Triploid and tetraploid hybrids from diploid x tetraploid crosses in Grindella (compositae)

Crosses between the diploids G. oxylepis var. eligulata Steyermark (Mexico) and G. havardii Steyermark (New Mexico) and the tetraploid G. aphanactis Rydb. (New Mexico) were made. With G. aphanactis as the pistillate parent and G. havardii as the pollen parent a triploid hybrid was obtained in which the maximum meiotic configuration observed was 6m. The plant was 10 % fertile. Two triploid hybrid plants were also obtained when G. aphanactis was used as the pistillate parent and G. oxylepis var. eligulata was the pollen parent. One plant was about 20 % fertile and the other had a maximum configuration of 3_II + 4_III. The reciprocal cross produced a tetraploid which had a maximum configuration of 6_II + 3_IV and was 8 % fertile. The tetraploid plant undoubtedly resulted from the union of an unreduced gamete from the 2n parent and a normally reduced gamete from the tetraploid. Morphology of the hybrids was usually intermediate when compared with the parental species, although some characters in the triploids were those of the diploid parent. Chromosome end arrangements of the respective genomes and putative pairing relationships are presented and phylogenetic implications are discussed. It is concluded that G. aphanactis is more closely related to G. havardii than to G. oxylepis var. eligulata.     

The genetics of diploid x tetraploid and reciprocal cyclamen crosses

Zusammenfassung Bis heute wurden bei Cyclamen 5 Kreuzungen diploid x tetraploid und 2 Kreuzungen tetraploid x diploid untersucht. Eine lieferte nur 2 weitgehend sterile triploide F₁-Pflanzen. In allen anderen Fällen waren F₁, F₂ und — soweit untersucht — F₃ vollständig tetraploid.In 3 Kreuzungen konnte die Blumenfarbe analysiert werden. Es stellte sich heraus, daß die F₂- und F₃-Spaltungen einem autotetraploiden Schema entsprechen. Hieraus geht hervor, daß bei den Kreuzungen diploid x tetraploid die Eizelle unreduziert oder verdoppelt vor der Befruchtung ist, während bei den Kreuzungen tetraploid x diploid der generative Kern unreduziert oder verdoppelt vor der Befruchtung sein muß.

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Herrn Prof.H. Kappert zum 65. Geburtstag gewidmet     

Heterofertilization exhibited by trifluralin-induced bicellular pollen on diploid and tetraploid maize crosses.

The heterofertilization rates and fertility of trifluralin-induced bicellular pollen were investigated in maize (Zea mays L.). A diploid inbred line, Oh43 (r1/r1), and a tetraploid line, Q28-1 (r1/r1/r1/r1), were pollinated with a trifluralin treated diploid stock heterozygous for R1-scm2. The gene R1-scm2 conditions purple pigmentation in both the embryo and the aleurone layer. Heterofertilized kernels were detected as discordant kernels, i.e., yellow kernel with purple embryo or purple kernel with white embryo. The diploid-diploid crosses treated with 0.2-0.3% Trefanocide solution (0.09-0.13% trifluralin) resulted in incidences of discordant kernels (3.7-4.8%) that were significantly higher than the control (2.3%). Most of the seedlings (86%) of the discordant kernels in the 0.3% treatment were triploids or triploid-class aneuploids. In tetraploid-diploid crosses, trifluralin treatments increased the number of plump kernels on the tetraploid ears. In the 0.3% treatment, 5.2% of ovaries produced plump kernels on the ears and most of the seedlings (92%) were tetraploids or tetraploid-class aneuploids, whereas in the control, only 1.5% ovaries produced plump kernels and most of the seedlings (98%) were triploids or triploid-class aneuploids. A high rate of discordance was observed among the plump kernels both in the treated plots (36.1-48.0%) and in the control (33.3%). Consequently, almost all of the plump kernels from the tetraploid-diploid crosses were considered to be the results of heterofertilization.     

