Genome instability in interspecific cell hybrids I. Unstable expression of genes in divergent cell hybrids

Somatic cell hybrids between cells of widely divergent mammalian species display a range of chromosomal and genetic anomalies which may be the equivalent of the “genomic shock” phenomena observed in many plant and animal interspecific hybrids. Mouse-kangaroo hybrids show extreme segregation and fragmentation of the kangaroo chromosomes. Here 1 show that, in addition to the chromosomal instability, some hybrids display unstable expression of three genes borne on the kangaroo active maternal X chromosome. These genes (HPRT, G6PD andPGK) may be co-ordinately inactivated at high frequency, then reactivated once more. I suggest that this reversible inactivation in interspecific hybrids may be the result of an unstable change at an X inactivation centre located in the kangaroo X_q.     

Interspecific hybridization in the grain legumes—a review

The current position regarding reports in the literature on interspecific hybridization in the pulses is confused. Two major reasons can he advanced in explanation. The first is that frequently conspecific wild and cultivated forms have received different binomials and crosses between such are frequently regarded as interspecific hybrids. The second arises from mistaken interpretation of results of attempted interspecific hybridization when progeny strongly resembling the maternal parent are produced. It is suggested that these progeny might have been produced by failures in emasculation or rare accidental apomixis. All cultigens of the generaArachis, Cajanus, Cicer, Phaseolus andPisum are able to some extent to produce viable true interspecific hybrids as are the Asiatic forms ofVigna. True interspecific hybrids have not been produced withVicia faba nor withVigna unguiculata the cowpea.     

A systematic study of the genus hulsea (Asteraceae)

A systematic treatment based on genetic, biochemical, and morphological studies is presented for the 11 recognized taxa ofHulsea. A uniform diploid complement of 38 chromosomes was found in the 37 populations examined, including all species and representing first reports forH. brevifolia, H. californica, andH. mexicana. Thirteen flavonoid compounds, based on the aglycones quercetin, apigenin, cyanidin, and the 4′-O-methyl ether of luteolin, were detected inHulsea. Patterns of the flavonoid compounds are discussed with respect to the systematic treatment. The results of 595 synthetic crosses indicate thatHulsea species are self-incompatible, relatively cross-compatible, and that their F₁ hybrids are relatively fertile. Heterozygosity for one or two translocations is reported for 11 interspecific and intersubspecific hybrid combinations.     

Reproductive relationships between annual species ofCalendula (Compositae)

Breeding experiments were carried out inCalendula species. In the annuals, which are selfers, rarely some outcrossing was observed only in the most peripheral flowers. In experimental crosses fruit was produced in all combinations. Fertile F₁ and F₂ hybrids could be grown from crosses between parents with similar chromosome numbers:C. palaestina ×C. pachysperma and crosses of different morphological forms ofC. arvensis. In crosses of species with different chromosome numbers at least partly fertile F₁ hybrids were obtained fromC. tripterocarpa ×C. stellata andC. tripterocarpa ×C. arvensis and crosses of the latter withC. palaestina. Fertile F₂ plants were grown from the combination ofC. arvensis ×C. tripterocarpa. Considering this information and previously obtained data, a scheme is proposed for explaining speciation in the genusCalendula.     

Chromosome pairing affinities in interspecific hybrids reflect phylogenetic distances among lady's slipper orchids (Paphiopedilum)

Background and Aims

Lady''s slipper orchids (Paphiopedilum) are of high value in floriculture, and interspecific hybridization has long been used for breeding improved cultivars; however, information regarding the genome affinities of species and chromosome pairing behaviour of the hybrids remains almost unknown. The present work analyses the meiotic behaviour of interspecific hybrids by genomic in situ hybridization and cytologically evaluates the genomic relationships among parental species.

Methods

Eight interspecific F₁ hybrids of Paphiopedilum species in various subgenera or sections were investigated in this study. The chromosome behaviour in meiosis of these interspecific hybrids was analysed and subjected to genomic in situ hybridization and fluorescent in situ hybridization.

Key Results

Genomic in situ hybridization was demonstrated as an efficient method to differentiate between Paphiopedilum genomes and to visualize the chromosome pairing affinities in interspecific F₁ hybrids, clarifying the phylogenetic distances among these species. Comparatively regular chromosome pairing observed in the hybrids of P. delenatii × P. bellatulum, P. delenatii × P. rothschildianum and P. rothschildianum × P. bellatulum suggested high genomic affinities and close relationships between parents of each hybrid. In contrast, irregular chromosome associations, such as univalents, trivalents and quadrivalents occurred frequently in the hybrids derived from distant parents with divergent karyotypes, such as P. delenatii × P. callosum, P. delenatii × P. glaucophyllum, P. rothschildianum × P. micranthum and P. rothschildianum × P. moquetteanum. The existence of multivalents and autosyndesis demonstrated by genomic in situ hybridization in this study indicates that some micro-rearrangements and other structural alterations may also play a part in differentiating Paphiopedilum species at chromosomal level, demonstrated as different chromosome pairing affinities in interspecific hybrids.

