berries with badly developed seeds. Embryo rescue has so far failed to produce backcross progeny.     

Regeneration and analysis of interspecific asymmetric potato –Solanum ssp hybrid plants selected by micromanipulation or fluorescence-activated cell sorting (FACS)

 Recipient protoplasts from three Solanum tuberosum genotypes, cv ‘Folva’ (2n=4x=48), cv ‘Matilda’ (4n) and ‘161 : 14’ (2n), were electrofused with X-ray-irradiated donor protoplasts from two wild species S. spegazzinii (2n) or S. microdontum×S. vernei (2n). Prior to fusion, protoplasts were fluorescence-labelled with either fluorescein diacetate or scopoletin. Fusion products were identified by dual fluorescence and selected by micromanipulation or fluorescence-activated cell sorting (FACS). All putative hybrid plants were analysed by the random amplified polymorphic DNA (RAPD) technique. Our analysis demonstrates that each asymmetric hybrid plant has an individual and stable profile of donor-specific RAPD bands. The irradiation of donor protoplasts hampered the growth of selected heterofusion products in a dose-dependent way. Irradiation resulted in donor chromosome elimination, but not in a dose dependent way, in the tested interval. In asymmetric hybrids with the S. spegazzinii donor 33–68% of the donor-specific RAPD bands were missing, indicating a similar level of chromosome elimination. In asymmetric hybrid plants with the S. microdontum×S. vernei donor 74–95% of the donor RAPD bands were missing. Chromosome countings revealed that these hybrids had chromosome numbers equal to or below the chromosome numbers found in the tetraploid recipients. This is the first time that highly asymmetric hybrid plants between two tetraploid potato recipients and the donor S. microdontum×S. vernei have been obtained. Received : 16 December 1996 / Accepted: 21 February 1997     

Phenotypic characterization and bulk segregant analysis of anther culture response in two backcross families of diploid potato

Two diploid (2n=2x=24) backcross potato populations (PBCp, and CBC) were characterized for anther culture response (ACR). PBCp (Solanum phureja Juz. & Buk. genotype 1-3 × CP2) and CBC (CP2 × S. chacoense Bitt. genotype 80-1) resulted from a cross between CP2 (intermediate ACR) and its parents, S. chacoense 80-1(low ACR) and S. phureja 1-3 (high ACR). Three components of ACR were initially investigated: embryos per anther (EPA), embryo regeneration rate and percent monoploids (2n=1x=12) among regenerants. EPA was selected for further characterization because of its relative stability. In a series of studies of EPA on a total of 44 genotypes within CBC, nine high (mean EPA=2.5) and ten low (mean EPA=0.02) selections were made. In PBCp, ten high (mean EPA= 4.7) and ten low (mean EPA= 0.05) selections were made from 67 genotypes. High and low selections were used for bulk segregant analysis to screen 214 RAPD primers as candidate markers linked to EPA. Bands amplified by OPQ-10 and OPZ-4 were associated in coupling and repulsion, respectively, to ACR in PBCp. A band amplified by OPW-14 primer was associated in coupling to ACR in CBC. One-way ANOVAs using presence/absence of each candidate band to classify additional genotypes in each population verified association of the markers with EPA.     

Ploidy level manipulations in potato through sexual hybridisation

There is no better use of sexual reproduction in regard to breeding and genetic research than the ploidy level manipulations possible in the potato and its relatives. Unique reproductive characteristics of tuber‐bearing Solanum species make possible: the production of gametes with unreduced chromosome number; the presence of an endosperm dosage system that regulates success of interploidy/interspecific crosses; the possibility to easily extract maternal haploids following crosses with S. phureja. This paper reviews results obtained in scaling genomic multiples up and down in potato, and relates these manipulations to breeding strategies for the genetic improvement of the cultivated potato. Several ploidy series have been developed, ranging from the monoploid to the hexaploid level. Cultivated tetraploids were scaled down to the diploid and monoploid level by haploidy. Scaling upward was achieved by sexual polyploidisation via 2n gametes that resulted in triploid, tetraploid, pentaploid, and hexaploid genotypes with a broad genetic base. Altogether, the success of ploidy level manipulations constitutes further proof that sexual polyploidisation played an important role in the polyploid evolution of Solanum species, and supports the idea that gene flow can be relatively easily accomplished through interploid and bridge crosses.     

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     

Anther culture of Solanum genotypes

Anther culture response was examined in five Solanum genotypes. Large genotypic differences exist in response to culture and liquid culture media were found to be superior to agar solidified media systems. Pre-treatment effects on embryoid induction were also investigated and two of the genotypes displayed differential response to temperature stress pretreatment. In general the beneficial effect of charcoal in the media was confirmed. Successful embryoid production and plantlet regeneration was obtained from the wild potato species, Solanum papita. The influence of sucrose concentration on embryoid production was also investigated. Large genotype by sucrose interactions were detected and this was mainly due to the differential response of the tuberosum clones to increasing sucrose concentration when compared with S. papita. Embryoids were produced from this species in media containing 15% sucrose although the optimum concentration of sucrose for embryoid production was 9%. The possible role of anther culture techniques in gene introgression and potato improvement is discussed.