Tetraploids,triploids, and 2n pollen from diploid interspecific crosses with cassava

Summary Interspecific crosses of five cultivated cassava varieties (2n=36) were made with two related Manihot species, M. epruinosa (2n=36) and M. glaziovii (2n=36). From these diploid interspecific crosses, four spontaneous tetraploids (2n=4x=72) and two triploids (2n=3x=54) were isolated for the first time in cassava. Occurrence of relatively high frequencies (0.1%–35.6%) of 2n pollen and of apomixis seems to be associated with sexual polyploidization. The tetraploids and triploids were very vigorous and one of the tetraploids performed as well as the best variety in uniform yield trials conducted in Nigeria. These spontaneous polypoloids provide greater genetic variation and offer an opportunity to breed radically new cassava varieties. Approaches for isolating and utilizing the polyploid cassava clones for varietal and population improvement are discussed.     

Isozyme gene duplication in diploid and tetraploid potatoes

Summary Isozyme techniques allow the study of gene redundancy in different ploidy levels of potato (Solanum tuberosum). In tetraploid potatoes all isozyme loci are duplicated. No sign of structural or regulatory divergence was found, as is expected due to their tetrasomic inheritance patterns. In addition to this genetic redundancy, produced by a relatively recent polyploidization event, some additional redundancy was found for at least three enzymes even in diploid groups and species. These older duplicate genes show structural and regulatory divergence, indicating they appeared by a separate polyploidization event far in the past. Their common origin is still recognizable by both their expression in the same subcellular compartment and by the dimerizing ability of the isozymes they encode. To account for the present chromosome number x = 12 of the Solanaceae family, the most frequently found among the species, a hypothetical polyploidization event is proposed.     

Estimation of preferential pairing in tetraploid x diploid hybridizations

Summary Crosses made between tetraploid and diploid, 2n pollen-producing species directly transfer from one-half to the entire diploid genome from the diploid to the tetraploid level, depending on the mechanism of 2n pollen formation and the amount of crossing-over that occurs. Tetraploid plants that result from tetraploid x diploid hybridizations can be further utilized in a breeding program. It is postulated that preferential pairing between homologous chromosomes derived from the original tetraploid or diploid parent occurs in the tetraploid x diploid hybrid. Depending on the genetic divergence of the species involved, preferential pairing of homologous chromosomes may range from zero to one. Theoretical estimates of the amount of preferential pairing and the standard errors of these estimates are derived for cases where the diploid parent produces 2n gametes by either a first division or a second division restitution mechanism.     

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.     

Megagametophyte organization in diploid alfalfa meiotic mutants producing 4n pollen and 2n eggs

Megagametogenesis was studied in five diploid alfalfa mutants producing 4n pollen and 2n eggs, using a stain-clearing technique. All mutants produced embryo sacs with a variable number of supernumerary nuclei both at the early (bi- and tetra-nucleate) and at the late (eight-nucleate) stages of development. The presence of supernumerary nuclei is considered to be a consequence of the production of coenocytic megaspores. The production of 2n eggs was confirmed through cytological investigation by means of the diameter of the egg-cell nucleolus. The frequency of 2n eggs was lower than the frequency of binucleated macrospores as previously determined. This discrepancy may be due to environmental effects but also to the fact that binucleated macrospores may degenerate or may, after two mitotic divisions, give rise to eight-nucleated embryo sacs counted as normals.     

Occurrence and cytological mechanism of 2n pollen formation in a tetraploid accession of Ipomoea batatas (sweet potato)

We evaluated a 4x accession of Ipomoea batatas (L.) Lam. and four 2x accessions of Ipomoea triloba for 2n pollen production. Approximately 90% of the genotypes of accession 81.2 (I. batatas, 4x) produced 2n pollen with different frequencies. In contrast, none of the genotypes of I. triloba produced 2n pollen. The diameter of the 2n pollen was approximately 30% ((3) sqrt 2) larger than that of the n pollen, making it easy to identify, measure, and quantify. The correlation (r = 0.93**) between the frequency of giant pollen and the frequency of dyads and triads was highly significant, strongly suggesting that the giant pollen grains were 2n pollen. The 2n pollen producers presented either a parallel or tripolar spindle arrangement (Y shaped) at anaphase II instead of the normal 60 degrees crossed spindle orientation. These two abnormal spindle configurations produced dyads and triads, with different frequencies (13-67%), instead of tetrads. Occasionally a metaphase II spindle variation was found with a single fused spindle, which also forms a dyad. The correlation (r = 0.89**) between the frequency of 2n pollen and the frequency of parallel, fused, and tripolar spindle arrangements was also highly significant, suggesting that these abnormal spindle configurations are involved in the production of 2n pollen in I. batatas. When we evaluated the efficiency of 2n pollen in polyploidization using 4x x 4x (2n) crosses, all progenies were 4x, suggesting the existence of barriers to crossability between 4x genotypes and their 2n pollen-producer counterparts.     

