Flexible chromosome painting based on multiplex PCR of oligonucleotides and its application for comparative chromosome analyses in Cucumis

The high-quality genome of Cryptotaenia japonica and comparative genomics analysis reveals anthocyanin biosynthesis in Apiaceae

Cryptotaenia japonica, a traditional medicinal and edible vegetable crops, is well-known for its attractive flavors and health care functions. As a member of the Apiaceae family, the evolutionary trajectory and biological properties of C. japonica are not clearly understood. Here, we first reported a high-quality genome of C. japonica with a total length of 427 Mb and N50 length 50.76 Mb, was anchored into 10 chromosomes, which confirmed by chromosome (cytogenetic) analysis. Comparative genomic analysis revealed C. japonica exhibited low genetic redundancy, contained a higher percentage of single-cope gene families. The homoeologous blocks, Ks, and collinearity were analyzed among Apiaceae species contributed to the evidence that C. japonica lacked recent species-specific WGD. Through comparative genomic and transcriptomic analyses of Apiaceae species, we revealed the genetic basis of the production of anthocyanins. Several structural genes encoding enzymes and transcription factor genes of the anthocyanin biosynthesis pathway in different species were also identified. The CjANSa, CjDFRb, and CjF3H gene might be the target of Cjaponica_2.2062 (bHLH) and Cjaponica_1.3743 (MYB). Our findings provided a high-quality reference genome of C. japonica and offered new insights into Apiaceae evolution and biology.

Translocations and inversions: major chromosomal rearrangements during Vigna (Leguminosae) evolution

KEY MESSAGE

Inversions and translocations are the major chromosomal rearrangements involved in Vigna subgenera evolution, being Vigna vexillata the most divergent species. Centromeric repositioning seems to be frequent within the genus. Oligonucleotide-based fluorescence in situ hybridization (Oligo-FISH) provides a powerful chromosome identification system for inferring plant chromosomal evolution. Aiming to understand macrosynteny, chromosomal diversity, and the evolution of bean species from five Vigna subgenera, we constructed cytogenetic maps for eight taxa using oligo-FISH-based chromosome identification. We used oligopainting probes from chromosomes 2 and 3 of Phaseolus vulgaris L. and two barcode probes designed from V. unguiculata (L.) Walp. genome. Additionally, we analyzed genomic blocks among the Ancestral Phaseoleae Karyotype (APK), two V. unguiculata subspecies (V. subg. Vigna), and V. angularis (Willd.) Ohwi & Ohashi (V. subg. Ceratotropis). We observed macrosynteny for chromosomes 2, 3, 4, 6, 7, 8, 9, and 10 in all investigated taxa except for V. vexillata (L.) A. Rich (V. subg. Plectrotropis), in which only chromosomes 4, 7, and 9 were unambiguously identified. Collinearity breaks involved with chromosomes 2 and 3 were revealed. We identified minor differences in the painting pattern among the subgenera, in addition to multiple intra- and interblock inversions and intrachromosomal translocations. Other rearrangements included a pericentric inversion in chromosome 4 (V. subg. Vigna), a reciprocal translocation between chromosomes 1 and 5 (V. subg. Ceratotropis), a potential deletion in chromosome 11 of V. radiata (L.) Wilczek, as well as multiple intrablock inversions and centromere repositioning via genomic blocks. Our study allowed the visualization of karyotypic patterns in each subgenus, revealing important information for understanding intrageneric karyotypic evolution, and suggesting V. vexillata as the most karyotypically divergent species.

Development, identification, and utilization of wheat-tetraploid Thinopyrum elongatum 4EL translocation lines resistant to stripe rust

KEY MESSAGE

The new stripe rust resistance gene Yr4EL in tetraploid Th. elongatum was identified and transferred into common wheat via 4EL translocation lines. Tetraploid Thinopyrum elongatum is a valuable genetic resource for improving the resistance of wheat to diseases such as stripe rust, powdery mildew, and Fusarium head blight. We previously reported that chromosome 4E of the 4E (4D) substitution line carries all-stage stripe rust resistance genes. To optimize the utility of these genes in wheat breeding programs, we developed translocation lines by inducing chromosomal structural changes through 60Co-γ irradiation and developing monosomic substitution lines. In total, 53 plants with different 4E chromosomal structural changes were identified. Three homozygous translocation lines (T4DS·4EL, T5AL·4EL, and T3BL·4EL) and an addition translocation line (T5DS·4EL) were confirmed by the genomic in situ hybridization (GISH), fluorescence in situ hybridization (FISH), FISH-painting, and wheat 55 K SNP array analyses. These four translocation lines, which contained chromosome arm 4EL, exhibited high stripe rust resistance. Thus, a resistance gene (tentatively named Yr4EL) was localized to the chromosome arm 4EL of tetraploid Th. elongatum. For the application of marker-assisted selection (MAS), 32 simple sequence repeat (SSR) markers were developed, showing specific amplification on the chromosome arm 4EL and co-segregation with Yr4EL. Furthermore, the 4DS·4EL line could be selected as a good pre-breeding line that better agronomic traits than other translocation lines. We transferred Yr4EL into three wheat cultivars SM482, CM42, and SM51, and their progenies were all resistant to stripe rust, which can be used in future wheat resistance breeding programs.

