Tuber-bearing Species Research Articles

Overcoming crossing barriers between nontuber-bearing and tuber-bearingSolanumspecies: towards potato germplasm enhancement with a broad spectrum of solanaceous genetic resources

The first direct sexual hybrids between diploid nontuber-bearing species and diploid potato breeding lines are reported here. Three nontuberous species of Solanum, S. brevidens, S. etuberosum, and S. fernandezianum, were used for sexual crosses, achieved by a combination of rescue pollinations and embryo rescue. Initial hybrid selection was made using an embryo spot marker, followed by the evaluation of morphological and reproductive traits. Putative hybrids were first tested for resistance to potato leaf roll virus derived from the wild species, and then were tested with molecular markers using species-specific DNA probes. Finally, the tuberization of several 2x hybrids was tested for actual potato germplasm enhancement. These hybrids are unique in terms of their potential to enhance recombination between chromosomes of wild species and those of cultivated potatoes in germplasm utilization, and to exploit the genetic nature of tuber formation. The finding that nontuber-bearing Solanum spp. can be directly crossed with tuber-bearing species also has important implications for the regulatory aspects of the use of genetically modified organisms.

Stolon characteristics and tuber initiation in a wild potato species (Solanum demissum Lindl.)

Solanum demissum Lindl. is a short-day dependent, tuber-bearing species. In vitro propagated genetically identical plants were grown under long-day (16 h) conditions at 20°C day and 15°C night temperatures. Subsequently, from day 0 onwards, plants were exposed to shortday (10 h) conditions and grown in a multi-compartment system that allowed stolon initiation, stolon branching, stolon growth and tuber initiation to be monitored twice weekly. These frequent measurements resulted in detailed life-histories of all 841 stolon tips formed until day 46. Tuber initiation was irregular under these well-defined conditions; at day 46 only 106 stolon tips showed subapical swelling. This irregularity of tuber initiation was also observed within stolons. It is therefore suggested that tuber initiation depends on characteristics of the stolon or stolon branch, and several such characteristics were analyzed to obtain information on their potentially determining role in tuber initiation. No positive correlations were found between subapical swelling and branching order, stolon and stolon-branch age, longitudinal growth rates of stolons and stolon branches, and attachment of the stolon to the main stem.

Wild tuber-bearing species ofSolanum and incidence ofPhytophthora infestans (Mont.) de Bary on the Western slopes of the volcano Nevado de Toluca. 4. Plant development in relation to late blight infection

Development of plants ofSolanum demissum (dms) andSolanum verrucosum was followed in three sites at altitudes of 2950, 3250 and 3500 m, and the incidence of late blight and physiological races determined. No plants were found that had grown from true seed although berry formation was abundant indms.

Species-specific sequences in the genus Solanum: identification, characterization, and application to study somatic hybrids of S. brevidens and S. tuberosum

To aid in the identification and analysis of somatic hybrids between potato (Solanum tuberosum, dihaploid line PDH 40) and the non tuber-bearing wild species S. brevidens, a series of species-specific repetitive DNA sequences have been isolated. This was accomplished by making libraries of HaeIII-digested total DNA of S. tuberosum and S. brevidens, by cloning fragments into the SmaI site of plasmid pUC18 and transforming them into E. coli (JM83). The S. brevidens library consisted of 1,000 recombinant clones, and that of S. tuberosum, 700. Nitrocellulose filters with recombinant clones were hybridised to nick-translated total DNA of S. brevidens and also S. tuberosum, and, following autoradiography, clones that hybridised strongly to the DNA of only one of the species were chosen. Two highly repeated S. brevidens clones (pSB1, 400 bp and pSB7,210 bp), one highcopy-number s. tuberosum clone (pST10, 200 bp) and one low-copy-number sequence of S. tuberosum (pST3, 1.5 kbp) were selected for further analysis by Southern hybridisation to digested total DNA. Clone pSB7 gave a ladder pattern on hybridisation to EcoR1-digested total DNA of S. brevidens, with signals at multiples of 200 bp DNA. Using these probes it was possible to verify the hybridity of putative hybrids of dihaploid S. tuberosum and S.brevidens, and to confirm by Southern analysis and by slot blots the parental genome dosage of hexaploid hybrids (two s. brevidens: one S. tuberosum, and vice-versa). The S. tuberosum-specific probe, pSTIO, hybridised with DNA of three other tuber-bearing wild species (S. hjertingii, S. capsicibaccatum and S. berthaultii). A squash-blot procedure was developed using the probes that would allow early identification of somatic hybrid callus. There are a number of useful applications of such species-specific probes in the identification and analysis of somatic hybrids.

