Tissue-specific elimination of alternative whole parental genomes in one barley hybrid

Haploid Induction in Indica Rice: Exploring New Opportunities

Haploid plants are of significant interest to crop breeders due to their ability to expedite the development of inbred lines. Chromosome-doubling of haploids, produced by either in vitro or in vivo methods, results in fully homozygous doubled haploids. For nearly five decades, in vitro methods of anther and microspore culture have been attempted in many crops. In rice, in vitro methods are used with some success in japonica cultivars, although indica types have remained recalcitrant to a large extent. This review aims to explore the reasons for the lack of success of in vitro methods in indica rice and discuss new advancements in in vivo haploid induction protocols in other cereals and their relevance to rice. In particular, the current level of understanding of in vivo haploid inducer systems that utilize MTL and CENH3 mutants is analyzed in detail. One notable advantage of in vivo haploid induction systems is that they do not require tissue culture competence. This makes these methods more accessible and potentially transformative for research, offering a pragmatic approach to improving indica rice cultivars. By embracing these in vivo methods and harnessing the power of gene editing technologies like CRISPR/Cas9 systems, breeders can reshape their approach to indica rice improvement.

Accelerated Breeding for Helianthus annuus (Sunflower) through Doubled Haploidy: An Insight on Past and Future Prospects in the Era of Genome Editing

The aim of any breeding process is to fully express the targeted, superior/desirable parent characteristic in the progeny. Hybrids are often used in this dynamic, and complex process for which homozygous parents-which may require up to eight generations of back crossing and selection-are required. Doubled haploid (DH) technologies can facilitate the production of true breeding lines faster and in a more efficient manner than the traditional back crossing and selection strategies. Sunflower is the third most important oilseed crop in the world and has no available double haploid induction procedure/technique that can be efficiently used in breeding programs. A reproducible and efficient doubled haploid induction method would be a valuable tool in accelerating the breeding of new elite sunflower varieties. Although several attempts have been made, the establishment of a sunflower doubled haploid induction protocol has remained a challenge owing recalcitrance to in vitro culture regeneration. Approaches for haploid development in other crops are often cultivar specific, difficult to reproduce, and rely on available tissue culture protocols-which on their own are also cultivar and/or species specific. As an out-crossing crop, the lack of a double haploid system limits sunflower breeding and associated improvement processes, thereby delaying new hybrid and trait developments. Significant molecular advances targeting genes, such as the centromeric histone 3 (CenH3) and Matrilineal (MTL) gene with CRISPR/Cas9, and the successful use of viral vectors for the delivery of CRISPR/Cas9 components into plant cells eliminating the in vitro culture bottleneck, have the potential to improve double haploid technology in sunflower. In this review, the different strategies, their challenges, and opportunities for achieving doubled haploids in sunflower are explored.

Understanding and exploiting uniparental genome elimination in plants: insights from Arabidopsis thaliana

Uniparental genome elimination (UGE) refers to the preferential exclusion of one set of the parental chromosome complement during embryogenesis following successful fertilization, giving rise to uniparental haploid progeny. This artificially induced phenomenon was documented as one of the consequences of distant (wide) hybridization in plants. Ten decades since its discovery, attempts to unravel the molecular mechanism behind this process remained elusive due to a lack of genetic tools and genomic resources in the species exhibiting UGE. Hence, its successful adoption in agronomic crops for in planta (in vivo) haploid production remains implausible. Recently, Arabidopsis thaliana has emerged as a model system to unravel the molecular basis of UGE. It is now possible to simulate the genetic consequences of distant crosses in an A. thaliana intraspecific cross by a simple modification of centromeres, via the manipulation of the centromere-specific histone H3 variant gene, CENH3. Thus, the experimental advantages conferred by A. thaliana have been used to elucidate and exploit the benefits of UGE in crop breeding. In this review, we discuss developments and prospects of CENH3 gene-mediated UGE and other in planta haploid induction strategies to illustrate its potential in expediting plant breeding and genetics in A. thaliana and other model plants.

Centromere Engineering as an Emerging Tool for Haploid Plant Production: Advances and Challenges

Haploid production is of great importance in plant breeding programs. Doubled haploid technology accelerates the generation of inbred lines with homozygosity in all loci in a single year. Haploids can be induced in vitro via cultivating the haploid gametes or in vivo through inter- and intraspecific hybridization. Haploid induction through centromere engineering is a novel system that is theoretically applicable to many plant species. The present review chapter discusses the proposed molecular mechanisms of selective chromosome elimination in early embryogenesis and the effects of kinetochore component modifications on proper chromosome segregation. Finally, the advantages and limitations of the CENH3-mediated haploidization approach and its applications are highlighted.

