Allotetraploid Hybrids Research Articles

Induced interspecific androgenesis using diploid sperm from allotetraploid hybrids of common carp×red crucian carp

In the present study, viable diploid androgenetic individuals (AT-ag) (2 n = 100) have been developed in UV-radiated eggs of goldfish (GF) ( Carassius auratus) following fertilization with diploid spermatozoa from male allotetraploid hybrid (AT) F 13 (4 n = 200) of red crucian carp ( C. auratus red var.) (♀) × common carp ( Cyprinus carpio L.) (♂). In the experimental groups, eggs were inactivated by UV-rays, the range of UV-radiation dose tested varied from 240 to 600 mJ/cm 2, the relative high hatching rate (4.1 ± 0.4%) and survival from hatching to start of feeding (41.3 ± 2.6%) were given at UV dose of 300 mJ/cm 2. The androgenetic nature was confirmed by chromosome observation and DNA content measurement of somatic cells by a flow cytometer. The AT-ag were diploid (2 n = 100) whereas the controls (GF × AT) were triploids (3 n = 150). The diploid AT-ag grew slower than the triploid GF × AT. The AT-ag and GF × AT were also morphologically distinguishable. The AT-ag began to sexually mature at the age of 2 years, and their fertility further increased at the age of 3 years. The tetraploids were obtained in the offspring (F 1ag × ag ) produced by mating the males with females of AT-ag and in the progeny (AT-ag × AT) generated by crossing the males of AT with the females of AT-ag, indicating that diploid AT-ag could produce diploid gametes. All the sex ratios of males to females in AT-ag, F 1ag × ag , AT-ag × AT were not significant ( P > 0.05) from the ratio of 1:1, suggesting the XXXY sex-determining mechanism in the males of AT. The present study indicated that bisexual fertile diploid AT-ag were successfully induced from diploid spermatozoa of AT.

What is Paeonia officinalis subsp. banatica (Rochel) Soó 1960?

The taxon currently named P. officinalis subsp. banatica has been treated as a subspecies of P. officinalis by most authors. Recent genetic studies revealed that this taxon is an ancient allotetraploid hybrid carrying the genes of P. mairei and the ancestor of P. officinalis. Based on its current polyphyletic status, degree of genetic and morphological divergence, its geographical distribution and unique genetic constitution I argue that this viewpoint is incorrect, and this taxon should be recognised as an independent species.

Inheritance and interactions of incompatibility alleles in the tetraploid sour cherry

Three progenies of sour cherry (Prunus cerasus) were analysed to correlate self-(in)compatibility status with S-RNase phenotype in this allotetraploid hybrid of sweet and ground cherry. Self-(in)compatibility was assessed in the field and by monitoring pollen tube growth after selfing. The S-RNase phenotypes were determined by isoelectric focusing of stylar proteins and staining for RNase activity and, for the parents, confirmed by PCR. Seedling phenotypes were generally consistent with disomic segregation of S-RNase alleles. The genetic arrangements of the parents were deduced to be 'Köröser' (self-incompatible) S1S4.S(B) S(D), 'Schattenmorelle' (self-compatible) S6S13.S(B)S(B), and clone 43.87 (self-compatible) S4S13.S(B)S(B), where "." separates the two homologous genomes. The presence of S4 and S6 alleles at the same locus led to self-incompatibility, whereas S13 and S(B) at homologous loci led to self-compatibility. The failure of certain heteroallelic genotypes in the three crosses or in the self-incompatible seedlings indicates that S4 and S6 are dominant to S(B). However, the success of S13S(B) pollen on styles expressing corresponding S-RNases indicates competitive interaction or lack of pollen-S components. In general, the universal compatibility of S13S(B) pollen may explain the frequent occurrence of S13 and S(B) together in sour cherry cultivars. Alleles S(B) and S(D), that are presumed to derive from ground cherry, and S13, presumably from sweet cherry, were sequenced. Our findings contribute to an understanding of inheritance of self-(in)compatibility, facilitate screening of progenies for self-compatibility and provide a basis for studying molecular interactions in heteroallelic pollen.