Solanum serratoris: a new wild ecuadorian potato species

A new wild Ecuadorian tuber-bearing species ofSolanum,S. serratoris (SeriesTuberosa, Sect.Petota is described and illustrated. This species from the eastern-Andean region of provincia Morona-Pastaza has been named in honor of Dr. Richard L. Sawyer, founder of the International Potato Center, Lima, Peru.

Interspecific incompatibility due to stylar barriers in tuber-bearing and closely related non-tuber-bearing Solanums

The phenomenon of interspecific incompatibility between various wild tuber-bearing and closely related non-tuber-bearing Solanum species was studied. One area of investigation included an examination of possible protein interactions in the incompatibility reaction using SDS electrophoresis. Pollen tube inhibition and morphology were examined in conjunction with biochemical analysis. Two sets of crosses were examined: interspecific tuber-bearing species crosses and interspecific tuber-bearing × non-tuber-bearing species crosses. These crosses had consistent pollen tube inhibition in the upper one-third of the style. The upper third of the styles of incompatibly pollinated, compatibly pollinated, and unpollinated styles was studied under fluorescence microscopy to observe pollen tube growth and morphology. Interspecific tuber-bearing × non-tuber-bearing species crosses demonstrated consistent pollen tube inhibition just below the stigma with frequent pollen tube swelling and bursting and extensive callose deposition along the pollen tube wall. Interspecific tuber-bearing species crosses had pollen tube inhibition further down the style with pollen tube tip tapering and extensive callose deposition. Stylar proteins of the lower two-thirds of the styles were analyzed with SDS electrophoresis. No unique protein differences were found to be specifically associated with the interspecific incompatibility reaction in this portion of the style.

Solanum salasianum: New wild tuber-bearing species from Peru

A new wild Peruvian diploid (2n=2x=24) tuber-bearing species ofSolanum, S. salasianum (seriesConicibaccata, Sect.Petota) from the moist montane forest near Monocanca, province Pachitea, department of Huanuco is described and illustrated. It is named in honor of my assistant, agronomist Alberto Salas Lopez.

Resistance to late blight (Phytophthora infestans) in ten wild Solanum species

Nineteen accessions of the tuber-bearing species Solanum berthaultii, S. chacoense, S. leptophyes, S. microdontum, S. sparsipilum, S. sucrense, S. venturii, S. vernei and S. verrucosum were tested for their resistance to late blight in two years of field experiments. Plants were artifically inoculated with zoospores of race 1.2.3.4.5.7.10.11 and the development of the disease was followed. Resistance ratings, calculated as the areas under the disease progress curves (ADPC), demonstrated a high resistance in all accessions except in S. sparsipilum, S. leptophyes and their interspecific hybrid. Segregations suggest that major genes for resistance are present in S. sucrense and S. venturii, and may also play a role in S. verrucosum. It is not yet certain wether the resistance of the other accessions is comparable to the partial and durable resistance of S. tuberosum cultivars like Pimpernel, as inheritance and mechanism have yet to be established. However, segregations suggesting the presence of single major genes with complete dominance were not found in these other accessions. Tuber initiation in the field occurred in only one accession, S. tuberosum ssp. andigena, and maturity of the clones was not related to their resistance. In the other accessions maturity types could not be assessed, as the clones require short day conditions for tuber initiation.

Resistance toPhytophthora infestans in tuber-bearing species ofSolanum and its geographical distribution

During the last 10 years, 1962 accessions in the German-Netherlands Potato Department of the Gene Bank in the FAL Braunschweig-Volkenrode have been evaluated for race non-specific resistance toPhytophthora infestans, by using different tests. A survey of the results shows that, in addition to the well-known sources of late blight resistance from Mexico, there are high levels of field resistance in several interesting species from the Andes. Species with high levels of race non-specific resistance are concentrated in the mexican and Bolivian-Argentinian parts of the centres of diversity of the potato, particularly near the northern and southern tropics. We suggest that some species of Andean origin should be given more attention in breeding for partial resistance.

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

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.

Solanum bombycinum, a new tuber-bearing tetraploid species from Bolivia

It is probable that the wild potatoes from Bolivia are the least known of all the Andean region. Although approximately 50 species have been described from this country since Dunal, 1852, a great majority of these are synonyms. Nevertheless, in some remote and unexplored regions of Bolivia, it is still possible to find new material, as demonstrated by the new species described herein.