Transcriptome analysis revealed misregulated gene expression in blastoderms of interspecific chicken and Japanese quail F1 hybrids

Hybrid incompatibility, such as sterility and inviability, prevents gene flow between closely-related populations as a reproductive isolation barrier. F₁ hybrids between chickens and Japanese quail (hereafter, referred to as quail), exhibit a high frequency of developmental arrest at the preprimitive streak stage. To investigate the molecular basis of the developmental arrest at the preprimitive streak stage in chicken–quail F₁ hybrid embryos, we investigated chromosomal abnormalities in the hybrid embryos using molecular cytogenetic analysis. In addition, we quantified gene expression in parental species and chicken- and quail-derived allele-specific expression in the hybrids at the early blastoderm and preprimitive streak stages by mRNA sequencing. Subsequently, we compared the directions of change in gene expression, including upregulation, downregulation, or no change, from the early blastoderm stage to the preprimitive streak stage between parental species and their hybrids. Chromosome analysis revealed that the cells of the hybrid embryos contained a fifty-fifty mixture of parental chromosomes, and numerical chromosomal abnormalities were hardly observed in the hybrid cells. Gene expression analysis revealed that a part of the genes that were upregulated from the early blastoderm stage to the preprimitive streak stage in both parental species exhibited no upregulation of both chicken- and quail-derived alleles in the hybrids. GO term enrichment analysis revealed that these misregulated genes are involved in various biological processes, including ribosome-mediated protein synthesis and cell proliferation. Furthermore, the misregulated genes included genes involved in early embryonic development, such as primitive streak formation and gastrulation. These results suggest that numerical chromosomal abnormalities due to a segregation failure does not cause the lethality of chicken–quail hybrid embryos, and that the downregulated expression of the genes that are involved in various biological processes, including translation and primitive streak formation, mainly causes the developmental arrest at the preprimitive streak stage in the hybrids.

Composition and random elimination of paternal chromosomes in a large population of wheat × barley (Triticum aestivum L. × Hordeum vulgare L.) hybrids

Statistical analysis of the chromosomal composition in a population of 210 primary plants regenerated from two intergeneric wheat-barley cross combinations revealed the random nature of uniparental elimination for barley chromosomes. Uniparental chromosome elimination is a common process in interspecific and intergeneric cereal hybrids. To characterize the frequency of paternal chromosomes, a population of 218 independent green plants was generated from two wheat (♀) × barley (♂) cross combinations via embryo rescue. The chromosomal composition of 210 primary plants was analyzed with chromosome-specific DNA markers representing all seven barley chromosomes. The analysis revealed an equal proportion of haploid and full hybrids (20.5% and 19.5%, respectively), while the rest of the population contained hypoploids (partial hybrids) with no preference for any possible numbers (one to six) of barley chromosome additions. Contrary to the previous reports, there was no statistical bias or preferential elimination for any individual barley chromosome (1H-7H) in this population. The reasons for the apparent contradiction and the implications of the above findings for cereal breeding are discussed.

State-of-the-art and novel developments of in vivo haploid technologies

The ability to generate (doubled) haploid plants significantly accelerates the crop breeding process. Haploids have been induced mainly through the generation of plants from cultivated gametophic (haploid) cells and tissues, i.e., in vitro haploid technologies, or through the selective loss of a parental chromosome set upon inter- or intraspecific hybridization. Here, we focus our review on the mechanisms responsible for the in vivo formation of haploids in the context of inter- and intraspecific hybridization. The application of a modified CENH3 for uniparental genome elimination, the IG1 system used for paternal as well as the BBM-like and the patatin-like phospholipase essential for maternal haploidy induction are discussed in detail.

