Takayoshi Ishii

Chromosome elimination by wide hybridization between Triticeae or oat plant and pearl millet: pearl millet chromosome dynamics in hybrid embryo cells

Wide crossing is one of a number of practical methods that can be used to expand genetic variation in common wheat (Triticum aestivum). However, in crosses between wheat and distantly related species such as maize (Zea mays) and pearl millet (Pennisetum glaucum), non-wheat chromosomes are often eliminated from the hybrid during embryogenesis. In this study, we used pearl millet pollen to pollinate the pistils of a range of plants in the tribe Triticeae, as well as oat. Seven days after pollination, the dynamics of the pearl millet chromosomes in the embryos were observed using in situ hybridization, probing both the pearl millet genomic DNA and its centromere-specific repeats. In embryos from the crosses with oat, all seven of the pearl millet chromosomes were retained. However, in hybrids with the Triticeae species, chromosome elimination occurred during embryogenesis. Pearl millet chromosome showed chromosome rearrangements and non-disjunction together with micronuclei. These rearranged chromosomes and micronuclei derived from the breakage of bridges and retention of acentric fragments in anaphase, respectively. The cause of the chromosome elimination of wheat–pearl millet hybrid is not malfunction of the kinetochores binding to the spindles but the malfunction of the sister chromatids segregation at anaphase especially of chromosome arm.

Point mutation impairs centromeric CENH3 loading and induces haploid plants

Significance The generation of haploids is the most powerful means to accelerate the plant-breeding process. We elucidated whether point mutations in the centromere-specific histone H3 variant CENH3 could be harnessed for the induction of haploids. We identified plants with impaired centromere loading caused by a mutation in the centromere-targeting domain (CATD). The same mutation results in reduced loading of CENH3 in transgenic Arabidopsis and sugar beet. Arabidopsis plants carrying this single point mutation in wild-type CENH3 were used as haploid inducers. Because the identified mutation site is highly conserved and because point mutations can be generated by mutagenesis or genome editing, the described method offers opportunities for application in a wide range of crop species. The chromosomal position of the centromere-specific histone H3 variant CENH3 (also called “CENP-A”) is the assembly site for the kinetochore complex of active centromeres. Any error in transcription, translation, modification, or incorporation can affect the ability to assemble intact CENH3 chromatin and can cause centromere inactivation [Allshire RC, Karpen GH (2008) Nat Rev Genet 9 (12):923–937]. Here we show that a single-point amino acid exchange in the centromere-targeting domain of CENH3 leads to reduced centromere loading of CENH3 in barley, sugar beet, and Arabidopsis thaliana. Haploids were obtained after cenh3 L130F-complemented cenh3-null mutant plants were crossed with wild-type A. thaliana. In contrast, in a noncompeting situation (i.e., centromeres possessing only mutated or only wild-type CENH3), no uniparental chromosome elimination occurs during early embryogenesis. The high degree of evolutionary conservation of the identified mutation site offers promising opportunities for application in a wide range of crop species in which haploid technology is of interest.

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.

The differential loading of two barley CENH3 variants into distinct centromeric substructures is cell type- and development-specific.

The organization of centromeric chromatin of diploid barley (Hordeum vulgare) encoding two (α and β) CENH3 variants was analysed by super-resolution microscopy. Antibody staining revealed that both CENH3 variants are organized in distinct but intermingled subdomains in interphase, mitotic and meiotic centromeres. Artificially extended chromatin fibres illustrate that these subdomains are formed by polynucleosome clusters. Thus, a CENH3 variant-specific loading followed by the arrangement into specific intermingling subdomains forming the centromere region appears. The CENH3 composition and transcription vary among different tissues. In young embryos, most interphase centromeres are composed of both CENH3 variants, while in meristematic root cells, a high number of nuclei contain βCENH3 mainly dispersed within the nucleoplasm. A similar distribution and no preferential arrangement of the two CENH3 variants in relationship to the spindle poles suggest that both homologs meet the same function in metaphase cells.