Citrus somatic allotetraploid hybrids exhibit a differential reduction of leaf sesquiterpenoid biosynthesis compared with their parents

Leaves of 13 interspecific and intergeneric citrus somatic allotetraploid hybrids were analysed and com- pared with those of their respective parents (lime, lemons, citron, orange, kumquat, poncirus and mandarin) with regards to their sesquiterpenoid contents (µg/g dry weight). The hybrids do not fully retain their parental traits. Sesquiterpenoid production is much reduced (by ~65-90%) in hybrids compared with the sum of their parents. The peculiar behaviours of some compounds were described. Statistical analyses were conducted on a qualitative (the presence/absence of each compound in parents and hybrids) and quantitative (the recovery of each compound in hybrids expressed as a percent- age of the sum of its concentration in their parents) basis. Mandarin exerts a strong dominance over its derived hybrids by greatly lowering the production of most compounds, while the expression of parental genomes was more balanced in hybrids having grapefruit or sweet orange as a parent. This decrease in the ability to synthesize sesquiterpenoids could be due to both the tetraploid nature of the hybrids and the dominance of one or other parental genome. Copyright © 2005 John Wiley & Sons, Ltd.

FCM, SSR and CAPS Analysis of Intergeneric Somatic Hybrid Plants between Changshou Kumquat and Dancy Tangerine

Flow cytometry (FCM), simple sequence repeats (SSR) and cleaved amplified polymorphic sequence (CAPS) were used to characterize the ploidy level and the nuclear and cytoplasmic compositions of two intergeneric somatic hybrids of Changshou kumquat (Fortunella obovata), leaf parent, and Dancy tangerine (Citrus reticulata), suspension parent. FCM showed that both somatic hybrids were tetraploids. One out of two SSR primer pairs was able to detect polymorphisms between the fusion parents, and the somatic hybrids showed additive banding profiles, showing them to be allotetraploid hybrids with inherited nuclear DNA from both parents. CAPS showed that 4 and 1 polymorphic loci were obtained for parental chloroplast and mitochondrial genomes, respectively. The somatic hybrid plants had the same banding patterns as the suspension parent for all of the polymorphic chloroplast markers, and their mitochondrial banding patterns approximated those of the suspension parent with loss of some mtDNA fragments. The present research plays a pivotal role in understanding the relationship between nuclear and cytoplasmic compositions and the likely field performance of somatic hybrids in the future.

Leaf Volatile Compounds of Six Citrus Somatic Allotetraploid Hybrids Originating from Various Combinations of Lime, Lemon, Citron, Sweet Orange, and Grapefruit

Volatile compounds were extracted by a pentane/ether (1:1) mixture from the leaves of six citrus somatic allotetraploid hybrids resulting from various combinations of lime, lemon, citron, sweet orange, and grapefruit. Extracts were examined by gas chromatography-mass spectrometry (GC-MS) and compared with those of their respective parents. All hybrids having an acid citrus parent exhibit the same relative contents in hydrocarbons and oxygenated compounds as the acid citrus, while the (grapefruit + orange) hybrid behaves similarly to its two parents. When volatile compound contents (microg g(-1)) are examined in detail, several behaviors are encountered in hybrids and seem to depend on the presence/absence of the considered parental compound and on the corresponding hybrid combination. Meanwhile, the sesquiterpene hydrocarbons are present in all hybrids at concentrations systematically lower than those of the highest parental producers. Statistical analyses show that hybrids exhibit hardly discriminable aromatic profiles, meaning that no strong dominance of one or the other parent was observed in hybrids with regards to the leaf volatile compound production.