Chromosome-specific painting in Thinopyrum species using bulked oligonucleotides

Chromosome-specific painting probes were developed to identify the individual chromosomes from 1 to 7E in Thinopyrum species and detect alien genetic material of the E genome in a wheat background. The E genome of Thinopyrum is closely related to the ABD genome of wheat (Triticum aestivum L.) and harbors genes conferring beneficial traits to wheat, including high yield, disease resistance, and unique end-use quality. Species of Thinopyrum vary from diploid (2n = 2x = 14) to decaploid (2n = 10x = 70), and chromosome structural variation and differentiation have arisen during polyploidization. To investigate the variation and evolution of the E genome, we developed a complete set of E genome-specific painting probes for identification of the individual chromosomes 1E to 7E based on the genome sequences of Th. elongatum (Host) D. R. Dewey and wheat. By using these new probes in oligonucleotide-based chromosome painting, we showed that Th. bessarabicum (PI 531711, EbEb) has a close genetic relationship with diploid Th. elongatum (EeEe), with five chromosomes (1E, 2E, 3E, 6E, and 7E) maintaining complete synteny in the two species except for a reciprocal translocation between 4 and 5Eb. All 14 pairs of chromosomes of tetraploid Th. elongatum have maintained complete synteny with those of diploid Th. elongatum (Thy14), but the two sets of E genomes have diverged. This study also demonstrated that the E genome-specific painting probes are useful for rapid and effective detection of the alien genetic material of E genome in wheat-Thinopyrum derived lines.

Oligonucleotide Fluorescence In Situ Hybridization: An Efficient Chromosome Painting Method in Plants

Fluorescence in situ hybridization (FISH) is an indispensable technique for studying chromosomes in plants. However, traditional FISH methods, such as BAC, rDNA, tandem repeats, and distributed repetitive sequence probe-based FISH, have certain limitations, including difficulties in probe synthesis, low sensitivity, cross-hybridization, and limited resolution. In contrast, oligo-based FISH represents a more efficient method for chromosomal studies in plants. Oligo probes are computationally designed and synthesized for any plant species with a sequenced genome and are suitable for single and repetitive DNA sequences, entire chromosomes, or chromosomal segments. Furthermore, oligo probes used in the FISH experiment provide high specificity, resolution, and multiplexing. Moreover, oligo probes made from one species are applicable for studying other genetically and taxonomically related species whose genome has not been sequenced yet, facilitating molecular cytogenetic studies of non-model plants. However, there are some limitations of oligo probes that should be considered, such as requiring prior knowledge of the probe design process and FISH signal issues with shorter probes of background noises during oligo-FISH experiments. This review comprehensively discusses de novo oligo probe synthesis with more focus on single-copy DNA sequences, preparation, improvement, and factors that affect oligo-FISH efficiency. Furthermore, this review highlights recent applications of oligo-FISH in a wide range of plant chromosomal studies.

Development and Characterization of a Novel Wheat-Tetraploid Thinopyrum elongatum 6E (6D) Disomic Substitution Line with Stripe Rust Resistance at the Adult Stage

Stripe rust, which is caused by Puccinia striiformis f. sp. tritici, is one of the most devastating foliar diseases of common wheat worldwide. Breeding new wheat varieties with durable resistance is the most effective way of controlling the disease. Tetraploid Thinopyrum elongatum (2n = 4x = 28, EEEE) carries a variety of genes conferring resistance to multiple diseases, including stripe rust, Fusarium head blight, and powdery mildew, which makes it a valuable tertiary genetic resource for enhancing wheat cultivar improvement. Here, a novel wheat-tetraploid Th. elongatum 6E (6D) disomic substitution line (K17-1065-4) was characterized using genomic in situ hybridization and fluorescence in situ hybridization chromosome painting analyses. The evaluation of disease responses revealed that K17-1065-4 is highly resistant to stripe rust at the adult stage. By analyzing the whole-genome sequence of diploid Th. elongatum, we detected 3382 specific SSR sequences on chromosome 6E. Sixty SSR markers were developed, and thirty-three of them can accurately trace chromosome 6E of tetraploid Th. elongatum, which were linked to the disease resistance gene(s) in the wheat genetic background. The molecular marker analysis indicated that 10 markers may be used to distinguish Th. elongatum from other wheat-related species. Thus, K17-1065-4 carrying the stripe rust resistance gene(s) is a novel germplasm useful for breeding disease-resistant wheat cultivars. The molecular markers developed in this study may facilitate the mapping of the stripe rust resistance gene on chromosome 6E of tetraploid Th. elongatum.

