Breeding for 2n egg production in haploid × species 2x potato hybrids

The identification of diploid(2x), Tuberosum haploid × wild species hybrids (HS) which produce high frequencies of 2n eggs and have good tuber appearance is important for obtaining 4x progeny from 2x × 2x crosses in potato. More than 1700 HS clones were screened for the occurrence and frequency of 2n eggs through 2x × 4x crosses during two pollination periods. The resulting seed/fruit provides an estimate of 2n egg frequency. About 650 HS clones produced 2n eggs; 63 had 10–114 seeds/fruit; and 27 were selected for good tuber type. There was a large variation in seed set after 2x × 4x crosses, both within and between clones. Comparison of fruit and seed set between the two pollination periods using the same 595 clones indicated that environmental factor significandy affected 2n egg frequency. The mode of 2n egg formation was mainly due to omission of the second meiotic division controlled by a recessive gene (os). The gene frequency foros varied from 0.28 to 0.76 among six, 2x taxa. Both additive and dominance variance were significant for seed set after 2x × 4x crosses. Narrow sense heritability was estimated as 0.24. The average degree of dominance was equal to 1.66 which could indicate overdominance. Alternatively, this could be due to linkage disequilibrium,i.e. pseudooverdominance. However, dominance variance was greater than the additive variance indicating recurrent selection can be useful for improving the frequency of 2n egg production in the 2x population. The selected individuals, based on their phenotypic performance, should be progeny tested to identify the best parents for use in the next cycle of recombination.     

对马铃薯4x-2x,2x-4x及2x-2x杂种四倍体进行花药培养获得转育2n配子基因的双单倍体新个体

利用3个2n配子材料(2x)在马铃薯(S.tuberosum L.)中进行4x—2x,2x—4x和2x—2x的杂交,获得了4个四倍体杂种材料;然后对它们进行花药培养,共得到32个双单倍体植株。检查其中23个植株,有2株是具5％以上2n花粉粒的双单倍体,1株是重组了2n卵基因的双单倍体。由此证明花药培养的倍性操作技术是转育马铃薯2n配子性状给双单倍体的有效方法之一。     

Comparisons between 4x×2x hybrid and open-pollinated true-potato-seed families

Summary TPS progenies from open-pollinated and 4x×2x crosses were compared on the basis of seed quality, seedling vigor, seedling tuber production and tuber yield. The hybrids from 4x×2x crosses were superior in all the traits. Even though the hybrid families may have these outstanding features, the relatively high cost of producing seeds can be a limitation to their widespread utilization in developing countries. Seeds from open pollination are much less expensive; their cost of production is only 10–20 % of that of hybrid seeds. Thus, the use of TPS families from open pollination may be an economical method for the production of potatoes from true seed until a suitable method of producing low-cost hybrid seed is available.