Allotetraploid hybrids produced by protoplast fusion for seedless triploid Citrus breeding

Protoplast fusion has been used to produce novel allotetraploid hybrids for use as parents in crosses with diploids to produce easy-peel, seedless, triploid citrus cultivars. Forty three plants were produced from fusions between protoplasts of ‘Encore’ mandarin and ‘Valencia’ sweet orange, and of ‘Encore’ and ‘Caffin’ clementine mandarin in which protoplasts of each of the donors were isolated from embryogenic callus. Forty plants were confirmed as tetraploid by flow cytometry and chromosome counting. Three (less than 10% of all regenerants) were non-hybrid diploids. DNA fingerprinting confirmed that both parents contributed DNA to the hybrids, which were therefore allotetraploid. The hybrids showed morphological features typical of other tetraploid citrus, including broad, thick, dark green leaves. However, they grew much faster than colchicine-induced autotetraploids. The allotetraploid hybrids will be used in crosses with the diploid cultivar ‘Encore’ mandarin and other clementine-type mandarins in an effort to generate seedless, triploid, easy-peel cultivars with ‘Encore-type’ flavor.

Chromosomal locus rearrangements are a rapid response to formation of the allotetraploid Arabidopsis suecica genome.

Allopolyploidy is a significant evolutionary process, resulting in new species with diploid or greater chromosome complements derived from two or more progenitor species. We examined the chromosomal consequences of genomic merger in Arabidopsis suecica, the allotetraploid hybrid of Arabidopsis thaliana and Arabidopsis arenosa. Fluorescence in situ hybridization with centromere, nucleolus organizer region (NOR), and 5S rRNA gene probes reveals the expected numbers of progenitor chromosomes in natural A. suecica, but one pair of A. thaliana NORs and one pair of A. arenosa-derived 5S gene loci are missing. Similarly, in newly formed synthetic A. suecica-like allotetraploids, pairs of A. thaliana NORs are gained de novo, lost, and/or transposed to A. arenosa chromosomes, with genotypic differences apparent between F(3) siblings of the same F(2) parent and between independent lines. Likewise, pairs of A. arenosa 5S genes are lost and novel linkages between 5S loci and NORs arise in synthetic allotetraploids. By contrast, the expected numbers of A. arenosa-derived NORs and A. thaliana-derived 5S loci are found in both natural and synthetic A. suecica. Collectively, these observations suggest that some, but not all, loci are unstable in newly formed A. suecica allotetraploids and can participate in a variety of alternative rearrangements, some of which resemble chromosomal changes found in nature.

RAPD and microsatellite analysis of diploid gynogens from allotetraploid hybrids of red crucian carp ( Carassius auratus)×common carp ( Cyprinus carpio)

Genetic variation was comparatively analyzed between the artificially induced diploid gynogen population from F 10 allotetraploid hybrids of red crucian carp (♀) ( Carassius auratus red var., 2n=100)×common carp (♂) ( Cyprinus carpio L., 2n=100) and the normal F 10 allotetraploid hybrid population used as the control, using random amplified polymorphic DNA (RAPD) assay and microsatellite analysis. The specific 600-bp fragment for diploid gynogen population was detected by S45 and the specific 900-bp fragment for allotetraploid F 10 hybrid population was detected by S134. The results from RAPD assay and microsatellite analysis were in agreement with each other, that is to say, the diploid gynogens presented lower level of polymorphism than allotetraploid F 10 hybrids. Furthermore, as expected, microsatellite analysis revealed more detailed information on genetic diversity than RAPD assay. The mean percentage of polymorphic loci (12.71%) and Shannon's index of phenotypic diversity (0.25) from RAPD data for diploid gynogen population were significantly lower than those (30.69% and 0.63, respectively) for F 10 allotetraploid hybrid population. The mean number of alleles per microsatellite locus (1.73) in diploid gynogen population was considerably lower than that (2.55) in F 10 allotetraploid hybrid population. The average observed (0.36) and expected heterozygosity (0.26) in diploid gynogen population were lower than those (0.58 and 0.40, respectively) in F 10 allotetraploid hybrid population, indicating that the diploid gynogens presented lower genetic diversity than the allotetraploids. In addition, the mean effective number of alleles at 11 microsatellite loci (1.60) in diploid gynogen population was lower than that (1.88) in F 10 allotetraploid hybrid population. The significant differences between two populations in the average observed and expected heterozygosity, mean number of alleles and effective number of alleles, suggested that the effect of gynogenesis resulted in rather higher genetic homogeneity in diploid gynogens. The comparative RAPD analysis of diploid gynogens and their parents was performed with 34 primers. The identical RAPD pattern was detected between diploid gynogens and their female parent, however, some clear specific RAPD bands were detected between diploid gynogens and their male parents, but not detected in their female parent. The result indicated that heterologous genetic material had incorporated into diploid gynogenetic fish (G 1).