Oligo-FISH of Populus simonii Pachytene Chromosomes Improves Karyotyping and Genome Assembly

Poplar was one of the first woody species whose individual chromosomes could be identified using chromosome specific painting probes. Nevertheless, high-resolution karyotype construction remains a challenge. Here, we developed a karyotype based on the meiotic pachytene chromosome of Populus simonii which is a Chinese native species with many excellent traits. This karyotype was anchored by oligonucleotide (oligo)-based chromosome specific painting probes, a centromere-specific repeat (Ps34), ribosomal DNA, and telomeric DNA. We updated the known karyotype formula for P. simonii to 2n = 2x = 38 = 26m + 8st + 4t and the karyotype was 2C. The fluorescence in situ hybridization (FISH) results revealed some errors in the current P. simonii genome assembly. The 45S rDNA loci were located at the end of the short arm of chromosomes 8 and 14 by FISH. However, they were assembled on pseudochromosomes 8 and 15. In addition, the Ps34 loci were distributed in every centromere of the P. simonii chromosome in the FISH results, but they were only found to be present in pseudochromosomes 1, 3, 6, 10, 16, 17, 18, and 19. Our results reveal that pachytene chromosomes oligo-FISH is a powerful tool for constructing high-resolution karyotypes and improving the quality of genome assembly.

Molecular cytogenetic map visualizes the heterozygotic genome and identifies translocation chromosomes in Citrus sinensis

Citrus sinensis is the most cultivated and economically valuable Citrus species in the world, whose genome has been assembled by three generation sequencings. However, chromosome recognition remains a problem due to the small size of chromosomes, and difficulty in differentiating between pseudo and real chromosomes because of a highly heterozygous genome. Here, we employ fluorescence in situ hybridization (FISH) with 9 chromosome painting probes, 30 oligo pools, and 8 repetitive sequences to visualize 18 chromosomes. Then, we develop an approach to identify each chromosome in one cell through single experiment of oligo-FISH and Chromoycin A3 (CMA) staining. By this approach, we construct a high-resolution molecular cytogenetic map containing the physical positions of CMA banding and 38 sequences of FISH including centromere regions, which enable us to visualize significant differences between homologous chromosomes. Based on the map, we locate several highly repetitive sequences on chromosomes and estimate sizes and copy numbers of each site. In particular, we discover the translocation regions of chromosomes 4 and 9 in C. sinensis "Valencia." The high-resolution molecular cytogenetic map will help improve understanding of sweet orange genome assembly and also provide a fundamental reference for investigating chromosome evolution and chromosome engineering for genetic improvement in Citrus.

Reconstruction of karyotypic evolution in Saccharum spontaneum species by comparative oligo-FISH mapping

BACKGROUND

Karyotype dynamics driven by chromosomal rearrangements has long been considered as a fundamental question in the evolutionary genetics. Saccharum spontaneum, the most primitive and complex species in the genus Saccharum, has reportedly undergone at least two major chromosomal rearrangements, however, its karyotypic evolution remains unclear.

RESULTS

In this study, four representative accessions, i.e., hypothetical diploid sugarcane ancestor (sorghum, x = 10), Sa. spontaneum Np-X (x = 10, tetraploid), 2012-46 (x = 9, hexaploid) and AP85-441 (x = 8, tetraploid), were selected for karyotype evolution studies. A set of oligonucleotide (oligo)-based barcode probes was developed based on the sorghum genome, which allowed universal identification of all chromosomes from sorghum and Sa. spontaneum. By comparative FISH assays, we reconstructed the karyotype evolutionary history and discovered that although chromosomal rearrangements resulted in greater variation in relative lengths of some chromosomes, all chromosomes maintained a conserved metacentric structure. Additionally, we found that the barcode oligo probe was not applicable for chromosome identification in both Sa. robustum and Sa. officinarum species, suggesting that sorghum is more distantly related to Sa. robustum and Sa. officinarum compared with Sa. spontaneum species.

CONCLUSIONS

Our study demonstrated that the barcode oligo-FISH is an efficient tool for chromosome identification and karyotyping research, and expanded our understanding of the karyotypic and chromosomal evolution in the genus Saccharum.

Accurate Chromosome Identification in the Prunus Subgenus Cerasus (Prunus pseudocerasus) and its Relatives by Oligo-FISH

A precise, rapid and straightforward approach to chromosome identification is fundamental for cytogenetics studies. However, the identification of individual chromosomes was not previously possible for Chinese cherry or other Prunus species due to the small size and similar morphology of their chromosomes. To address this issue, we designed a pool of oligonucleotides distributed across specific pseudochromosome regions of Chinese cherry. This oligonucleotide pool was amplified through multiplex PCR with specific internal primers to produce probes that could recognize specific chromosomes. External primers modified with red and green fluorescence tags could produce unique signal barcoding patterns to identify each chromosome concomitantly. The same oligonucleotide pool could also discriminate all chromosomes in other Prunus species. Additionally, the 5S/45S rDNA probes and the oligo pool were applied in two sequential rounds of fluorescence in situ hybridization (FISH) localized to chromosomes and showed different distribution patterns among Prunus species. At the same time, comparative karyotype analysis revealed high conservation among P. pseudocerasus, P. avium, and P. persica. Together, these findings establish this oligonucleotide pool as the most effective tool for chromosome identification and the analysis of genome organization and evolution in the genus Prunus.