Preferential recruitment of the maternal centromere-specific histone H3 (CENH3) in oat (Avena sativa L.) × pearl millet (Pennisetum glaucum L.) hybrid embryos

Chromosome elimination occurs frequently in interspecific hybrids between distantly related species in Poaceae. However, chromosomes from both parents behave stably in a hybrid of female oat (Avena sativa L.) pollinated by pearl millet (Pennisetum glaucum L.). To analyze the chromosome behavior in this hybrid, we cloned the centromere-specific histone H3 (CENH3) genes of oat and pearl millet and produced a pearl millet-specific anti-CENH3 antibody. Application of this antibody together with a grass species common anti-CENH3 antibody revealed the dynamic CENH3 composition of the hybrid cells before and after fertilization. Despite co-expression of CENH3 genes encoded by oat and pearl millet, only an oat-type CENH3 was incorporated into the centromeres of both species in the hybrid embryo. Oat CENH3 enables a functional centromere in pearl millet chromosomes in an oat genetic background. Comparison of CENH3 genes among Poaceae species that show chromosome elimination in interspecific hybrids revealed that the loop 1 regions of oat and pearl millet CENH3 exhibit exceptionally high similarity.

Centromere location in Arabidopsis is unaltered by extreme divergence in CENH3 protein sequence

During cell division, spindle fibers attach to chromosomes at centromeres. The DNA sequence at regional centromeres is fast evolving with no conserved genetic signature for centromere identity. Instead CENH3, a centromere-specific histone H3 variant, is the epigenetic signature that specifies centromere location across both plant and animal kingdoms. Paradoxically, CENH3 is also adaptively evolving. An ongoing question is whether CENH3 evolution is driven by a functional relationship with the underlying DNA sequence. Here, we demonstrate that despite extensive protein sequence divergence, CENH3 histones from distant species assemble centromeres on the same underlying DNA sequence. We first characterized the organization and diversity of centromere repeats in wild-type Arabidopsis thaliana We show that A. thaliana CENH3-containing nucleosomes exhibit a strong preference for a unique subset of centromeric repeats. These sequences are largely missing from the genome assemblies and represent the youngest and most homogeneous class of repeats. Next, we tested the evolutionary specificity of this interaction in a background in which the native A. thaliana CENH3 is replaced with CENH3s from distant species. Strikingly, we find that CENH3 from Lepidium oleraceum and Zea mays, although specifying epigenetically weaker centromeres that result in genome elimination upon outcrossing, show a binding pattern on A. thaliana centromere repeats that is indistinguishable from the native CENH3. Our results demonstrate positional stability of a highly diverged CENH3 on independently evolved repeats, suggesting that the sequence specificity of centromeres is determined by a mechanism independent of CENH3.

Tissue-specific genome instability in synthetic interspecific hybrids of Pennisetum purpureum (Napier grass) and Pennisetum glaucum (pearl millet) is caused by micronucleation

Genome instability is observed in several species hybrids. We studied the mechanisms underlying the genome instability in hexaploid hybrids of Napier grass (Pennisetum purpureum R.) and pearl millet (Pennisetum glaucum L.) using a combination of different methods. Chromosomes of both parental genomes are lost by micronucleation. Our analysis suggests that genome instability occurs preferentially in meristematic root tissue of hexaploid hybrids, and chromosome elimination is not only caused by centromere inactivation. Likely, beside centromere dysfunction, unrepaired DNA double-strand breaks result in fragmented chromosomes in synthetic hybrids.

Analysis of grain elements and identification of best genotypes for Fe and P in Afghan wheat landraces

This study was carried out with the aim of developing the methodology to determine elemental composition in wheat and identify the best germplasm for further research. Orphan and genetically diverse Afghan wheat landraces were chosen and EDXRF was used to measure the content of some of the elements to establish elemental composition in grains of 266 landraces using 10 reference lines. Four elements, K, Mg, P, and Fe, were measured by standardizing sample preparation. The results of hierarchical cluster analysis using elemental composition data sets indicated that the Fe content has an opposite pattern to the other elements, especially that of K. By systematic analysis the best wheat germplasms for P content and Fe content were identified. In order to compare the sensitivity of EDXRF, the ICP method was also used and the similar results obtained confirmed the EDXRF methodology. The sampling method for measurement using EDXRF was optimized resulting in high-throughput profiling of elemental composition in wheat grains at low cost. Using this method, we have characterized the Afghan wheat landraces and isolated the best genotypes that have high-elemental content and have the potential to be used in crop improvement.

Wide Hybridization Between Oat and Pearl Millet

Wide hybridization is a one of the important techniques in plant breeding. Oat (Avena sativa L.) and pearl millet (Pennisetum glaucum L.) belong to different subfamilies of Poaceae. In generally, such distant relative species show uniparental chromosome elimination after successful fertilization. However, all seven pearl millet chromosomes are retained beside the genome of oat during embryogenesis. Hybrid seedlings develop, but show necrosis after light irradiation. Here, a detailed protocol for wide hybridization between oat and pearl millet is described.