Production of gynogenetic progeny from allotetraploid hybrids red crucian carp×common carp

Following activation by UV-irradiated sperm, diploid eggs of the allotetraploid hybrids (red crucian carp ♀×common carp ♂) developed into normal live gynogens. Cold-shocking these activated diploid eggs failed to induce the expected tetraploidy in them. All these gynogenetic progenies, generated with or without the cold shock, were female diploids with 100 chromosomes, indicating the XXXX genotype of the tetraploid females. The gynogenetic females matured at the age of 2+ suggesting delayed maturity, in comparison to the diploid females of the red crucian carp. Thirty percent of these gynogens were fertile with normal ovary and mature eggs. These gynogens were not morphologically distinguishable from the F 2 hybrids. Like the diploid F 2 hybrids, these diploid gynogens produced diploid eggs, for these eggs developed into triploids with 150 chromosomes, when crossed with red crucian carp. Without the treatment for doubling the chromosome number, the diploid eggs, produced by the F 1 gynogens, developed into the F 2 gynogens following activation by UV-irradiated sperm. Production of these diploid eggs from the diploid gynogens may have resulted from the pre-meiotic endoreduplication of chromosomes. For aquaculture, these diploid eggs remain an important gamete source for the production of tetraploids and triploids, and establishment of diploid gynogenetic hybrid line.

Natural variation in nucleolar dominance reveals the relationship between nucleolus organizer chromatin topology and rRNA gene transcription in Arabidopsis.

In genetic hybrids, nucleolus formation on chromosomes inherited from only one parent is the epigenetic phenomenon, nucleolar dominance. By using Arabidopsis suecica, the allotetraploid hybrid of Arabidopsis thaliana and Arabidopsis arenosa, natural variation in nucleolar dominance was found to occur, providing a unique opportunity to examine homologous nucleolus organizer regions (NORs) in their active and inactive states. In A. suecica strain LC1, NORs derived from A. arenosa are active, whereas A. thaliana-derived NORs are silenced. In A. suecica strain 9502, NORs of both parental species are active. When active, NORs are partially, but not fully, decondensed. Both active and inactive LC1 NORs colocalize with the nucleolus, contradicting the long-standing assumption that rRNA gene transcription drives nucleolus association. Collectively, these observations clarify the relationships among NOR chromatin topology, rRNA gene transcription, and NOR-nucleolus associations. A. suecica strains LC1 and 9502 have each lost one pair of A. thaliana NORs during evolution, and amplified fragment-length polymorphism analysis further indicates that these strains are genetically very similar. These data suggest that nucleolar dominance can result from subtle genetic or epigenetic variation but is not a trait fundamental to a given interspecies hybrid combination.

Transgene-induced RNA interference: a strategy for overcoming gene redundancy in polyploids to generate loss-of-function mutations.