Chromosome painting reveals inter-chromosomal rearrangements and evolution of subgenome D of wheat

Aegilops species represent the most important gene pool for breeding bread wheat (Triticum aestivum). Thus, understanding the genome evolution, including chromosomal structural rearrangements and syntenic relationships among Aegilops species or between Aegilops and wheat, is important for both basic genome research and practical breeding applications. In the present study, we attempted to develop subgenome D-specific fluorescence in situ hybridization (FISH) probes by selecting D-specific oligonucleotides based on the reference genome of Chinese Spring. The oligo-based chromosome painting probes consisted of approximately 26 000 oligos per chromosome and their specificity was confirmed in both diploid and polyploid species containing the D subgenome. Two previously reported translocations involving two D chromosomes have been confirmed in wheat varieties and their derived lines. We demonstrate that the oligo painting probes can be used not only to identify the translocations involving D subgenome chromosomes, but also to determine the precise positions of chromosomal breakpoints. Chromosome painting of 56 accessions of Ae. tauschii from different origins led us to identify two novel translocations: a reciprocal 3D-7D translocation in two accessions and a complex 4D-5D-7D translocation in one accession. Painting probes were also used to analyze chromosomes from more diverse Aegilops species. These probes produced FISH signals in four different genomes. Chromosome rearrangements were identified in Aegilops umbellulata, Aegilops markgrafii, and Aegilops uniaristata, thus providing syntenic information that will be valuable for the application of these wild species in wheat breeding.

Genome sequence of Gossypium anomalum facilitates interspecific introgression breeding

Crop wild relatives are an important reservoir of natural biodiversity. However, incorporating wild genetic diversity into breeding programs is often hampered by reproductive barriers and a lack of accurate genomic information. We assembled a high-quality, accurately centromere-anchored genome of Gossypium anomalum, a stress-tolerant wild cotton species. We provided a strategy to discover and transfer agronomically valuable genes from wild diploid species to tetraploid cotton cultivars. With a (Gossypium hirsutum × G. anomalum)2 hexaploid as a bridge parent, we developed a set of 74 diploid chromosome segment substitution lines (CSSLs) of the wild cotton species G. anomalum in the G. hirsutum background. This set of CSSLs included 70 homozygous substitutions and four heterozygous substitutions, and it collectively contained about 72.22% of the G. anomalum genome. Twenty-four quantitative trait loci associated with plant height, yield, and fiber qualities were detected on 15 substitution segments. Integrating the reference genome with agronomic trait evaluation of the CSSLs enabled location and cloning of two G. anomalum genes that encode peroxiredoxin and putative callose synthase 8, respectively, conferring drought tolerance and improving fiber strength. We have demonstrated the power of a high-quality wild-species reference genome for identifying agronomically valuable alleles to facilitate interspecific introgression breeding in crops.

Rapid and visual monitoring of alien sequences using crop wild relatives specific oligo-painting: The case of cucumber chromosome engineering

Wild species related to domesticated crops (crop wild relatives, or CWRs) represent a high level of genetic diversity that provides a practical gene pool for crop pre-breeding employed to address climate change and food demand challenges globally. Nevertheless, rapid identifying and visual tracking of alien chromosomes and sequences derived from CWRs have been a technical challenge for crop chromosome engineering. Here, a species-specific oligonucleotide (oligo) pool was developed by using the reference genome of Cucumis hystrix (HH, 2n = 2x = 24), a wild species carrying many favorable traits and interspecific compatibility with cultivated cucumber (C. sativus, CC, 2n = 2x = 14). These synthetic double-stranded oligo probes were applied to validate the assembly and characterize the chromosome architectures of C. hystrix, as well as to rapidly identify C. hystrix-chromosomes in diverse C. sativus-hystrix chromosome-engineered germplasms, including interspecific hybrid F1 (HC), synthetic allopolyploids (HHCC, CHC, and HCH) and alien additional lines (CC-H). Moreover, a ∼2Mb of C. hystrix-specific sequences, introduced into cultivated cucumber, were visualized by CWR-specific oligo-painting. These results demonstrate that the CWR-specific oligo-painting technique holds broad applicability for chromosome engineering of numerous crops, as it allows rapid identification of alien chromosomes, reliable detection of homoeologous recombination, and visual tracking of the introgression process. It is promising to achieve directed and high-precision crop pre-breeding combined with other breeding techniques, such as CRISPR/Cas9-mediated chromosome engineering.

Development and application of specific FISH probes for karyotyping Psathyrostachys huashanica chromosomes

Background

Psathyrostachys huashanica Keng has long been used as a genetic resource for improving wheat cultivar because of its genes mediating the resistance to various diseases (stripe rust, leaf rust, take-all, and powdery mildew) as well as its desirable agronomic traits. However, a high-resolution fluorescence in situ hybridization (FISH) karyotype of P. huashanica remains unavailable.