Centromere histone H3- and phospholipase-mediated haploid induction in plants

Simple and consistent production of haploid is always an appealing pursuit for both crop breeders and researchers. Although diverse strategies have been developed to produce haploids over the past decades, most of them are applicable in only a limited number of plant species. In 2010, Ravi and Chan reported that haploid Arabidopsis thaliana plants can be efficiently induced through the introduction of a single genetic alteration in centromere histone H3 (CENH3). Subsequent studies demonstrated that haploids can be efficiently induced either through genetic engineering of CENH3 N-terminal tail or histone fold domain or by replacing CENH3 with an ortholog. The mutation of a pollen-specific phospholipase gene, MATRILINEAL (MTL) has been revealed to trigger the haploid induction (HI) in maize, which present another promising HI approach by the editing of MTL in plant. Here, we review the progress of the CENH3-medialed HI and propose a revised centromere-size model by suggesting a competitive loading process between wild-type and mutant CENH3 during HI. This model can explain both the findings of HI failure when wild-type and mutant CENH3 genes are coexpressed and the alien centromere loading of CENH3 in stable hybrids. In addition, we review the current understanding of MTL-mediated HI in plant. The conservation of CENH3 and MTL in plants indicates wide potential application for HI. We discuss the utility and potential of these two methods in crops by comparing their mechanisms and applications to date in plants. This review will promote the study and application of both CENH3- and MTL-mediated haploid induction in plants.

Effect of Hybridization on Somatic Mutations and Genomic Rearrangements in Plants

Hybridization has been routinely practiced in agriculture to enhance the crop yield. Principally, it can cause hybrid vigor where hybrid plants display increased size, biomass, fertility, and resistance to diseases, when compared to their parents. During hybridization, hybrid offspring receive a genomic shock due to mixing of distant parental genomes, which triggers a myriad of genomic rearrangements, e.g., transpositions, genome size changes, chromosomal rearrangements, and other effects on the chromatin. Recently, it has been reported that, besides genomic rearrangements, hybridization can also alter the somatic mutation rates in plants. In this review, we provide in-depth insights about hybridization triggered genomic rearrangements and somatic mutations in plants.

The importance of reproductive barriers and the effect of allopolyploidization on crop breeding

Inter-specific hybrids are a useful source for increasing genetic diversity. Some reproductive barriers before and/or after fertilization prevent production of hybrid plants by inter-specific crossing. Therefore, techniques to overcome the reproductive barrier have been developed, and have contributed to hybridization breeding. In recent studies, identification of molecules involved in plant reproduction has been studied to understand the mechanisms of reproductive barriers. Revealing the molecular mechanisms of reproductive barriers may allow us to overcome reproductive barriers in inter-specific crossing, and to efficiently produce inter-specific hybrids in cross-combinations that cannot be produced through artificial techniques. Inter-specific hybrid plants can potentially serve as an elite material for plant breeding, produced through the merging of genomes of parental species by allopolyploidization. Allopolyploidization provides some benefits, such as heterosis, increased genetic diversity and phenotypic variability, which are caused by dynamic changes of the genome and epigenome. Understanding of allopolyploidization mechanisms is important for practical utilization of inter-specific hybrids as a breeding material. This review discusses the importance of reproductive barriers and the effect of allopolyploidization in crop breeding programs.

Haploidization via Chromosome Elimination: Means and Mechanisms

The ability to generate haploids and subsequently induce chromosome doubling significantly accelerates the crop breeding process. Haploids have been induced through the generation of plants from haploid tissues (in situ gynogenesis and androgenesis) and through the selective loss of a parental chromosome set via inter- or intraspecific hybridization. Here, we focus on the mechanisms responsible for this selective chromosome elimination. CENH3, a variant of the centromere-specific histone H3, has been exploited to create an efficient method of haploid induction, and we discuss this approach in some detail. Parallels have been drawn with chromosome-specific elimination, which occurs as a normal part of differentiation and sex determination in many plant and animal systems.

Large genetic screens for gynogenesis and androgenesis haploid inducers in Arabidopsis thaliana failed to identify mutants

Gynogenesis is a process in which the embryo genome originates exclusively from female origin, following embryogenesis stimulation by a male gamete. In contrast, androgenesis is the development of embryos that contain only the male nuclear genetic background. Both phenomena are of great interest in plant breeding as haploidization is an efficient tool to reduce the length of breeding schemes to create varieties. Although few inducer lines have been described, the genetic control of these phenomena is poorly understood. We developed genetic screens to identify mutations that would induce gynogenesis or androgenesis in Arabidopsis thaliana. The ability of mutant pollen to induce either gynogenesis or androgenesis was tested by crossing mutagenized plants as males. Seedlings from these crosses were screened with recessive phenotypic markers, one genetically controlled by the female genome and another by the male genome. Positive and negative controls confirmed the unambiguous detection of both gynogenesis and androgenesis events. This strategy was applied to 1,666 EMS-mutagenised lines and 47 distant Arabidopsis strains. While an internal control suggested that the mutagenesis reached saturation, no gynogenesis or androgenesis inducer was found. However, spontaneous gynogenesis was observed at a frequency of 1/10,800. Altogether, these results suggest that no simple EMS-induced mutation in the male genome is able to induce gynogenesis or androgenesis in Arabidopsis.