Conclusions

The results indicate that genome homology and the interaction of genetic factors, but not chromosome number nor karyotype similarity, determine the chromosome pairing behaviour in Paphiopedilum hybrids.     

Biosystematic study of Berlandiera (Compositae)

Berlandiera is revised to includeB. subacaulis, B. pumila, B. texana, andB. lyrata with its varietieslyrata andmacrophylla. All species may be induced to interbreed readily, producing F₁ progeny which are vigorous but with reduced fertility. Sympatric species produce natural hybrids which are proposed here asB. Xhumilis (=B. pumila XB. subacaulis) andB. Xbetonicifolia (=B. pumila XB. texana). All taxa investigated hadn = 15 chromosomes, including new counts forB. subacaulis and all six F₁ hybrids.     

Chromosome studies in species and hybrids ofPetrorhagia sect.Kohlrauschia (Caryophyllaceae)

Cytogenetic investigations have been made in the fourPetrorhagia species and hybrids of the sectionKohlrauschia. The three diploid species show close similarities in chromosome number, size and morphology, with the exception ofP. velutina, where one pair of metacentric chromosomes is represented by a pair of telocentrics. Meiotic studies in hybrids indicate close genomic homology between the diploid species and also between the two floral forms ofP. prolifera. The tetraploidP. nanteuilii behaves as an allotetraploid forming only bivalents at meiosis and results suggest thatP. velutina andP. prolifera are the diploid progenitors of this species. Since meiosis in diploid and triploid hybrids results in extensive intergenomic pairing it is concluded that the natural tetraploid has a bivalent promoting mechanism that prevents pairing between the genomes of its diploid progenitors.     

Response of sunflower (Helianthus annuus L) genotypes to PEG-mediated water stress

Drought tolerance of two sunflower (Helianthus annuus L.) genotypes, cultivated cultivar 1114 and interspecific line H. annuus × H. mollis, was studied under laboratory conditions using PEG-6000. Four levels of osmotic stress (?0.4, ?0.6, ?0.8 and ?1.0 MPa) were created and performances were monitored against a control. Physiological and biochemical stress determining parameters such as malondialdechyde (MDA), proline content, and hydrogen peroxide (H₂O₂) were compared between seedlings of both genotypes. The results indicated that both genotypes have similar responses at four osmotic potentials for all traits studied. All seedling growth parameters such as germination percentage, root length, shoot length, root and shoot dry weight decreased with increasing osmotic stress. MDA, proline, and H₂O₂ were found to be increased at different osmotic gradients in comparison to control. Cultivar 1114 was less affected than the interspecific line under these stress conditions. The data observed in the experiments revealed that perennial wild H. mollis can hardly be considered to be an excellent candidate of drought tolerance genes.     

Is the insular endemic Psidium socorrense (Myrtaceae) at risk of extinction through hybridization?

Natural hybridization between an insular endemic species and a widely distributed congener may endanger the endemic through genetic assimilation or outbreeding depression. Furthermore, hybrids can exhibit complex morphological variation, causing taxonomic problems in the identification of the involved taxa. In this work, we used a combination of leaf morphological and molecular markers (RAPD) to establish the differentiation between Psidium sp. aff. sartorianum and the insular endemic P. socorrense. It was also determined if hybridization between these taxa occurs in the southern slope of Isla Socorro, Mexico. Plant collection was carried along an altitudinal gradient (100–800 m). We collected eight populations separated 100 m a.s.l. apart from each other; 25 individuals were collected per population. Psidium socorrense and P. sp. aff. sartorianum differed significantly in all but two morphological characters measured. Also, a high number of diagnostic RAPD markers were found for each taxon. These results suggest that two Psidium species occur at Isla Socorro. Furthermore, both morphological and RAPD markers revealed a hybrid zone located in the southern slope of Isla Socorro (400–700 m a.s.l.) with an asymmetrical pattern of gene flow towards P. socorrense. We suggest that the disturbance caused by the sheep population in the mixed stand favors the establishment of hybrids. We further discuss whether hybridization represents a threat to the insular endemic P. socorrense.     