Gene redundancy in polyploid species complicates genetic analyses by making the generation of recessive, loss-of-function alleles impractical. We show that this problem can be circumvented using RNA interference (RNAi) to achieve dominant loss of function of targeted genes. Arabidopsis suecica is an allotetraploid (amphidiploid) hybrid of A. thaliana and A. arenosa. We demonstrate that A. suecica can be genetically transformed using the floral dip method for Agrobacterium-mediated transformation. Transgenes segregate as in a diploid, indicating that chromosome pairing occurs exclusively (or almost so) among homologs and not among homeologs. Expressing a double-stranded (ds) RNA corresponding to the A. thaliana gene, decrease in DNA methylation 1 (DDM1) caused the elimination of DDM1 mRNAs and the loss of methylation at both A. thaliana- and A. arenosa-derived centromere repeats. These results indicate that a single RNAi-inducing transgene can dominantly repress multiple orthologs.

Studies in the genus Hypericum L. (Guttiferae) 4(2). Section 9. Hypericum sensu lato (part 2): subsection 1. Hypericum series 1. Hypericum

Introduction 61Sect. 9. Hypericum 61Subdivision 61Characters and variation 61Distribution and evolution 62Sect. 9. Hypericum subsect. 1. Hypericum series 1. Hypericum 62Characters and variation 62Hybrids 64Distribution and evolution 65Systematic treatment 66Sect. 9. Hypericum 66Sect. 9. Hypericum subsect. 1. Hypericum series 1. Hypericum 67References 119Systematic index 121The subdivision of the large sect. 9. Hypericum and four of its segregated sections having been treated in Part 4(1), Part 4(2) is concerned with those members of sect. 9, viz. Hypericum sensu stricto, in which the stem internodes have glandiferous raised lines – subsect. Hypericum series Hypericum. The series comprises 11 species and can be divided into (i) a group centred in Europe and the Mediterranean (4 species) and (ii) one centred in north-east Asia (6 species), one species of which is confined to western North America and another has spread westward into Europe. The remaining species (H. perforatum) is morphologically and geographically intermediate between two taxa, one from each group, and behaves as an allotetraploid hybrid. It is divided into four subspecies: subspp. perforatum, songaricum (Ledeb. ex Rchb.) N. Robson stat. nov., veronense (Schrank) H. Lindb. and chinense N. Robson subsp. nov. The hybrids of H. perforatum are treated in detail and include H. × desetangsii nothosubsp. balcanicum N. Robson, nothosubsp. nov. (H. maculatum subsp. immaculatum × perforatum). Other new hybrids proposed are: H. × laschii nothoforma froelichii N. Robson, nothoforma nov. (H. maculatum subsp. obtusiusculum × tetrapterum), H. undulatum × tetrapterum and H. elegans × perforatum.

Salinity reduces growth, gas exchange, chlorophyll and nutrient concentrations in diploid sour orange and related allotetraploid somatic hybrids

SummaryDiploid seedlings of sour orange (SO) (Citrus aurantium (L.)) (2×) and allotetraploid (4×) somatic hybrid plants of SO with Carrizo citrange (SO+CC) (Citrus sinsensis × Poncirus trifoliata), Palestine sweet lime (SO+PSL) (Citrus aurantifolia. (Christm.) Swing) and Flying dragon (SO+FD) (Poncirus trifoliata), were grown under greenhouse conditions and irrigated with nutrient solution containing 0, 60, or 90 mM NaCl. Net gas exchange and leaf CI concentration were measured 21,28 and 35 d after beginning the salinity treatment. At the end of the experiment (day 35), Ca, Mg, K, total N and P concentrations in leaves and roots, growth parameters and chlorophyll concentrations were measured. Diploid plants of sour orange (SO) accumulated less CI and Na in leaves than did the tetraploid plants. However, CI and Na concentrations in roots were similar or higher in SO than in tetraploid plants. At day 35 in the 60 mM treatment, accumulation of salts in leaves was negatively correlated with growth. Thus, SO was more salt tolerant than the tetraploids and SO+PSL was the least tolerant to salinity. After 35 d, salinity reduced leaf chlorophyll, photosynthesis, stomatal conductance and whole plant transpiration in all selections. Salinity decreased the concentration of K in leaves and roots but increased the concentration of tissue N. At 90 mM NaCl, there was no correlation between CI and Na accumulations in leaves and shoot fresh weight reduction.