Unrepaired DNA damage facilitates elimination of uniparental chromosomes in interspecific hybrid cells

Elimination of uniparental chromosomes occurs frequently in interspecific hybrid cells. For example, human chromosomes are always eliminated during clone formation when human cells are fused with mouse cells. However, the underlying mechanisms are still elusive. Here, we show that the elimination of human chromosomes in human-mouse hybrid cells is accompanied by continued cell division at the presence of DNA damage on human chromosomes. Deficiency in DNA damage repair on human chromosomes occurs after cell fusion. Furthermore, increasing the level of DNA damage on human chromosomes by irradiation accelerates human chromosome loss in hybrid cells. Our results indicate that the elimination of human chromosomes in human-mouse hybrid cells results from unrepaired DNA damage on human chromosomes. We therefore provide a novel mechanism underlying chromosome instability which may facilitate the understanding of carcinogenesis.

Genetic mechanisms of allopolyploid speciation through hybrid genome doubling: novel insights from wheat (Triticum and Aegilops) studies

Polyploidy, which arises through complex genetic and ecological processes, is an important mode of plant speciation. This review provides an overview of recent advances in understanding why plant polyploid species are so ubiquitous and diverse. We consider how the modern framework for understanding genetic mechanisms of speciation could be used to study allopolyploid speciation that occurs through hybrid genome doubling, that is, whole genome doubling of interspecific F1 hybrids by the union of male and female unreduced gametes. We outline genetic and ecological mechanisms that may have positive or negative impacts on the process of allopolyploid speciation through hybrid genome doubling. We also discuss the current status of studies on the underlying genetic mechanisms focusing on the wheat (Triticum and Aegilops) hybrid-specific reproductive phenomena that are well known but deserve renewed attention from an evolutionary viewpoint.

Loss of centromeric histone H3 (CENH3) from centromeres precedes uniparental chromosome elimination in interspecific barley hybrids

Uniparental chromosome elimination occurs in several interspecific hybrids of plants. We studied the mechanism underlying selective elimination of the paternal chromosomes during the early development of Hordeum vulgare × Hordeum bulbosum embryos. The following conclusions regarding the role of the centromere-specific histone H3 variant (CENH3) in the process of chromosome elimination were drawn: (i) centromere inactivity of H. bulbosum chromosomes triggers the mitosis-dependent process of uniparental chromosome elimination in unstable H. vulgare × H. bulbosum hybrids; (ii) centromeric loss of CENH3 protein rather than uniparental silencing of CENH3 genes causes centromere inactivity; (iii) in stable species combinations, cross-species incorporation of CENH3 occurs despite centromere-sequence differences, and not all CENH3 variants get incorporated into centromeres if multiple CENH3s are present in species combinations; and (iv) diploid barley species encode two CENH3 variants, the proteins of which are intermingled within centromeres throughout mitosis and meiosis.

Centromere conversion and retention in somatic cell hybrids

The generation of somatic cell hybridization-derived cell lines between highly divergent species affords the opportunity to examine the concept of 'genome dominance' in the context of genetic and epigenetic changes. While whole-scale genome dominance has been well documented in natural hybrids among closely related species, an examination of centromere position and sequence retention in 2 marsupial-eutherian hybrids has revealed a mechanism for 'centromere dominance' as a driving force in the generation of stable somatic cell hybrids following an initial period of genomic instability. While one somatic cell hybrid cell line appeared to retain marsupial centromere sequences which remained competent to recruit the centromere-specific histone variant CENP-A in a Chinese hamster background, fusion events between marsupial and mouse-derived chromosomes in another hybrid line led to a centromere sequence conversion from one species to the other. We postulate that the necessity to maintain an epigenetically defined centromere following genome hybridization may be responsible for retention of specific chromosomes and may result in rapid sequence turnover to facilitate the recruitment of CENP-A containing histones.

Different genome-specific chromosome stabilities in synthetic Brassica allohexaploids revealed by wide crosses with Orychophragmus

BACKGROUND AND AIMS

In sexual hybrids between cultivated Brassica species and another crucifer, Orychophragmus violaceus (2n = 24), parental genome separation during mitosis and meiosis is under genetic control but this phenomenon varies depending upon the Brassica species. To further investigate the mechanisms involved in parental genome separation, complex hybrids between synthetic Brassica allohexaploids (2n = 54, AABBCC) from three sources and O. violaceus were obtained and characterized.