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Zusammenfassung Die Hauptgruppen von Kartoffelsamen-Familien (TPS, ‘true-potato-seed’) die für eine Kartoffelproduktion verwendbar erscheinen, sind: a) Hybriden von 4x×2x-, 2x×4x- und 2x×2x-Kreuzungen (Peloquin, 1982) sowie 4x×4x-Kreuzungen (Peloquin, 1979; Mendoza, 1979), b) offen best?ubende Nachkommenschaft. Diese beiden Gruppen von TPS-Nachkommenschaften haben deutlich unterschiedliche Eigenschaften im Hinblick auf ihre Eignung für die Kartoffelproduktion. Zweck dieser Studie war ein Vergleich zwischen TPS-Nachkommen einerseits und 4x×2x-Kreuzungen und offen best?ubenden andererseits auf der Basis von Saatqualit?t, S?mlingsvitalit?t, S?mlingsknollen-Produktion und Knollenertrag. Zwei-und-fünfzig TPS-Familien, 35 von offen best?ubenden und 17 von 4x×2x-Kreuzungen wurden verwendet. Die Hybridfamilien entstanden aus Kreuzungen zwischen tetraploiden Klonen und 2x Phureja-haploiden Tuberosum-Hybriden mit der F?higkeit zur Erzeugung von 2n-Pollen durch FDR. Mit wenigen Ausnahmen entstanden alle OP-Nachkommen aus denselben tetraploiden Eltern, die in den 4x×2x-Kreuzungen verwendet wurden. S?mtliche Samen- und S?mlingstests liefen im Laboratorium und im Gew?chshaus. Die S?mlinge für die Feldversuche wurden im Gew?chshaus angezogen und dann im Alter von etwa 6 Wochen in das Feld ausgepflanzt. Alle Ertragsversuche wurden mittels RCBD und 2 Wiederholungen durchgeführt. Die Ergebnisse zeigen betr?chtliche Unterschiede bei Samengr?sse, Keimungsrate, S?mlings-Vitalit?t und S?mlings-Knollenproduktion zwischen den 4x×2x-Hybriden und OP-Nachkommen (Tabellen 1 und 2). Die Hybriden-Nachkommen waren bei allen diesen Merkmalen besser. Desweiteren erkl?ren die mittleren Knollenertr?ge der beiden Gruppen von TPS-Familien die überlegenheit der 4x×2x-Familien (Tabelle 3). Die Heterosis-Reaktion in den Nachkommen der 4x×2x-Kreuzungen wird auf der Basis der FDR-Weise für 2n-Pollenbildung erkl?rt. Bei diesem Mechanismus wird mehr als 80 Prozent der Heterozygotie der 2x-Eltern auf die Nachkommen übertragen. Ungeachtet der noch ausstehenden Eigenschaften der Hybrid-Familien k?nnten die relativ hohen Kosten für die Hybridsaat-Produktion eine Begrenzung für eine verbreitete Verwendung in Entwicklungsl?ndern bedeuten. Folglich kann die Verwendung von TPS-Familien von Offen-Best?ubenden eine ?konomische Methode für die Produktion von Kartoffeln aus Samen sein, bis eine brauchbare Methode zur kostengünstigen Produktion von Hybridsamen vorhanden ist.

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Résumé Les graines recherchées pour la production directe de pommes de terre proviennent des principaux types de familles suivants: a) les hybrides obtenus par croisément 4x×2x, 2x×4x, 2x×2x (Peloquin, 1982) et 4x×4x (Peloquin, 1979; Mendoza, 1979), b) les descendances issues de la pollinisation naturelle. Les lignées de ces deux groupes ont des caractéristiques distinctes sur le plan de leur aptitude à produire des pommes de terre. Le sujet de cette étude a été de comparer les descendances issues du croisement 4x×2x et celles provenant de la pollinisation naturelle, en se basant sur la qualité des semences, la vigueur germinative, la production de tubercules et le rendement en tubercules. Cinquante-et-deux familles, dont 35 issues de pollinisation naturelle et 17 du croisement 4x×2x ont été utilisées. Ces dernières étaient obtenues par le croisement entre cl?nes tétraplo?des et des hybrides Phureja 2x×Tuberoseum haplo?de capables de produire du pollen 2n. A quelques exceptions près, toutes les lignées issues de pollinisation naturelle étaient issus des mêmes parents tétraplo?des utilisés dans les croisements 4x×2x. Tous les tests sur graines et de germination étaient effectués en laboratoire et en serre. Les semences pour les essais au champ étaient réalisées en serre puis les plantes étaient transplantées dans le champ après six mois de croissance, selon un dispositif RCBD à 2 répétitions. Les résultats obtenus montrent des différences importantes au niveau de la taille des graines, des pourcentages de germination, de la vigueur germinative et de la production de tubercules entre les hybrides 4x×2x et les lignées issues de pollinisation naturelle (tableaux 1 et 2). La descendance des hybrides étaient meilleure pour tous les caractères. Le rendement moyen en tubercules dans les deux groupes souligne la supériorité des familles de 4x×2x (tableau 3). Le comportement des lignées 4x×2x s'explique par le mode de formation du pollen 2n. Dans ce mécanisme, plus de 80 pour cent de l'hétérozygotie du parent 2x est transmis à la descendance. En dépit des caractéristiques intéressantes des familles d'hybrides, leur co?t relativement élevé peut être une limite à la diffusion dans les régions de grande culture. Par conséquent, l'utilisation de graines issues de pollinisation naturelle peut être économique pour la production de pomme de terre jusqu'à ce qu'une méthode de création d'hybrides moins onéreuse soit mise au point.