Results

To develop chromosome-specific FISH markers for P. huashanica, repetitive sequences, including pSc119.2, pTa535, pTa713, pAs1, (AAC)₅, (CTT)₁₂, pSc200, pTa71A-2, and Oligo-44 were used for a FISH analysis. The results indicated that the combination of pSc200, pTa71A-2 and Oligo-44 probes can clearly identify all Ns genomic chromosomes in the two P. huashanica germplasms. The homoeologous relationships between individual P. huashanica chromosomes and common wheat chromosomes were clarified by FISH painting. Marker validation analyses revealed that the combination of pSc200, pTa71A-2, and Oligo-44 for a FISH analysis can distinguish the P. huashanica Ns-genome chromosomes from wheat chromosomes, as well as all chromosomes (except 4Ns) from the chromosomes of diploid wheat relatives carrying St, E, V, I, P and R genomes. Additionally, the probes were applicable for discriminating between the P. huashanica Ns-genome chromosomes in all homologous groups and the corresponding chromosomes in Psathyrostachys juncea and most Leymus species containing the Ns genome. Furthermore, six wheat–P. huashanica chromosome addition lines (i.e., 2Ns, 3Ns, 4Ns, 7Ns chromosomes and chromosomal segments) were characterized using the newly developed FISH markers. Thus, these probes can rapidly and precisely detect P. huashanica alien chromosomes in the wheat background.

Conclusions

The FISH karyotype established in this study lays a solid foundation for the efficient identification of P. huashanica chromosomes in wheat genetic improvement programs.

Oligo-FISH Paints in Triticeae

We developed seven oligonucleotide (oligo) pools based on single-copy sequences, targeting chromosomes 1 to 7 of barley (Hordeum vulgare L.) and wheat (Triticum aestivum) for chromosomal Oligo-FISH painting methods. The probes were applied to high-throughput karyotyping for the Triticeae tribe of over 350 species including 30 genera such as Triticum, Hordeum, Secale, Aegilops, Thinopyrum, and Dasypyrum, as well as several wheat alien-derived lines. In combination with other nondenaturing FISH (ND-FISH) procedures using tandem-repeat oligos, the newly developed Oligo-FISH painting technique provides an efficient tool for the identification of individual chromosomes with homologous linkage groups to establish standard karyotypes, particularly with any wild Triticeae species having nonsequenced genomes for chromosome evolutionary analysis. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Oligo-pool probe development Basic Protocol 2: Nondenaturing FISH Basic Protocol 3: Oligo-FISH painting.

A comprehensive molecular cytogenetic analysis of the genome architecture in modern sugarcane cultivars

Modern sugarcane cultivars are derived from the hybridization of Saccharum officinarum (2n = 80) and S. spontaneum (2n = 40-128), leading to a variety of complex genomes with highly polyploid and varied chromosome structures. These complex genomes have hindered deciphering the genome structure and marker-assisted selection in sugarcane breeding. Ten cultivars were analyzed by fluorescence in situ hybridization adopting chromosome painting and S. spontaneum-specific probes. The results showed six types of chromosomes in the studied cultivars, including S. spontaneum or S. officinarum chromosomes, interspecific recombinations from homoeologous or nonhomoeologous chromosomes, and translocations of S. spontaneum or S. officinarum chromosomes. The results showed unexpectedly high proportions of interspecific recombinations in these cultivars (11.9-40.9%), which renew our knowledge that less than 13% of chromosomes result from interspecific exchanges. Also, the results showed a high frequency of translocations (an average of 2.15 translocations per chromosome) between S. officinarum chromosomes. The diverse types of chromosomes in cultivars imply that hybrid gametes of S. spontaneum and S. officinarum may form unusual chromosome pairs, including homoeologous or nonhomoeologous chromosomes either between or within S. spontaneum and S. officinarum. Moreover, we consistently observed 11 or 12 copies for the four studied chromosomes, i.e., chromosomes 1, 2, 7, and 8, suggesting steady transmission during the breeding program. By comparison, we found a relatively fewer copies of S. spontaneum chromosome 1 than those of S. spontaneum chromosomes 2, 7, and 8. These results provide deep insights into the structure of cultivars and may facilitate chromosome-assisted selection in sugarcane breeding.

Identification of a Wheat-Psathyrostachys huashanica 7Ns Ditelosomic Addition Line Conferring Early Maturation by Cytological Analysis and Newly Developed Molecular and FISH Markers

Early maturation is an important objective in wheat breeding programs that could facilitate multiple-cropping systems, decrease disaster- and disease-related losses, ensure stable wheat production, and increase economic benefits. Exploitation of novel germplasm from wild relatives of wheat is an effective means of breeding for early maturity. Psathyrostachys huashanica Keng f. ex P. C. KUO (2n=2x=14, NsNs) is a promising source of useful genes for wheat genetic improvement. In this study, we characterized a novel wheat-P. huashanica line, DT23, derived from distant hybridization between common wheat and P. huashanica. Fluorescence in situ hybridization (FISH) and sequential genomic in situ hybridization (GISH) analyses indicated that DT23 is a stable wheat-P. huashanica ditelosomic addition line. FISH painting and PCR-based landmark unique gene markers analyses further revealed that DT23 is a wheat-P. huashanica 7Ns ditelosomic addition line. Observation of spike differentiation and the growth period revealed that DT23 exhibited earlier maturation than the wheat parents. This is the first report of new earliness per se (Eps) gene(s) probably associated with a group 7 chromosome of P. huashanica. Based on specific locus-amplified fragment sequencing technology, 45 new specific molecular markers and 19 specific FISH probes were developed for the P. huashanica 7Ns chromosome. Marker validation analyses revealed that two specific markers distinguished the Ns genome chromosomes of P. huashanica and the chromosomes of other wheat-related species. These newly developed FISH probes specifically detected Ns genome chromosomes of P. huashanica in the wheat background. The DT23 line will be useful for breeding early maturing wheat. The specific markers and FISH probes developed in this study can be used to detect and trace P. huashanica chromosomes and chromosomal segments carrying elite genes in diverse materials.