METHODS

Genomic in situ hybridization, amplified fragment length polymorphism (AFLP) and single-strand conformation polymorphism (SSCP) were used to explore chromosomal/genomic components and rRNA gene expression of the complex hybrids and their progenies.

KEY RESULTS

Complex hybrids with variable fertility exhibited phenotypes that were different from the female allohexaploids and expressed some traits from O. violaceus. These hybrids were mixoploids (2n = 34-46) and retained partial complements of allohexaploids, including whole chromosomes of the A and B genomes and some of the C genome but no intact O. violaceus chromosomes; AFLP bands specific for O. violaceus, novel for two parents and absent in hexaploids were detected. The complex hybrids produced progenies with chromosomes/genomic complements biased to B. juncea (2n = 36, AABB) and novel B. juncea lines with two genomes of different origins. The expression of rRNA genes from B. nigra was revealed in all allohexaploids and complex hybrids, showing that the hierarchy of nucleolar dominance (B. nigra, BB > B. rapa, AA > B. oleracea, CC) in Brassica allotetraploids was still valid in these plants.

CONCLUSIONS

The chromosomes of three genomes in these synthetic Brassica allohexaploids showed different genome-specific stabilities (B > A > C) under induction of alien chromosome elimination in crosses with O. violaceus, which was possibly affected by nucleolar dominance.

Cytological evidence for chromosome elimination in wheat x Imperata cylindrica hybrids

Haploid induction of wheat by crossing with Imperata cylindrica pollen is an efficient method for doubled haploid breeding. We investigated the process of wheat haploid formation after crossing with I. cylindrica. Our cytological observations of zygotes showed the successful fertilization of parental gametes. Wheat haploids were formed by complete elimination of I. cylindrica chromosomes. Missegregation of I. cylindrica chromosomes was observed in the first cell division of zygote. At metaphase I. cylindrica chromosomes did not congress onto the equatorial plate. The sister chromosomes did not move toward the poles during anaphase, though their cohesion was released normally. I. cylindrica chromosomes were still in the cytoplasm at telophase and eliminated from daughter nuclei. After two-celled stage, we could find no I. cylindrica chromosome in the nuclei but micronuclei containing I. cylindrica chromatin in the cytoplasm. These observations indicate that I. cylindrica chromosomes are completely eliminated from nuclei in the first cell division probably due to lack of functional kinetochores.

Simple sequence repeat analyses of interspecific hybrids and MAALs of Oryza officinalis and Oryza sativa

Wild rice is a valuable resource for the genetic improvement of cultivated rice (Oryza sativa L., AA genome). Molecular markers are important tools for monitoring gene introgression from wild rice into cultivated rice. In this study, Simple sequence repeat (SSR) markers were used to analyze interspecific hybrids of O. sativa-O. officinalis (CC genome), the backcrossing progenies and the parent plants. Results showed that most of the SSR primers (335 out of 396, 84.6%) developed in cultivated rice successfully amplified products from DNA samples of wild rice O. officinalis. The polymorphism ratio of SSR bands between O. sativa and O. officinalis was as high as 93.9%, indicating differences between the two species with respect to SSRs. When the SSR markers were applied in the interspecific hybrids, only a portion of SSR primers amplified O. officinalis-specific bands in the F(1) hybrid (52.5%), BC(1) (52.5%), and MAALs (37.0%); a number of the bands disappeared. Of the 124 SSR loci that detected officinalis-specific bands in MAAL plants, 96 (77.4%) showed synteny between the A and C-genomes, and 20 (16.1%) showed duplication in the C-genome. Sequencing analysis revealed that indels, substitution and duplication contribute to the diversity of SSR loci between the genomes of O. sativa and O. officinalis.