     

Male sterility and 2n pollen in 4x progenies derived from 4x×2x and 4x×4x crosses in potatoes

Summary The objectives of these experiments were to evaluate male sterility (MS) and 2n pollen frequency in 77 4x×2x families and 26 4x×4x families. The 2x parents were haploid-species hybrids and the 4x parents mainly clones withS. tuberosum ssp.tuberosum (tbr) andS. tuberosum ssp.andigena (adg) in their genetic background. The female parents had different cytoplasms: adg, tbr,S. demissum (dms), andS. stoloniferum (sto). Families from female parents with adg or dms cytoplasm did not have MS plants. However, families derived from crosses of many tbr females with 2x hybrids and a 4x adg clone had a high percentage of MS plants; all the progenies from cv. Serrana (sto cytoplasm) possessed tetrad sterility. These results can be explained as due to an interaction between a dominant male sterility (Ms) gene with tbr and sto cytoplasms. Some MS 4x hybrids (tbr×adg) used as females had the Ms gene, but male fertility was restored in some plants of their progenies when they were crossed with 4x tbr clones. This indicates that some tbr clones have a fertility restorer gene (Rt). The results from both 4x×2x and 4x×4x families fit the expected ratios assuming chromatid segregation for both the Ms and Rt loci. The gene frequency of parallel spindles was estimated in the 4x population as 0.74.     

马铃薯4x×2x杂种无性一代产量及产量性状的表现

产生２ｎ花粉的Ｎｅｏ－ｔｕｂｅｒｏｓｕｍ双单倍体×Ｓｏｌａｎｕｍｐｈｕｒｅｊａ的二倍体杂种以及其与Ｎｅｏ－ｔｕｂｅｒｏｓｕｍ双单倍体的回交后代与普通马铃薯（Ｓ．ｔｕｂｅｒｏｓｕｍｓｓｐ．ｔｕｂｅｒｏ ｓｕｍ品种东农３０３进行了杂交。４ｘ杂种在株高方面表现出了很强的杂种优势。商品薯产量与４ｘ亲本无显著差异，但两个杂种群体的最高值分别超４ｘ亲本的２６０ｇ／株和６０ｇ／株。总产量杂种优势明显，但杂种单株结薯数多，平均块茎重小，这些是限制４ｘ×２ｘ杂种利用的不利因素，需要进一步地改善。     

Yield and tuber characteristics of 4x progeny from 2x×2x crosses

Summary Unrelated, unselected 2x haploid-species hybrids that produce 2n eggs, by second division restitution and 2n pollen due to first division restitution were intercrossed to generate 4x progeny. The 19, 4x families were compared with seven cultivars for tuber yield, tuber appearance and tuber set. The mean yield of the 4x families was 30% higher than the mean yield of the cultivars. The best five 4x families outyielded the best five cultivars by 57% and 69% at two locations. Cultivars were superior for tuber appearance and tuber set. The high yields of the 4x families are due to genetic diversity of the 2x parents and their ability to transmit that diversity to the 4x progeny utilizing 2n gametes. The potential of the 2x×2x breeding method is dependent on developing 2x hybrid clones that form 2n eggs and possess desirable tuber types; clones producing 2n pollen and good tuber quality are available.     

Performance and stability of nine 4x clones from 4x−2x crosses and four commercial cultivars

Summary Nine clones from 4x−2x crosses were evaluated along with four cultivars in six Wisconsin environments. The 4x clones from 4x−2x crosses had as their 2x parents hybrids with haploid Tuberosum and either Group Phureja orSolanum tarijense. The 2x hybrids produced 2n pollen by first division restitution. The traits evaluated were marketable yield, mass density and chip color. Stability analysis was conducted by determining the linear relationships between mean yield of individual clones and environmental mean yields. Mean square deviations from linear regression (s² _d) were used to evaluate phenotypic stability and regression coefficients (b) as environmental response parameters. Three 4x clones from 4x-2x crosses were more stable and not significantly different for yield than cv. Atlantic; some also had superior specific gravity and chip color. The results suggest that the 4x×2x breeding method is very efficient for the production of high yielding, stable clones with superior processing qualities, particularly since the 4x clones from 4x-2x crosses originated from very small source populations.     