Cytology of 2N pollen formation and pollen morphology in diploid lotus tenuis (fabaceae)

Two genotypes of Lotus tenuis Waldst & Kit. ex Willd. PI 204882, a diploid (2n = 2x = 12), were identified as producing 2n pollen (maximum = 6%). The objectives of this research were: (1) to determine the mechanism(s) of 2n pollen formation in the L. tenuis genotypes and (2) to morphologically describe n and 2n pollen using light and scanning electron microscopy. Meiotic studies revealed that 2n pollen resulted from tripolar spindles during anaphase II of microsporogenesis. The 2n pollen germinated well, although abnormal pollen tubes were observed. The genetic constitution of 2n pollen resulting from tripolar spindles is equivalent to first division restitution (FDR) of meiosis. Fresh and air-dried pollen samples exhibited differences in size and shape under light and scanning electron microscopy. The size of diploid (2n) pollen was larger than that of haploid (1n) pollen. Normal haploid (1n) pollen was globose-prolate in shape, while diploid (2n) pollen was tetrahedral in shape.     

QTL mapping in outbred tetraploid (and diploid) diallel populations

Over the last decade, multiparental populations have become a mainstay of genetics research in diploid species. Our goal was to extend this paradigm to autotetraploids by developing software for quantitative trait locus (QTL) mapping in connected F1 populations derived from a set of shared parents. For QTL discovery, phenotypes are regressed on the dosage of parental haplotypes to estimate additive effects. Statistical properties of the model were explored by simulating half-diallel diploid and tetraploid populations with different population sizes and numbers of parents. Across scenarios, the number of progeny per parental haplotype (pph) largely determined the statistical power for QTL detection and accuracy of the estimated haplotype effects. Multiallelic QTL with heritability 0.2 were detected with 90% probability at 25 pph and genome-wide significance level 0.05, and the additive haplotype effects were estimated with over 90% accuracy. Following QTL discovery, the software enables a comparison of models with multiple QTL and nonadditive effects. To illustrate, we analyzed potato tuber shape in a half-diallel population with three tetraploid parents. A well-known QTL on chromosome 10 was detected, for which the inclusion of digenic dominance lowered the Deviance Information Criterion (DIC) by 17 points compared to the additive model. The final model also contained a minor QTL on chromosome 1, but higher-order dominance and epistatic effects were excluded based on the DIC. In terms of practical impacts, the software is already being used to select offspring based on the effect and dosage of particular haplotypes in breeding programs.     

Assessment of genetic variability of haploids extracted from tetraploid (2n = 4x = 48) Solanum tuberosum.

The objectives of this study were to assess the genetic variability of haploids (2n = 2x = 24) extracted from tetraploid Solanum tuberosum through 4x x 2x crosses with Solanum phureja. Molecular and phenotypic analyses were performed to fingerprint the genotypes used and to evaluate their potential use in breeding programs. AFLP analysis revealed the presence of specific bands derived from the tetraploid seed parent S. phureja, as well as ex novo originated bands. On average, 210 bands were visualized per genotype, 149 (70%) of which were common to both parental genotypes. The percentage of S. tuberosum specific bands ranged from 25.1% to 18.6%, with an average of 22%. The fraction of genome coming from S. phureja ranged from 1.9% to 6.5%, with an average value of 4%. The percentage of ex novo bands varied from 1.9% to 9.0%. The presence of S. phureja DNA is very interesting because it indicated that S. phureja pollinator is involved in the mechanism of haploid formation. The characterization for resistance to Erwinia carotovora subsp. carotovora and potato virus X (PVX) provided evidence that haploids may express traits that are lacking in the tetraploids they come from, which can be useful for both genetic studies and breeding purposes. It is noteworthy that genotypes combining resistance to both diseases and good pollen stainability were identified. Other possible breeding implications owing to the presence of S. phureja genome in the haploids analyzed are discussed.     