Transplantation and immunoincompatibility reactions among reef-building corals

Intracolony or syngeneic transplants of pieces of living coral, either occurring naturally on reefs or produced experimentally, were consistently compatible. In contrast, intercolony allografts performed extensively inAcropora formosa and inPorites andrewsii were incompatible, as were allografts occurring naturally in additional species observed in reef communities. Diverse interspecific combinations of coral xenografts generally displayed greater incompatibilities. Four levels of immuno-reactivity, with manifestations ranging from mild to severe, were distinguishable as follows: a) contact avoidance reactions, b) allogeneic contact incompatibility, c) chronic xenogeneic incompatibility, and d) acute interspecific aggression.Allogeneic incompatibility, most studied inA. formosa, was characterized by soft tissue contact avoidance wherever possible; absence of tissue death despite extensive enforced contact (18–20 days); and late interfacial cementation terminating allogeneic soft tissue contact. The properties ofchronic xenogeneic incompatibility as found in diverse species combinations, are slow onset (>7 days), bidirectional or unidirectional killing in contact zones, and short-range or localized effectiveness.Acute interspecific aggression initiated byFungia fungites as a dominant “aggressor ” is distinguished by early occurrence (2–7 days) and unidirectional contact killing. The presence or absence of these acute xenogeneic reactions sharply discriminated between species within each of the generaAcropora, Pocillopora, andPorites. No hierarchy of aggressive interactions was evident at the generic level.     

Size class homogeneity of repeat lengths and evolutionary divergence of ribosomal RNA genes in fishes as studied by restriction fragment length analysis

Fish ribosomal RNA genes (rDNA) have been compared by restriction endonuclease digestion followed by Southern hybridization using rRNA or cloned rRNA genes as labelled probes. In several species belonging to the orders Cypriniformes and Perciformes, the simple restriction patterns revealed a high degree of size class homogeneity among the rDNA repeats and similar restriction map within a species. Different species have different restriction patterns and fragment lengths arising mostly out of different length of the nontranscribed spacer. Polymorphic restriction sites are present in some species. The species-specific differences in fragment lengths produced in rDNA by some restriction enzymes can thus be used to study interspecific fish hybrids.     

Limited effective gene flow between two interfertile red oak species

Key message

Highly elevated differentiation in different life stages between two interfertile oak species at a CONSTANS -like gene suggests a role of this gene in pre-zygotic isolation and adaptive divergence between species.

Abstract

Genome-wide differentiation patterns among oak species suggest that divergent selection can maintain species-specific adaptations and morphological integrity by reducing effective interspecific gene flow. While there is evidence for both pre- and post-zygotic isolation mechanisms in oaks (e.g., differences in flowering time, selection against hybrids), these mechanisms are rarely studied at each life stage from acorns to adult trees within the same forest. To assess the reproductive isolation mechanisms between two ecologically divergent species, we (1) quantified the number of hybrids in different life stages in Quercus rubra and Quercus ellipsoidalis, two interfertile red oaks with different adaptations to drought, and (2) assessed the timing of bud burst in both natural populations and in a seedling common garden trial. The low number of hybrids in all life stages suggested pre-zygotic isolation between species or selection in very early life stages that have not been sampled (e.g., early seed abortion). Significant differences in bud burst were found in two consecutive years between species in a common garden seedling trial but not in natural populations of the same provenance. In addition, we found evidence for divergent selection on several gene loci between species in each life stage. In particular, an SSR repeat located within the coding sequence of a CONSTANS-like gene, a locus involved in the photoperiodic regulation of flowering time and development, showed very high interspecific differentiation between species in all life stages (mean F _ST = 0.83), compared to the average neutral differentiation of 3.7 %.

     

Introgressive hybridization in two southern California species of Rhus (Anacardiaceae)

Five populations ofRhus integrifolia andR. ovata were studied and, based upon morphological characteristics and pollen viability, a number of putative hybrids were found in those areas where the two species are sympatric. Diagnostic characteristics most useful for distinguishing the species and their hybrids are leaf morphology, sepal pubescence, petiole length, and petal size. Introgression was evident in at least three populations, which suggested that hybridization between the two species is an ancient phenomenon. Evolutionarily, hybridization between the two taxa has probably been of little consequence.     