2n pollen in eleven 2x, 2EBN wild species and their haploid x wild species hybrids

Summary Sixty-three plants producing 2n pollen were identified among 65 introductions of 11 2x, 2EBN wild species. They were found in 27 of the 65 introductions representing ten of 11 species. The gene frequencies for the allele controlling 2n pollen formation ranged from 0.19 to 0.65. Seventyeight haploid x wild species hybrid families were produced using the previously identified 2n pollen plants as male parents. One hundred and thirty-two plants that produced 2n pollen were identified among the haploid x wild species hybrids at one location and 44 at another. The high number of haploid x wild species hybrids that produced 2n pollen and the large number of hybrids with acceptable pollen stainability demonstrate the potential of the hybrids in 4x × 2x germplasm transfer. Potato breeders can unleash the genetic diversity present in the 2x, 2EBN wild species by screening them for 2n pollen and crossing the 2n pollen producing plants to haploids.     

Erwinia resistance in backcross progenies ofSolanum tuberosum x S. tarijense andS. tuberosum (+) S. commersonii hybrids

Summary The resistance to tuber soft rot caused byErwinia carotovora ofSolanum tuberosum x S. tarijense andS. tuberosum (+) S. commersonii hybrids and their backcrosses is reported. A number of resistant diploid sexual hybrids and tetraploid/hexaploid somatic hybrids were selected. Backcross progenies were obtained through 2x×4x crosses involving a resistant diploid hybrid and tetraploidS. tuberosum, and through 4x×4x crosses between a resistant somatic hybrid andS. tuberosum. The hybrids showed high variability interms of resistance to tuber soft rot. The resistance of progeny from 2x×4x backcrosses was similar to that of the parental sexual hybrid. By contrast, the resistance of genotypes deriving from 4x×4x backcrosses was reduced compared with the resistant somatic hybrid. In general, tuber characteristics of the backcross hybrids improved considerably as compared with their parents, and tuber yield per plant was good.     

Potato haploid performance in 2x × 4x crosses

Nineteen haploids (2n = 2x = 24) extracted from four tetraploids ofSolanum tuberosum (2n = 4x = 48) with high 2n egg frequencies and known mode of 2n egg formation were evaluated via 4x progeny from 2x × 4x crosses (haploid × cultivar) for total tuber yield, tuber appearance and tuber set. Ninety-eight, 4x families (15-20 genotypes each) were planted in the randomized complete block design with two replications at two locations. Analyses of variance were conducted for all haploids over three common testers, and each of the four groups of haploids individually over a variable number of testers. The performance of haploids, based on general combining ability effects, was consistent for all traits, when tested with either three or more testers. Diploid clones can be evaluated in 2x × 4x crosses using three unrelated adapted 4x testers. Nonorthogonal comparisons indicated no significant differences for all traits among four groups of haploids, and between modes of 2n egg formation. Significant variation within each group of haploids indicates that selection at the haploid level should be carried out among haploids from either the same or different cultivars. Superior haploids can be utilized in 2x × 4x and 2x × 2x breeding schemes to generate 4x clones.     

The effect of direction of hybridization (4x × 2x vs. 2x × 4x) on yield of cultivated potatoes