FISH Going Meso-Scale: A Microscopic Search for Chromatin Domains

The intimate relationships between genome structure and function direct efforts toward deciphering three-dimensional chromatin organization within the interphase nuclei at different genomic length scales. For decades, major insights into chromatin structure at the level of large-scale euchromatin and heterochromatin compartments, chromosome territories, and subchromosomal regions resulted from the evolution of light microscopy and fluorescence in situ hybridization. Studies of nanoscale nucleosomal chromatin organization benefited from a variety of electron microscopy techniques. Recent breakthroughs in the investigation of mesoscale chromatin structures have emerged from chromatin conformation capture methods (C-methods). Chromatin has been found to form hierarchical domains with high frequency of local interactions from loop domains to topologically associating domains and compartments. During the last decade, advances in super-resolution light microscopy made these levels of chromatin folding amenable for microscopic examination. Here we are reviewing recent developments in FISH-based approaches for detection, quantitative measurements, and validation of contact chromatin domains deduced from C-based data. We specifically focus on the design and application of Oligopaint probes, which marked the latest progress in the imaging of chromatin domains. Vivid examples of chromatin domain FISH-visualization by means of conventional, super-resolution light and electron microscopy in different model organisms are provided.

Oligo-FISH barcode in beans: a new chromosome identification system

An Oligo-FISH barcode system was developed for two model legumes, allowing the identification of all cowpea and common bean chromosomes in a single FISH experiment, and revealing new chromosome rearrangements. The FISH barcode system emerges as an effective tool to understand the chromosome evolution of economically important legumes and their related species. Current status on plant cytogenetic and cytogenomic research has allowed the selection and design of oligo-specific probes to individually identify each chromosome of the karyotype in a target species. Here, we developed the first chromosome identification system for legumes based on oligo-FISH barcode probes. We selected conserved genomic regions between Vigna unguiculata (Vu, cowpea) and Phaseolus vulgaris (Pv, common bean) (diverged ~ 9.7-15 Mya), using cowpea as a reference, to produce a unique barcode pattern for each species. We combined our oligo-FISH barcode pattern with a set of previously developed FISH probes based on BACs and ribosomal DNA sequences. In addition, we integrated our FISH maps with genome sequence data. Based on this integrated analysis, we confirmed two translocation events (involving chromosomes 1, 5, and 8; and chromosomes 2 and 3) between both species. The application of the oligo-based probes allowed us to demonstrate the participation of chromosome 5 in the translocation complex for the first time. Additionally, we detailed a pericentric inversion on chromosome 4 and identified a new paracentric inversion on chromosome 10. We also detected centromere repositioning associated with chromosomes 2, 3, 5, 7, and 9, confirming previous results for chromosomes 2 and 3. This first barcode system for legumes can be applied for karyotyping other Phaseolinae species, especially non-model, orphan crop species lacking genomic assemblies and cytogenetic maps, expanding our understanding of the chromosome evolution and genome organization of this economically important legume group.

Chorus2: design of genome-scale oligonucleotide-based probes for fluorescence in situ hybridization

Oligonucleotide (oligo)-fluorescence in situ hybridization (FISH) has rapidly becoming the new generation of FISH technique in plant molecular cytogenetics research. Genome-scale identification of single-copy oligos is the foundation of successful oligo-FISH experiments. Here, we introduce Chorus2, a software that is developed specifically for oligo selection. We demonstrate that Chorus2 is highly effective to remove all repetitive elements in selection of single-copy oligos, which is critical for the development of successful FISH probes. Chorus2 is more effective than Chorus, the original version of the pipeline, and OligoMiner for repeat removal. Chorus2 allows to select oligos that are conserved among related species, which extends the usage of oligo-FISH probes among phylogenetically related plant species. We also implemented a new function in Chorus2 that allows development of FISH probes from plant species without an assembled genome. We anticipate that Chorus2 can be used in plants as well as in mammalian and other non-plant species. Chorus2 will broadly facilitate the design of FISH probes for various types of application in molecular cytogenetics research.