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

Summary 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.     

Genetic diversity in diploid vs. tetraploid Rorippa amphibia (Brassicaceae)

The frequency of polyploidy increases with latitude in the Northern Hemisphere, especially in deglaciated, recently colonized areas. The cause or causes of this pattern are largely unknown, but a greater genetic diversity of individual polyploid plants due to a doubled genome and/or a hybrid origin is seen as a likely factor underlying selective advantages related to life in extreme climates and/or colonization ability. A history of colonization in itself, as well as a recent origin, and possibly a limited number of polyploidization events would all predict less genetic diversity in polyploids than in diploids. The null hypothesis of higher gene diversity in polyploids has to date hardly been quantified and is here tested in self-incompatible Rorippa amphibia (Brassicaceae). The species occurs in diploid and tetraploid forms and displays clear geographical polyploidy in Europe. On the basis of eight microsatellite loci it can be concluded that the level of gene diversity is higher in tetraploids than in diploids, to an extent that is expected under neutral evolution when taking into account the larger effective population size in the doubled cytotype. There is thus no evidence for reduced genetic diversity in the tetraploids. The evidence presented here may mean that the tetraploids' origin is not recent, has not been affected by bottlenecks and/or that tetraploids were formed multiple times while an effect of introgression may also play a role.     

Duplicate gene expression in diploid and tetraploid loaches (Cypriniformes,Cobitidae)

Botia macracantha and B. modesta have been demonstrated to be tetraploid species on the basis of their karyotypes and on the basis of the expression of a number of isozymes encoded by duplicate loci. A rather low percentage of duplicate loci was detected by electrophoresis, compared to that for other tetraploid Cypriniformes. Several hypotheses have been advanced to account for the low levels of duplicate gene expression observed. Lastly, many of the duplicate loci have diverged to unique patterns of expressions in different tissues or different levels of activity within a single tissue.     

Cytogenetics, geographical distribution, and pollen fertility of diploid and tetraploid cytotypes of Santolina pectinata Lag. (Asteraceae: Anthemideae)

A cytogenetic study of 62 populations of Santolina pectinata in Spain shows the existence of two ploidy levels. The diploid cytotypes with 2 n  = 18 occupy the eastern Betic mountains, and the tetraploid cytotypes with 2 n  = 36 are located on the spurs of the Iberian System. The former show a much wider ecological spectrum than the latter. Mixed cytotypes were observed in two diploid populations, with one tetraploid in each, showing different karyotypes. Three trisomic individuals were detected, one in a diploid population and the other two in a tetraploid population. Also, three hypotetraploid individuals were detected in a tetraploid population. Polyploidy is shown to be spontaneous and recurrent, promoting partial sterility in the pollen. Structural chromosomal changes, principally translocations, and local speciation through autopolyploidy are the principal factors in the evolution and diversification of this species.  © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society , 2008, 156 , 657–667.     

Marker-assisted selection of diploid and tetraploid potatoes carryingRpi-phu1, a major gene for resistance toPhytophthora infestans

TheRpi-phu1 gene originates from an interspecific hybrid betweenSolanum stenotomum andS. phureja, and confers a high level of resistance toPhytophthora infestans (late blight) in potato. TheRpi-phu1 was introduced by crossing at the diploid level into theS. tuberosum gene pool and then transferred to the tetraploid level by means of2n gametes. Tetraploid lines carrying theRpi-phu1 were selected for further crosses. A molecular marker GP94, linked in mapping population 97-30 with theRpi-phu1 (6.4 cM), was applied to other unselected populations (2 diploid and 1 tetraploid), and was shown to be useful in marker-assisted selection (MAS) of the resistant individuals. GP94 was applied also in commercial breeding in 2 tetraploid populations. Although the marker allele of GP94, characteristic for theRpi-phu1 presence, was rare in theS. tuberosum gene pool, it seemed to be common in 2 other sources of resistance to late blight:S. ruizceballosii andS. kurtzianum.