The types of reciprocal differences found in higher plants are generally limited to those characters which are controlled by one or two major genes. There are, however, a few examples where the progeny from reciprocal crosses differ in yield or other quantitative characters. Both have been ascribed mainly to cytoplasmic inheritance. In cultivated tetraploid potatoes (Solatium tuberosum Group Tuberosum) differences in yield of the progeny from reciprocal crosses have been found. These reciprocal differences are attributed to the mode of 2n gamete formation rather than cytoplasmic differences. Preliminary experiments involving reciprocal crosses between Tuberosum cultivars (2n=4x=48) and Phurejahaploid Tuberosum hybrids (2n=2x=24) indicated that some progenies from 4x x 2x crosses yield more than progenies from reciprocal crosses. It was hypothesized that the differences in tuber yield may be due to one or more of the following factors: (i) the effect of high fruit set on yield —i.e. almost 100% of the progenies from 2x X 4x crosses are male fertile and thus higher fruit set is expected than in the 4x X 2x progenies where only about 57% of the F₁ clones are male fertile; (ii) the advantage of being in 4x cytoplasm if the nucleus is 4x; and (iii) the difference in mode of 2n gamete formation-first division restitution (FDR) gives highly heterozygous gametes and second division restitution (SDR) results in highly homozygous gametes. Six families of reciprocal crosses between three 4x cultivars (Merrimack, Superior, and Wis 639) and three 2x Phureja-haploid clones (W5293.3, W5295.7, and W5845.1) were planted in 1970 in Rhinelander in a split-plot design with two tubers of each clone in each of two replicates. Flowers were removed from one of the paired plants in each family. Even though the fruit yields were higher in 2x X 4x progenies as expected, tuber yields were not affected by fruit formation. Therefore, factor (i) is apparently not involved in the reciprocal differences observed. Significantly higher tuber yields were obtained from 4x X 2x families only with the 2x clone W5295.7. This eliminated factor (ii), since no difference in reciprocals was obtained with the other 2x clones. Factor (iii) could thus be responsible for differences in reciprocal crosses, since cytological observations indicate only W5295.7 of the 2x clones used forms 2n pollen by FDR.     

Tuber yields of families from open pollinated and hybrid true potato seed

This work was undertaken to compare the tuber yields, vegetative vigor and uniformity of seedling transplants obtained from different breeding methods. Twenty hybrid families from different groups of crosses involving 4x clones and 2x Phureja-haploid Tuberosum hybrids; 4x × 4x, 4x × 2x (FDR), 4x × (4x × 2x), and (4x × 2x) × (4x × 2x) and eleven open pollinated families from 4x clones, and 4x hybrids from 4x × 2x crosses were compared in a replicated yield trial. Families from 4x × 2x (FDR) crosses were the highest yielding among the hybrid groups having approximately twice the yield of the open pollinated 4x clones. They also were the best group in relation to plant vigor and uniformity. Based on these results, it appears that a good breeding scheme for high-yielding and uniform progeny from true potato seeds is the 4x × 2x hybridization method where the male parent is capable of producing 2n pollen via a first division restitution mechanism.     

Male fertility and 2n pollen production in haploid-wild species hybrids

Hybrids (2n = 2x = 24) from crosses between Group Tuberosum haploids (2n = 2x = 24) and 24-chromosome, wild species must be fertile and produce 2n gametes if they are to be used in breeding schemes involving sexual polyploidization. Male fertility of the 2x hybrids depends primarily on the species parent. Hybrids involvingS. berthaultii, S. chacoense, S. kurtzianum, S. spegazzinii, andS. tarijense as parents are male fertile. The haploid parents are male sterile, so the species not only contribute genetic diversity and desirable traits, but also genes necessary for male fertility.S. infundibuliforme, S. raphanifolium, andS. sanctae-rosae parents produced male sterile hybrid progeny, while use ofS. boliviense, S. bukasovii, S. canasense, S. microdontum, andS. sparsipilum results in families containing both fertile and sterile plants. Sterility may be due to interactions between Tuberosum cytoplasm and dominant nuclear genes from the species. Genetic male sterility and environmental conditions may also be responsible for low pollen stainability in some hybrids. Many fertile, 2x hybrids produce 2n pollen. Selection for 2n pollen in the species is fast and efficient, but meiotic analysis is required in male sterile haploids. Fortunately, selection in the species parent alone results in a large number of 2n pollen-producing hybrids. These 2x hybrids have been used to generate tetraploids via 4x × 2x crosses.     