The <Emphasis Type="Italic">Drosophila bipectinata</Emphasis> species complex: phylogenetic relationship among different members based on chromosomal variations

Making interspecific hybridizations, where possible remains an unparalleled option for studying the intricacies of speciation. In the Drosophila bipectinata species complex comprising of four species, namely D. bipectinata, D. parabipectinata, D. malerkotliana and D. pseudoananassae, interspecific hybrids can be obtained in the laboratory, thus bequeathing an ideal opportunity for studying speciation and phylogeny. With the view of investigating the degree of divergence between each species pair, we planned to study the polytene chromosomes of the F ₁ hybrids, as it would mirror the level of compatibility between the genomes of the parental species. Two sets of crosses were made, one involving homozygous strains of all four species from India and the other including homozygous strains from different places across the globe. Polytene chromosomes of F ₁ larvae from both sets of crosses had similar configurations. In F ₁ larvae from crosses involving D. bipectinata, D. parabipectinata and D. malerkotliana, complex configurations (depicting overlapping inversions) could be detected in different arms. However, they were fairly synapsed, indicating that the differences are only at the level of gene arrangements. The polytene chromosomes of larvae obtained by crossing D. pseudoananassae with the other three species were very thin with gross asynapsis in all the arms, demonstrating that the genome of D. pseudoananassae is widely diverged from rest of the species. The overlapping inversions (reflected in complex configuration), are inferred in the light of earlier chromosomal studies performed in this complex.     

Unreduced gamete formation in wheat × <Emphasis Type="Italic">Aegilops</Emphasis> spp. hybrids is genotype specific and prevented by shared homologous subgenomes

Key message

The presence of homologous subgenomes inhibited unreduced gamete formation in wheat × Aegilops interspecific hybrids. Unreduced gamete rates were under the control of the wheat nuclear genome.

Abstract

Production of unreduced gametes is common among interspecific hybrids, and may be affected by parental genotypes and genomic similarity. In the present study, five cultivars of Triticum aestivum and two tetraploid Aegilops species (i.e. Ae. triuncialis and Ae. cylindrica) were reciprocally crossed to produce 20 interspecific hybrid combinations. These hybrids comprised two different types: T. aestivum × Aegilops triuncialis; 2n = ABDU^tC^t (which lack a common subgenome) and T. aestivum × Ae. cylindrica; 2n = ABDD^cC^c (which share a common subgenome). The frequency of unreduced gametes in F₁ hybrids was estimated in sporads from the frequency of dyads, and the frequency of viable pollen, germinated pollen and seed set were recorded. Different meiotic abnormalities recorded in the hybrids included precocious chromosome migration to the poles at metaphase I and II, laggards in anaphase I and II, micronuclei and chromosome stickiness, failure in cell wall formation, premature cytokinesis and microspore fusion. The mean frequency of restitution meiosis was 10.1 %, and the mean frequency of unreduced viable pollen was 4.84 % in T. aestivum × Ae. triuncialis hybrids. By contrast, in T. aestivum × Ae. cylindrica hybrids no meiotic restitution was observed, and a low rate of viable gametes (0.3 %) was recorded. This study present evidence that high levels of homologous pairing between the D and D^c subgenomes may interfere with meiotic restitution and the formation of unreduced gametes. Variation in unreduced gamete production was also observed between T. aestivum × Ae. triuncialis hybrid plants, suggesting genetic control of this trait.

     

Somatic hybridization between Nicotiana rustica and N. sylvestris

Protoplasts isolated from cell cultures of chlorophyll-deficient Nicotiana rustica cv. chlorotica and wild-type N. sylvestris were fused. The scheme for selection of somatic hybrids was based on the inability of the protoplast-derived colonies of the parental species to turn green; N. sylvestris protoplasts also had a very low plating efficiency in the medium employed. A total of 777 green colonies which were presumptive hybrids were isolated within four weeks of the fusion experiments. One hundred and eight green colonies formed shoots in vitro and 16 lines were rooted and grown in the greenhouse. Each of these hybrid plants displayed vegetative and floral traits intermediate to those of the parental species, except for plant height which in almost all cases was greater in the hybrids. Isozyme analyses by gel electrophoresis and isoelectric focussing of the small subunit of ribulose 1,5-bisphosphate carboxylase/oxygenase (RUBPCase) demonstrated that the nuclear genomes of both parents were expressed by the hybrids. Each of the eight somatic hybrid plants analyzed expressed only the N. rustica chloroplast genome as shown by isoelectric focussing of the large subunit of RUBPCase. This study demonstrated the value of N. rustica cv. chlorotica as a parental line in somatic hybridization with N. sylvestris and it might have widespread use with wild-type lines of other species.     

Identification of gibberellins from sugarcane plants

Five GAs, GA₁, GA₃, GA₁₉, GA₂₀, and GA₂₉, were identified in extracts from mature leaf and shoot apical meristem of flowering and non-flowering sugarcane (Saccharum spp. hybrids) by combined GC/MS. The presence of ABA was also confirmed.