Chromosome elimination in the interspecific hybrid medaka between Oryzias latipes and O. hubbsi

An interspecific hybrid medaka (rice fish) between Oryzias latipes and O. hubbsi is embryonically lethal. To gain an insight into the cellular and molecular mechanisms that cause the abnormalities occurring in the hybrid medaka, we investigated the behavior of chromosomes and the expression patterns of proteins responsible for the chromosome behavior. The number of chromosomes in the hybrid embryos gradually decreased to nearly half, since abnormal cell division with lagging chromosomes at anaphase eliminated the chromosomes from the cells. The chromosome lagging occurred at the first cleavage and continued throughout embryogenesis even after the midblastula transition. Fluorescent in-situ hybridization analyses revealed that the chromosomes derived from O. hubbsi are preferentially eliminated in both O. latipes-hubbsi and O. hubbsi-latipes embryos. Whole-mount immunocytochemical analyses using antibodies against alpha-tubulin, gamma-tubulin, inner centromere protein, Cdc20, Mad2, phospho-histone H3 and cohesin subunits (SMC1alpha, SMC3 and Rad21) showed that the expression patterns of these proteins in the hybrid embryos are similar to those in the wild-type embryos, except for phospho-histone H3. Phospho-histone H3 present on chromosomes at metaphase was lost from normally separated chromosomes at anaphase, whereas it still existed on lagging chromosomes at anaphase, indicating that the lagging chromosomes remain in the metaphase state even when the cell has proceeded to the anaphase state. On the basis of these findings, we discuss the cellular and molecular mechanisms of chromosome elimination in the hybrid medaka.

Elimination of chromosomes in Hordeum vulgare x H. bulbosum crosses at mitosis and interphase involves micronucleus formation and progressive heterochromatinization

Uniparental chromosome elimination occurs in several interspecific hybrids of plants. We studied the mechanism underlying selective elimination of the paternal chromosomes during the development of Hordeum vulgare x H. bulbosum hybrid embryos that is restricted to an early stage of development. In almost all embryos most of the H. bulbosum chromatin undergoes a fast rate of elimination within nine days after pollination. There are differences in the mitotic behaviour between the parental chromosomes, with H. bulbosum chromatids segregating asymmetrically at anaphase. We provide evidence for a chromosome elimination pathway that involves the formation of nuclear extrusions during interphase in addition to postmitotically formed micronuclei. The chromatin structure of nuclei and micronuclei differs and heterochromatinization and disintegration of the nuclear envelope of micronuclei are the final steps of chromosome elimination.

Genome conflict in the gramineae

The genomes of grasses and cereals include a diverse and large collection of selfish genetic elements, many of which are fossil relics of ancient origin. Some of these elements are active and, because of their selfish nature and the way in which they exist to perpetuate themselves, they cause a conflict for genomes both within and between species in hybrids and allopolyploids. The conflict arises from how the various elements may undergo 'drive', through transposition, centromere and neocentromere drive, and in mitotic and meiotic drive processes in supernumerary B chromosomes. Experimental and newly formed hybrids and polyploids, where new combinations of genomes are brought together for the first time, find themselves sharing a common nuclear and cytoplasmic environment, and they can respond with varying degrees of instability to adjust to their new partnerships. B chromosomes are harmful to fertility and to the physiology of the cells and plants that carry them. In this review we take a broad view of genome conflict, drawing together aspects arising from a range of genetic elements that have not hitherto been considered in their entirety, and we find some common themes linking these various elements in their activities.

Confocal analysis of chromosome behavior in wheat × maize zygotes

Herein, we profile the first embryonic mitosis in a hybrid of wheat and maize by using a whole-mount genomic in situ hybridization method and immunofluorescence staining with a tubulin-specific antibody. We have successfully captured the dynamics of each set of parental chromosomes in the first zygotic division of the hybrid embryo 24-28 h after crossing. During the first zygotic metaphase, although both sets of parental chromosomes congressed into the equatorial plate of the zygote, the maize chromosomes tended to lag in comparison with the wheat chromosomes. During anaphase, each parental chromosome separated into its sister chromosomes; however, some of the maize chromosomes lagged around the metaphase plate as segregants. The maize sister chromosomes that did move toward the pole showed delayed and asymmetric movement as compared with the wheat ones. Immunological staining of tubulin revealed a bipolar spindle structure in the first zygotic metaphase. The kinetochores of the maize chromosomes that lagged around the metaphase plate did not attach to the spindle microtubules. These results suggest that factors on the kinetochores of maize chromosomes that are required to control chromosome movement are deficient in the zygotic cell cycle.Key words: whole-mount, GISH, chromosome elimination, hybrid embryogenesis.