Restricted chromosomal silencing in nucleolar dominance.

Failure of one parent's chromosomes to organize nucleoli in an interspecific hybrid is an epigenetic phenomenon known as nucleolar dominance. Selective gene silencing on a scale of millions of bp is known to be involved, but the full extent to which nucleolus organizer region (NOR)-bearing chromosomes are inactivated beyond the NORs is unknown. Aided by genome sequence data for Arabidopsis thaliana, we have mapped the extent of nucleolar dominance-induced silencing in Arabidopsis suecica, the allotetraploid hybrid of A. thaliana and Arabidopsis arenosa. Using a sensitive reverse transcription PCR assay, we show that the four A. thaliana NORs, each approximately 4 Mbp in size, are approximately 99.5% silenced in A. suecica vegetative leaves, whereas the NORs inherited from A. arenosa remain fully active. The two A. thaliana NORs, NOR2 and NOR4, abut the telomeres on chromosomes 2 and 4, thus there are no genes distal to the NORs. The three protein-coding genes nearest NOR4 on its centromere-proximal side, the closest of which is only 3.1 kb from rRNA gene sequences, are shown to be transcribed in the hybrid despite the silencing of the adjacent approximately 4-Mbp NOR. These data argue against hypotheses in which NOR inactivation is attributed to the spread of silencing from adjacent chromosomal regions, but favor models in which NORs or rRNA genes are the targets of regulation.

Somatic hybridization in citrus: An effective tool to facilitate variety improvement

Citrus somatic hybridization and cybridization via protoplast fusion has become an integral part of citrus variety improvement programs worldwide. Citrus somatic hybrid plants have been regenerated from more than 200 parental combinations, and several cybrid combinations have also been produced. Applications of somatic hybridization to citrus scion improvement include the production of quality tetraploid breeding parents that can be used in interploid crosses to generate seedless triploids, and the direct production of triploids by haploid + diploid fusion. Applications of somatic hybridization to citrus rootstock improvement include the production of allotetraploid hybrids that combine complementary diploid rootstocks, and to combine citrus with sexually incompatible or difficult to hybridize genera that possess traits of interest for germplasm expansion. A few somatic hybrid tetraploid breeding parents have flowered, are fertile, and are being used as pollen parents to generate triploids. Several allotetraploid somatic hybrid rootstocks are performing well in commercial field trials, and show great promise for tree size control. Seed trees of most of these somatic hybrid rootstocks are producing adequate nucellar seed for standard propagation. Somatic hybridization is expected to have a positive impact on citrus cultivar improvement efforts.

ROOTSTOCK BREEDING BY SOMATIC HYBRIDISATION FOR THE MEDITERRANEAN CITRUS INDUSTRY

The predominance of sour orange rootstock for citrus in the Mediterranean area is currently threatened by the spread of citrus tristeza virus (CTV). As a result, the search for alternative rootstocks resistant to CTV and able to withstand other constraints, such as drought, alkalinity, salinity, Phytophthora and nematode problems, is now considered an urgent priority. Progress in genetic improvement of citrus rootstock by conventional sexual breeding is difficult, mainly because of the reproductive biology and heterozygosity of genitors. These induce important segregations of characters in progeny and a low probability of obtaining recombinant hybrids that combine all the desirable genes and traits of the two parents. In this context, the creation of somatic hybrids is very promising. Indeed, it allows for the accumulation of all the dominant genes of both parents for tolerance to biotic and abiotic factors, irrespective of their level of heterozygosity. This strategy has now been mastered by CIRAD in France with a view to launching a Mediterranean citrus rootstock challenge. Protoplast electrofusions were used for 11 interspecific and intergeneric combinations. Regenerated embryos and plants are studied by flow cytometry and molecular markers in order to select allotetraploid hybrids. 'Me somatic hybrids then undergo an accelerated propagation step, which uses softwood cuttings for further multilocal evaluation of tolerance to biotic and abiotic factors and agronomic rootstock trials. (Resume d'auteur)

A genetic map of tomato based on BC(1) Lycopersicon esculentum x Solanum lycopersicoides reveals overall synteny but suppressed recombination between these homeologous genomes.