Solanum Incamayoense: A new tuberous species of Argentina

Solarium incamayoense is described as a new wild tuber-bearing species from dept. Rosario de Lerma, prov. Salta, Argentina. This species is morphologically related toS. gourlayi Hawkes from which it differs by the shape, size and colour of the leaflets and by the habit of growth. The species is diploid (2n = 24).

Solanum Nemorosum: A new hexaploid tuber-bearing species from Peru

Up to the present time, the only representative Peruvian hexaploid tuber-bearing wild species isSolanum acaule Bitt. var.albicans Ochoa, from the series Acaulia of theSection Tuberarium. This entity was found by the author for the first time in the paramo prairies of Porcon, Cajamarca in northern Peru. Now it is my pleasure to present again another extremely interesting new wild species,Solanum nemorosum, which is also hexaploid, that is to say with a content of 2n = 6x = 72 chromosomes. It is native also from the north of Peru and belongs to the series Conicibaccata. It occurs primarily in humid forests of shrubs or small trees.

Natural hybridization between Solanum acaule Bitt. and S. megistacrolobum Bitt. in the province of jujuy, Argentina

Evidence is provided that hybridization of the tetraploid (2n=4x=48), self-fertile tuber-bearing species Solanum acaule Bitt. with the diploid (2n=2x=24), self-incompatible, tuber-bearing S. megistacrolobum Bitt. takes place in several localities of the province of Jujuy in the high, cold plateaux of the Argentine Puna. The triploid hybrids (2n=3x=36) closely resemble S. acaule in growth habit, leaf morphology and floral structures and for this reason they can be easily overlooked for that species in the field. Experimental data show that S. acaule can be crossed with S. megistacrolobum though the crossability is rather low and variable according to the particular cross considered. The artificial hybrid obtained compares well with the natural hybrid in morphology and chromosome number. The hybrids, though almost completely male sterile, are successful colonizers of disturbed areas around farmers' dwellings, cattle enclosures and other areas where the soil is rich in organic matter.

The occurrence and frequency of 2n pollen in three diploid solanums from Northwest Argentina

A group of wild, tuber-bearing species from Northwest Argentina, belonging to the series Tuberosa, Solarium spegazzini Bitt. (spg, 2n=2x=24), S. gourlayi Hawkes (grl, 2n=2x=24 and 2n=4x=48) and S. oplocense Hawkes (opl, 2n=6x=72), and Cuneolata, S. infundibuliforme Phil (ifd, 2n=2x=24), is being used to investigate the mode of origin of polyploids in the genus Solanum. 2n gametes have been detected in the diploid species ifd and spg and in a diploid race of grl, using cytological and breeding approaches. Twenty-two introductions of spg, 8 of grl and 26 of ifd have been tested for 2n pollen; 59%, 63% and 54% of them, respectively, had at least one 2n pollen producing plant. These introductions comprised 238, 76 and 235 plant respectively, of which 20, 16, and 32 plant produced 5% or more 2n pollen. The mechanism of 2n pollen formation was determined in several plant of 2x spg, 2x grl and 2x ifd. All of them were found to form diplandroids via parallel spindles. This mechanism, which gives meiotic products genetically equivalent to first division restitution gametes, is under control of the Mendelian recessive ps. The results suggest that the allele ps is widely distributed in natural populations of the three diploids, and that its frequency is very high. These species are seen as valuable material for population genetic studies, and for the eventual incorporation into a breeding scheme involving sexual polyploidization via 2n gametes.

Studies on the Breeding System of Oxalis tuberosa Mol.

A study has been made of the morphological basis and the compatibility relations of the tristylous flowers of Oxalis tuberosa M ol ., a cultivated tuber-bearing species from South America. Despite its long history of asexual propagation O. tuberosa has been found to retain the full complement of characters associated with tristyly, i.e. three corresponding style and stamen lengths and three pollen grain sizes which accord with three types of stigmatic papillae. Pollination experiments have shown that the physiological pollen - stigma self and own-type incompatibility reaction is also still functional so as to promote outcrossing and inhibit selfing.