A microsatellite sequence from the rice blast fungus (Magnaporthe grisea) distinguishes between the centromeres of Hordeum vulgare and H. bulbosum in hybrid plants

A TC/AG-repeat microsatellite sequence derived from the rice blast fungus (Magnaporthe grisea) hybridized to all of the centromeres of Hordeum vulgare chromosomes, but hybridized faintly or not at all to the chromosomes of Hordeum bulbosum. Using this H. vulgare centromere-specific probe, the chromosomes of four F1 hybrids between H. vulgare and H. bulbosum were analyzed. The chromosome constitution in the root tips of the hybrids was mosaic, i.e., 7 (7v, H. vulgare) and 14 (7v + 7b H. bulbosum), or 14 (7v + 7b) and 27 (14v + 13b), or 7 (7v), 14 (7v + 7b), and 27 (14v + 13b). The 27-chromosome tetraploid hybrid cells were revealed to have the NOR (nucleolus organizer region) bearing chromosome of H. bulbosum in a hemizygous state, which might indicate some role for this chromosome in the chromosome instability of the hybrid condition. The chromosomal distribution showed that the chromosomes of H. vulgare were concentric and chromosomes of H. bulbosum were peripheral in the mitotic squash. This non-random chromosome distribution and the centromere-specific repeated DNA differences in the two species were discussed in relation to H. bulbosum chromosome elimination. Meiotic chromosome analyses revealed a high frequency of homoeologous chromosome pairing in early prophase. However, this chromosome pairing did not persist until later meiotic stages and many univalents and chromosome fragments resulted. These were revealed to be H. bulbosum by fluorescence in situ hybridization (FISH) analysis with the H. vulgare centromere-specific probe. Because the chromosome segregation of H. vulgare and H. bulbosum chromosomes at anaphase I of meiosis was random, the possibility for obtaining chromosome substitution lines in diploid barley from the diploid hybrid was discussed.

Molecular changes at Rrn loci in barley (Hordeum vulgare L.) hybrids with H. bulbosum (L.)

Southern blots of restriction fragments of genomic DNAs from Hordeum vulgare (L.), H. bulbosum (L.), and interspecific hybrids and their derivatives were hybridized with rDNA probe to identify locus-specific modifications at Rrn loci. H. bulbosum rDNA revealed a single EcoRV site per repeat compared with two sites in H. vulgare rDNA repeats. H. bulbosum accessions possessed at least two rDNA repeat lengths, indicating heterozygosity at the Rrn locus. Hybrids possessed both H. vulgare and H. bulbosum rDNA repeats. Two of the hybrid derivatives possessed bulbosum-specific Sau3AI and HaeIII rDNA fragments, while amphiploid and doubled haploid derivatives lacked H. bulbosum rDNA repeat units and (or) fragments. Two hybrid derivatives, one amphiploid and a doubled haploid derived from the same parental combination, lacked the vulgare Rrn2-specific 9.0-kb rDNA repeat. This is the first conclusive evidence for the elimination of vulgare genetic material in vulgare-bulbosum hybrids. The ratios of 9.0- to 9.9-kb vulgare repeats and H. vulgare to H. bulbosum rDNA repeats indicate partial loss of the vulgare-specific 9.0-kb rDNA repeat among the hybrids. Differences in MboI and Sau3AI fragments and the ratios of 9.0 to 9.9 kb vulgare rDNA repeats revealed differential methylation at Rrn1 and Rrn2 loci. Hybrids and derivatives showed differential distribution of methylation of EcoRI, BglII, and SacI sites at the Rrn1 locus. Two of the hybrid derivatives exhibited extensive CpG-biased methylation. Data presented here are indicative of the differences in the onset of events triggered by the interaction of the component genomes and enabled detection of differential methylation among Rrn loci, loss of H. vulgare genetic material, and development of doubled haploids with the Rrn1 locus.

Parental genome separation and asynchronous centromere division in interspecific F1 hybrids in Lathyrus

Chromosomes were studied in root-tip metaphase cells of several F1 interspecific Lathyrus hybrids including: L. hirsutus x L. cassius (H x C), L. cassius x L. hirsutus (C x H), L. cassius x L. odoratus (C x O), and their parents, all with 2n = 2x = 14. Two types of morphologically distinct centromeres were identified in the hybrids on the basis of the degree of contraction of the primary constriction. At least 12 well-defined centromeres were seen in all cells of L. hirsutus, L. cassius and L. odoratus, and about 80% of cells had 14. The hybrids were more variable than the species. H x O contained between six and 14 well-defined centromeres, while cells of H x C, C x H and C x O all had seven well-defined and seven weakly defined centromeres. These were used as markers to plot their spatial disposition in two dimensions on metaphase spreads. In H x C, C x H and C x O the two types of centromeres showed a significant tendency to occupy two spatially distinct and concentrically arranged domains on the metaphase plate (P < 0.005). Owing to shortage of material subsequent work was restricted to H x C and C x H. Six or seven chromosomes of one parental genome were selectively labelled by in situ hybridization using biotinylated total genomic DNA from either parent as a probe. Moreover, there was a very strong correlation between centromere type and genomic origin (P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Nuclear architecture in plants