Germplasm transfer from the wild tetraploid speciesSolanum acaule bitt. to the cultivated potato,S. tuberosum L. using 2N eggs

Solanum acaule Bitt. (acl ¹, 2n=4x=48 and 2n=6x=72) is a wild potato species in which resistance to adverse biotic and abiotic agents has been found. The tetraploid subspeciesacaule andaemulans cannot be easily crossed with the common potato,S. tuberosum L. Grouptuberosum (tbr, 2n=4x=48) because the development of the endosperm in the hybrid seed is abnormal. Since 4xacl behaves as a diploid in intra and interploidy crosses, it was postulated that the hybridization barrier withtbr could be overcome if the wild species produced 2n gametes. One hundred and ninety-seven plants of 4xacl (22 introductions) were screened for pollen of heterogeneous size; only two plants produced this type of pollen, with 3% and 5% of pollen in the large class, respectively. On the other hand, 427 plants (47 introductions) were screened for 2n eggs through controlled crosses withtbr. Forty-one plants (13 introductions) produced 219 berries of which 46 contained at least one plump seed. Seed from berries with low seed set gave rise to hexaploid hybrids possibly through the functioning of 2n eggs from 4xacl. A sample of these hybrids was successfully backcrossed as female parents totbr, giving rise to 5x BC₁ progenies. These BC₁ progenies could be easily back-crossed totbr, yielding tetraploid or near tetraploid BC₂ plants.     

Haploids of tetraploid (2n=4x=48) Mexican potato species—Their extraction,cytology and crossability

Successful isolation of haploids (2n=2x=24) from 4x(2EBN) Mexican species was achieved by using Modified Wenzel (MW) and Nitsch and Nitsch (NN) media for anther culture. Both media, MW and NN, gave consistent regeneration of plantlets from 4x(2EBN) Mexican species, and responses to the media were both species and accession (PI) specific. Plantlets regenerated directly from microspores, by-passing the callus cycle, were mostly haploid (2n=2x=24). Haploid plants appeared smaller and weaker than their parents with a drastic reduction in both male and female fertility. Abnormalities observed during meiosis and the lack of success in crossing confirmed the effect of the meiotic irregularities observed in the haploids. A total of 290 seeds was obtained from the 4x × 2x crosses of 4x Mexican species with irradiated pollen of 2xS. cardiophyllum and 2xS. chancayense; however, no haploid (2n=2x=24) plants resulted. Extraction of haploids from colchicinedoubled 4x (2EBN) or natural 6x(4EBN) Mexican species using a Phureja pollinator was examined to provide clues to the potential of using 2x (1EBN) pollinators for haploid induction in 4x(2EBN) × 2x(1EBN) crosses.     

Characteristics of 2N pollen producing triploid hybrids betweenSolanum stoloniferum and cultivated diploid potatoes

Segregants which produce 2n pollen were identified inSolanum stoloniferum and hybridized with 2n pollen producing cultivated diploids. A number of the resulting triploids produced 2n pollen, ranging from less than 1 up to 7.6%. Parallel spindles at Anaphase II were shown to be the probable meiotic mechanism responsible for 2n pollen production. Thus the genetic consequences are equivalent to first division restitution. This 2n pollen produced by triploids should facilitate transfer of genes from wild allotetraploids to cultivated tetraploids. There should be increased gene exchange between nonhomologous wild and cultivated genomes during microsporogenesis of the triploids. This methodology should have merit in crosses with allotetraploid species in the Series Longipedicellata and Acaulia.     

Breeding value of 2n pollen diploid from hybrids and Phureja in 4x-2x crosses in potatoes

Breeding value of 2n pollen from Phureja — haploid Group Tuberosum diploid hybrids (DHs) and Phureja was compared in crosses with tetraploid cultivars. Mean total yield for Phureja — derived (PT) hybrids significantly exceeded that for DH — derived (DHT) hybrids, and did not differ from that for cultivar parents. PT hybrids had the largest range for total yield, with 13% of the population between 4200 and 5750 gm/plant (0.8 and 4.6% for DHT hybrids and cultivars, respectively). Moreover, the tail of the frequency distribution of total yield for PT hybrids extended 920 gm/plant above that for cultivars. Mean marketable yield for PT hybrids significantly exceeded that for DHT hybrids, but was significantly below that for cultivars. However, the tail of the distribution of marketable yields for PT hybrids exceeded that of the cultivars by 820 gm/plant. Significant general combining ability among 2x and 4x parents for total and marketable yield indicates that the potential of specific crosses can be predicted by average performance of parents in test-crosses. Predictability of parental breeding value and exceptional yield potential of hybrids make 4x-2x breeding an attractive alternative to conventional approaches.     