Reconstruction of ancestral karyotype illuminates chromosome evolution in the genus Cucumis

Karyotype dynamics driven by complex chromosome rearrangements constitute a fundamental issue in evolutionary genetics. The evolutionary events underlying karyotype diversity within plant genera, however, have rarely been reconstructed from a computed ancestral progenitor. Here, we developed a method to rapidly and accurately represent extant karyotypes with the genus, Cucumis, using highly customizable comparative oligo-painting (COP) allowing visualization of fine-scale genome structures of eight Cucumis species from both African-origin and Asian-origin clades. Based on COP data, an evolutionary framework containing a genus-level ancestral karyotype was reconstructed, allowing elucidation of the evolutionary events that account for the origin of these diverse genomes within Cucumis. Our results characterize the cryptic rearrangement hotspots on ancestral chromosomes, and demonstrate that the ancestral Cucumis karyotype (n = 12) evolved to extant Cucumis genomes by hybridizations and frequent lineage- and species-specific genome reshuffling. Relative to the African species, the Asian species, including melon (Cucumis melo, n = 12), Cucumis hystrix (n = 12) and cucumber (Cucumis sativus, n = 7), had highly shuffled genomes caused by large-scale inversions, centromere repositioning and chromothripsis-like rearrangement. The deduced reconstructed ancestral karyotype for the genus allowed us to propose evolutionary trajectories and specific events underlying the origin of these Cucumis species. Our findings highlight that the partitioned evolutionary plasticity of Cucumis karyotype is primarily located in the centromere-proximal regions marked by rearrangement hotspots, which can potentially serve as a reservoir for chromosome evolution due to their fragility.

A universal karyotypic system for hexaploid and diploid Avena species brings oat cytogenetics into the genomics era

BACKGROUND

The identification of chromosomes among Avena species have been studied by C-banding and in situ hybridization. However, the complicated results from several cytogenetic nomenclatures for identifying oat chromosomes are often contradictory. A universal karyotyping nomenclature system for precise chromosome identification and comparative evolutionary studies would be essential for genus Avena based on the recently released genome sequences of hexaploid and diploid Avena species.

RESULTS

Tandem repetitive sequences were predicted and physically located on chromosomal regions of the released Avena sativa OT3098 genome assembly v1. Eight new oligonucleotide (oligo) probes for sequential fluorescence in situ hybridization (FISH) were designed and then applied for chromosome karyotyping on mitotic metaphase spreads of A. brevis, A. nuda, A. wiestii, A. ventricosa, A. fatua, and A. sativa species. We established a high-resolution standard karyotype of A. sativa based on the distinct FISH signals of multiple oligo probes. FISH painting with bulked oligos, based on wheat-barley collinear regions, was used to validate the linkage group assignment for individual A. sativa chromosomes. We integrated our new Oligo-FISH based karyotype system with earlier karyotype nomenclatures through sequential C-banding and FISH methods, then subsequently determined the precise breakage points of some chromosome translocations in A. sativa.

CONCLUSIONS

This new universal chromosome identification system will be a powerful tool for describing the genetic diversity, chromosomal rearrangements and evolutionary relationships among Avena species by comparative cytogenetic and genomic approaches.

Whole-Genome Sequence of Synthesized Allopolyploids in Cucumis Reveals Insights into the Genome Evolution of Allopolyploidization

The importance of allopolyploidy in plant evolution has been widely recognized. The genetic changes triggered by allopolyploidy, however, are not yet fully understood due to inconsistent phenomena reported across diverse species. The construction of synthetic polyploids offers a controlled approach to systematically reveal genomic changes that occur during the process of polyploidy. This study reports the first fully sequenced synthetic allopolyploid constructed from a cross between Cucumis sativus and C. hystrix, with high-quality assembly. The two subgenomes are confidently partitioned and the C. sativus-originated subgenome predominates over the C. hystrix-originated subgenome, retaining more sequences and showing higher homeologous gene expression. Most of the genomic changes emerge immediately after interspecific hybridization. Analysis of a series of genome sequences from several generations (S0, S4-S13) of C. ×hytivus confirms that genomic changes occurred in the very first generations, subsequently slowing down as the process of diploidization is initiated. The duplicated genome of the allopolyploid with double genes from both parents broadens the genetic base of C. ×hytivus, resulting in enhanced phenotypic plasticity. This study provides novel insights into plant polyploid genome evolution and demonstrates a promising strategy for the development of a wide array of novel plant species and varieties through artificial polyploidization.

BAC- and oligo-FISH mapping reveals chromosome evolution among Vigna angularis, V. unguiculata, and Phaseolus vulgaris

Cytogenomic resources have accelerated synteny and chromosome evolution studies in plant species, including legumes. Here, we established the first cytogenetic map of V. angularis (Va, subgenus Ceratotropis) and compared this new map with those of V. unguiculata (Vu, subgenus Vigna) and P. vulgaris (Pv) by BAC-FISH and oligopainting approaches. We mapped 19 Vu BACs and 35S rDNA probes to the 11 chromosome pairs of Va, Vu, and Pv. Vigna angularis shared a high degree of macrosynteny with Vu and Pv, with five conserved syntenic chromosomes. Additionally, we developed two oligo probes (Pv2 and Pv3) used to paint Vigna orthologous chromosomes. We confirmed two reciprocal translocations (chromosomes 2 and 3 and 1 and 8) that have occurred after the Vigna and Phaseolus divergence (~9.7 Mya). Besides, two inversions (2 and 4) and one translocation (1 and 5) have occurred after Vigna and Ceratotropis subgenera separation (~3.6 Mya). We also observed distinct oligopainting patterns for chromosomes 2 and 3 of Vigna species. Both Vigna species shared similar major rearrangements compared to Pv: one translocation (2 and 3) and one inversion (chromosome 3). The sequence synteny identified additional inversions and/or intrachromosomal translocations involving pericentromeric regions of both orthologous chromosomes. We propose chromosomes 2 and 3 as hotspots for chromosomal rearrangements and de novo centromere formation within and between Vigna and Phaseolus. Our BAC- and oligo-FISH mapping contributed to physically trace the chromosome evolution of Vigna and Phaseolus and its application in further studies of both genera.