Induction of haploid plants by anther culture in tuber-bearing species and interspecific hybrids ofSolanum

Out of 46 species and 9 interspecific hybrids ofSolanum studied, 17 tuber-bearing species, 2 non-tuber-bearing species and 4 interspecific hybrids ofSolanum produced pollen plants through anther culture. Haploidy was confirmed in 167 plants of 11 species and 2 interspecific hybrids ofSolanum. Plantlets which were formed directly from pollen almost always proved to be haploids, while plantlets which were redifferentiated from pollen-calli gave haploid, diploid, triploid, tetraploid and even aneuploid plants.

Reaction of some wild and cultivated potato varieties to charcoal rot

With a view to select potato varieties possessing resistance to charcoal rot, laboratory tests were carried out at the Central Potato Research Institute, Simla (India). Five hundred and twenty six clones comprising tuber-bearing species ofSolanum and varieties and hybrids of potato were screened for their reaction toMacrophomina phaseoli (Maubl.) Ashby. A high degree of resistance was observed in six clones ofSolanum chacoense and four hybrids withS. chacoense germ plasm, while moderate susceptibility was observed in another six clones ofS. chacoense, three clones ofSolanum tuberosum ssp.andigena and nineteen hybrids produced at the Institute.

MORPHOLOGICAL VARIATION IN SOLANUM X EDINENSE, A HYBRID OF THE COMMON POTATO.

The wild tuber-bearing species of Solanum (Sect. Tuberarium) have long been recognized as a useful group for evolutionary and genetic studies (cf. Howard, 1961; Magoon et al., 1962; Dodds and Paxman, 1962; Hougas and Peloquin, 1960). The diversity of species and infraspecific taxa (Correll, 1962; Hawkes, 1963; Ochoa, 1962), together with the ease of seed and tuber propagation, makes this polyploid complex especially suitable for field and greenhouse investigations of experimental hybridizations between races and species. It is thus not surprising that much is known about the variability and crossability of wild and cultivated potatoes when grown under controlled conditions (Hawkes, 1958; Simmonds, 1964). In contrast, very few studies have concerned themselves with the dynamics of wild or cultivated potato populations in their native Mexican or Andean habitats. The investigations by Correll (1952) on the environmental-latitudinal modifications of S. demissum phenotypes and those of Hawkes (1962a) on introgression in S. chacoense and S. microdontum are of interest here, though neither study employed population concepts based upon mass sampling techniques (Anderson, 1949). Similarly, the experimental synthesis of S. X juzepczukii and S. x curtilobum from S. acaule and diploid and tetraploid clones of S. tuberosum (Hawkes, 1962b), while a milestone in potato systematics, nonetheless sheds little light on the true extent of morphological intergradation that occurs between these taxa in nature. One purpose of the present study is to explore the value of mass collection techniques and variability analyses in understanding the dynamics of natural hybrid potato populations. A series of small and local hybrid potato populations, sampled in Central Mexico during the summer of 1962, 1963, and 1965, will thus be analyzed following methods outlined by Anderson (1949). A second objective is to compare the results obtained from studies of native Mexican potato populations with those obtained from the analysis of plants cultivated under uniform field conditions at the University of Wisconsin Experimental Farm at Sturgeon Bay, Wisconsin. In this way both genetic and phenotypic variation will be compared and the responses of hybridizing plants to differing environmental conditions will be analyzed. The weedy Mexican potato, Solanum X edinense Berthault, is particularly wellsuited for these investigations as its partial sterility and pentaploid chromosome number (2n = 60) offer virtually indisputable proof of its hybrid origin (Hawkes, 1944, 1963). However, its postulated origin from a cross of S. tuberosum L. (Group Andigena, 2n = 48) with S. demissum Lindl. (2n = 72), as suggested by Hawkes (1944, 1966), has not hitherto been supported by comparative morphological studies.

Frost resistance among theSolanum species in the ir-1 potato collection

Five-hundred eighty-six wild and cultivated introductions representing 82Solanum species maintained in the IR-1 Potato Collection were evaluated in the field for frost resistance following freezing temperatures down to 27 F (−2.8 C). The most resistant introductions were those ofS. acaule, S. chomatophilum, S. commersonii, S. xcurtilobum, S. demissum, andS. sanctae-rosae. Resistance comparable to the best in the tuberbearing species was found in introductions of 2 closely related (nontuberous) species,S. brevidens andS. etuberosum. Seedling families of 102 interspecific crosses also were evaluated. Most of the hybrids involvingS. acaule were resistant to temperatures of 27 F. Some of the recent literature on frost resistance is reviewed and the species reported as resistant are listed according to the most recent taxonomic treatments of the tuber-bearingSolanum species.