Aneuploid hybrids from 5x − 4x crosses in potato: Chromosome number, fertility, morphology and yield

The objective of this study was to characterize sixS. commersonii - S. tuberosum progenies deriving from 5x ? 4x crosses to provide evidence that they can be used in potato breeding. Hybrids analyzed (coded PTH) had a chromosome number between the 4x and the 5x level. In particular, progeny means for chromosome number ranged from 50±0.5 to 54±0.7, with most (70%) genotypes exhibiting a low aneuploid level of 48 to 53 chromosomes. Despite being aneuploid, the hybrids did not generally show phenotypic aberrations or vigor reduction common to aneuploids of other species. Most genotypes resembledS. tuberosum in growth habit, whereas corolla type, eye depth, and stolon length varied between and within progenies. Aneuploid hybrids produced tubers under long-day conditions. However, tuber yields were not as high as expected, probably due to lack of adaptation of theS. commersonii genome to long-day conditions. Although aneuploidy has often been associated with reduced male and female fertility, many 5x ? 4x hybrids were fertile in crosses withS. tuberosum. The average berry set and number of seeds/berry were 33% and 34.7, respectively, following PTH ×S. tuberosum crosses, and 40% and 51.7, respectively, forS. tuberosum × PTH crosses. The useful variation for fertility and tuber traits found in this material is being used for further breeding efforts.     

The identification and evaluation of hybrid plants among open pollinated tkue seed families

An increase in the proportion ofhybrid vs. selfed plants improves the performance ofopen pollinated (OP) true potato seed (TPS) families. Strategies to increase the proportion ofhybrid seeds were investigated. A marker gene for yellow tuber flesh in a diploid clone, interplanted as the male parent, permitted identification of 4x × 2x hybrids among the OP progeny. Only 5% of the OP progeny were 4x × 2x hybrids. The proportion of 4x × 2x hybrid plants among the OP progeny was increased when the female parent had low male fertility and when only the largest 25% of the seeds were planted. The hybrid 4x × 2x plants were superior for all traits evaluated on the OP seedling transplant families. Traits included vigor, flowering, pollen stainability, OP fruit set, and tuber yield. Effects of the female parent were significant only for vigor and flowering, while seed size did not affect values for any trait consistently: With the limited outcrossing observed, the low seed set obtained when using parents with low male fertility and the work involved in selecting the largest seeds within individual fruits were not justifiable. However, research with OP true seed families asa low cost planting material should be continued, since high yielding OP families have been identified.     

Inheritance of potato chip color at the 24-chromosome level

Potato chip color is an important trait in determining quality and acceptability of cultivars for processing. Therefore, the genetics of potato chip color is important in varietal breeding. The use of 2xS. tuberosum Group Tuberosum haploid × 2x wild species hybrids provides disomic inheritance for genetic analysis of chip color. Two hundred forty haploid-species hybrids representing crosses between 15 haploids and 10 species were evaluated for potato chip color after 21 days reconditioning (18 to 20C) following 6 months at 4C. Phenotypic variation for chip color was found among the haploid-species hybrids. Crosses between good, medium, and poor performing clones were made in all combinations and 136 families were obtained. Progeny were evaluated for reversion resistance (chipping direct from 10C storage) and reconditioning (chipping from 4C after tuber warming) chip color. Variation for chip color was found among the progeny for each trait. Significant correlations between reversion resistance and reconditioning were found. Yet, the use of at least two chip tests (one for reversion resistance and one for reconditioning) is more informative than the use of one test and subsequent predictions made, regarding the other trait. A three locus hypothesis for both reversion resistance and reconditioning is proposed for the inheritance of each trait. One dominant allele at each of the three loci is necessary for good chipping. One or two loci may be common to both characteristics. Good chipping 2x hybrids that produce 2n pollen can be used to transfer this trait to the 4x level through use of 4x × 2x crosses.