Structure within the nucleus of plants is becoming increasingly clear in both metaphase and interphase nuclei, although there are conflicting data about the relative positions of individual and pairs of chromosomes. At interphase, individual chromosomes may generally occupy discrete domains that are not intermixed with other chromosomes. Aspects of mechanical chromosome behaviour and even of gene expression may correlate with interphase chromosome position, and imply that a better understanding of nuclear architecture is required.

Karyotype analysis of regenerated plants from callus cultures of interspecific hybrids of cultivated barley (Hordeum vulgare L.)

The karyotype of 82 regenerated plants from callus cultures of interspecific hybrids between cultivated barley (Hordeum vulgare L.) and seven polyploid wild barley species was examined by C-banding or Feulgen staining. The karyotypic changes observed in 46 plants included aneuploidy, double haploidy, amphidiploidy, deletions, inversions, extra C-bands, and extra euchromatic segments. Apparently, chromosome 5, 6, and 7 of H. vulgare were more frequently exposed to elimination or structural change than the other chromosomes of this species. Irradiation of calli seemed to enhance the occurrence of karyotypic variants.

Elimination and duplication of particular Hordeum vulgare chromosomes in aneuploid interspecific Hordeum hybrids

Seeds formed in crosses Hordeum lechleri (6x) x H. vulgare (2x and 4x), H. arizonicum (6x) x H. v. (2x), H. parodii (6x) x H. v. (2x), and H. tetraploidum (4x) x H. v. (2x) produced plants at high or rather high frequencies through embryo rescue. Giemsa C-banding patterns were used to analyze chromosomal constitutions and chromosomal locations on the methaphase plate. Among 100 plants obtained from H. vulgare (2x) crosses, 32 plants were aneuploid with 2n=29 (1), 28 (3), 27 (13), 26 (5), 25 (4), 24 (4), or 22 (2); 50 were euploid (12 analyzed), and 18 were polyhaploid (5 analyzed). Four plants had two sectors differing in chromosome number. Two of four hybrids with H. vulgare (4x) were euploid and two were aneuploid. Parental genomes were concentrically arranged with that of H. vulgare always found closest to the metaphase centre. Many plants showed a certain level of intraplant variation in chromosome numbers. Except for one H. vulgare (4x) hybrids, this variation was restricted to peripherally located non-H. vulgare genomes. This may reflect a less firm attachment of the chromosomes from these genomes to the spindle. Interplant variation in chromosome numbers was due to the permanent elimination or, far less common, duplication of the centrally located H. vulgare chromosomes in all 34 aneuploids, and in a few also to loss/gain of non-H, vulgare chromosomes. This selective elimination of chromosomes of the centrally located genome contrasts conditions found in diploid interspecific hybrids, which eliminate the peripherally located genome. The difference is attributed to changed "genomic ratios'. Derivatives of various H. vulgare lines were differently distributed among euploid hybrids, aneuploids, and polyhaploids. Chromosomal constitutions of hypoploid hybrids revealed a preferential elimination of H. vulgare chromosomes 1, 5, 6, and 7, but did not support the idea that H. vulgare chromosomes should be lost in a specific order. H. vulgare SAT-chromosomes 6 and 7 showed nucleolar dominance. Aneuploidy is ascribed to the same chromosome elimination mechanism that produces haploids in cross-combinations with H. vulgare (2x). The findings have implications for the utilization of interspecific Hordeum hybrids.

Chromosome elimination and chromosome pairing in tetraploid hybrids of Hordeum vulgare × H. bulbosum

The C0 tetraploid counterparts of diploid hybrids of Hordeum vulgare × H. bulbosum were meiotically analysed, and were found to be chromosomally less stable than the same genotypes had been as diploids. The 14 bulbosum chromosomes present in the tetraploid cytotypes were probably eliminated as pairs rather than randomly or one genome at the time. Development of the vulgare and bulbosum genomes was asynchronous in some hybrids, the bulbosum chromosomes appearing less advanced than the vulgare chromosomes in the same cell. This appeared to reduce pairing between bulbosum homologues and also suppressed homoeologous pairing.