F(1) hybrids between the cultivated tomato (Lycopersicon esculentum) and the wild nightshade Solanum lycopersicoides are male sterile and unilaterally incompatible, breeding barriers that impede further crosses to tomato. Meiosis is disrupted in 2x hybrids, with reduced chiasma formation and frequent univalents, but is normal in allotetraploid hybrids, indicating the genomes are homeologous. In this study, a partially male-fertile F(1) was backcrossed to tomato, producing the first BC(1) population suitable for genetic mapping from this cross. BC(1) plants were genotyped at marker loci to study the transmission of wild alleles and to measure rates of homeologous recombination. The pattern of segregation distortion, in favor of homozygotes on chromosomes 2 and 5 and heterozygotes on chromosomes 6 and 9, suggested linkage to a small number of loci under selection on each chromosome. Genome ratios nonetheless fit Mendelian expectations. Resulting genetic maps were essentially colinear with existing tomato maps but showed an overall reduction in recombination of approximately 27%. Recombination suppression was observed for all chromosomes except 9 and 12, affected both proximal and distal regions, and was most severe on chromosome 10 (70% reduction). Recombination between markers on the long arm of this chromosome was completely eliminated, suggesting a lack of colinearity between S. lycopersicoides and L. esculentum homeologues in this region. Results are discussed with respect to phylogenetic relationships between the species and their potential use for studies of homeologous pairing and recombination in a diploid plant genome.

Gene dosage and stochastic effects determine the severity and direction of uniparental ribosomal RNA gene silencing (nucleolar dominance) in Arabidopsis allopolyploids.

Nucleolar dominance is an epigenetic phenomenon in which one parental set of ribosomal RNA (rRNA) genes is silenced in an interspecific hybrid. In natural Arabidopsis suecica, an allotetraploid (amphidiploid) hybrid of Arabidopsis thaliana and Cardaminopsis arenosa, the A. thaliana rRNA genes are repressed. Interestingly, A. thaliana rRNA gene silencing is variable in synthetic Arabidopsis suecica F1 hybrids. Two generations are needed for A. thaliana rRNA genes to be silenced in all lines, revealing a species-biased direction but stochastic onset to nucleolar dominance. Backcrossing synthetic A. suecica to tetraploid A. thaliana yielded progeny with active A. thaliana rRNA genes and, in some cases, silenced C. arenosa rRNA genes, showing that the direction of dominance can be switched. The hypothesis that naturally dominant rRNA genes have a superior binding affinity for a limiting transcription factor is inconsistent with dominance switching. Inactivation of a species-specific transcription factor is argued against by showing that A. thaliana and C. arenosa rRNA genes can be expressed transiently in the other species. Transfected A. thaliana genes are also active in A. suecica protoplasts in which chromosomal A. thaliana genes are repressed. Collectively, these data suggest that nucleolar dominance is a chromosomal phenomenon that results in coordinate or cooperative silencing of rRNA genes.

Allotetraploid hybrids between citrus and seven related genera produced by somatic hybridization

We have developed an efficient protoplast-fusion method to produce somatic hybrid allopolyploid plants that combine Citrus with seven related genera, including four that are sexually incompatible. In this paper we report the creation of 18 new allotetraploid hybrids of Citrus, including ten among sexually incompatible related genera, that may have direct cultivar potential as improved citrus rootstocks. All hybrids were confirmed by cytological and RAPD analyses. If fertile, the attributes of these hybrids may be amenable to further genetic manipulation by breeding at the tetraploid level. Wide somatic hybridization of Citrus via protoplast fusion bypasses biological barriers to the natural allopolyploidization of Citrus, and creates new evolutionary opportunities that would be difficult or impossible to achieve by natural or conventional hybridization.