Chromosome-scale genome assembly of Cucumis hystrix-a wild species interspecifically cross-compatible with cultivated cucumber

Cucumis hystrix Chakr. (2n = 2x = 24) is a wild species that can hybridize with cultivated cucumber (C. sativus L., 2n = 2x = 14), a globally important vegetable crop. However, cucumber breeding is hindered by its narrow genetic base. Therefore, introgression from C. hystrix has been anticipated to bring a breakthrough in cucumber improvement. Here, we report the chromosome-scale assembly of C. hystrix genome (289 Mb). Scaffold N50 reached 14.1 Mb. Over 90% of the sequences were anchored onto 12 chromosomes. A total of 23,864 genes were annotated using a hybrid method. Further, we conducted a comprehensive comparative genomic analysis of cucumber, C. hystrix, and melon (C. melo L., 2n = 2x = 24). Whole-genome comparisons revealed that C. hystrix is phylogenetically closer to cucumber than to melon, providing a molecular basis for the success of its hybridization with cucumber. Moreover, expanded gene families of C. hystrix were significantly enriched in "defense response," and C. hystrix harbored 104 nucleotide-binding site-encoding disease resistance gene analogs. Furthermore, 121 genes were positively selected, and 12 (9.9%) of these were involved in responses to biotic stimuli, which might explain the high disease resistance of C. hystrix. The alignment of whole C. hystrix genome with cucumber genome and self-alignment revealed 45,417 chromosome-specific sequences evenly distributed on C. hystrix chromosomes. Finally, we developed four cucumber-C. hystrix alien addition lines and identified the exact introgressed chromosome using molecular and cytological methods. The assembled C. hystrix genome can serve as a valuable resource for studies on Cucumis evolution and interspecific introgression breeding of cucumber.

An efficient Oligo-FISH painting system for revealing chromosome rearrangements and polyploidization in Triticeae

A chromosome-specific painting technique has been developed which combines the most recent approaches of the companion disciplines of molecular cytogenetics and genome research. We developed seven oligonucleotide (oligo) pools derivd from single-copy sequences on chromosomes 1 to 7 of barley (Hordeum vulgare L.) and corresponding collinear regions of wheat (Triticum aestivum L.). The seven groups of pooled oligos comprised between 10 986 and 12 496 45-bp monomers, and these then produced stable fluorescence in situ hybridization (FISH) signals on chromosomes of each linkage group of wheat and barley. The pooled oligo probes were applied to high-throughput karyotyping of the chromosomes of other Triticeae species in the genera Secale, Aegilops, Thinopyrum, and Dasypyrum, and the study also extended to some wheat-alien amphiploids and derived lines. We demonstrated that a complete set of whole-chromosome oligo painting probes facilitated the study of inter-species chromosome homologous relationships and visualized non-homologous chromosomal rearrangements in Triticeae species and some wheat-alien species derivatives. When combined with other non-denaturing FISH procedures using tandem-repeat oligos, the newly developed oligo painting techniques provide an efficient tool for the study of chromosome structure, organization, and evolution among any wild Triticeae species with non-sequenced genomes.

Identification of 5S and 45S rDNA sites in Chrysanthemum species by using oligonucleotide fluorescence in situ hybridization (Oligo-FISH)

Fluorescence in situ hybridization (FISH) is a conventional method used to visualize the distribution of DNA elements within a genome. To examine the relationships within the Chrysanthemum genus, ribosomal DNA (rDNA), a popular cytogenetic marker, was utilized as a probe for FISH within this genus. Based on the genome data of Chrysanthemum nankingense, C. seticuspe and its allied genera in the Compositae(Asteraceae), we explored rDNA sequences to design oligonucleotide probes and perform oligonucleotide fluorescence in situ hybridization (Oligo-FISH) in eight Chrysanthemum accessions. The results showed that the majority of 5S rDNA signals were located in subterminal chromosome regions and that the number of 5S rDNA sites might be tightly associated with ploidy. For 45S rDNA sites, the number and intensity of signals differed from those of previously investigated Chrysanthemum resources. These findings may provide an optimally reliable method of examining the chromosome composition and structural variation of Chrysanthemum and its related species and allow researchers to understand the evolutionary history and phylogenetic relationships of Chrysanthemum.

Chromosome Painting in Cultivated Bananas and Their Wild Relatives (Musa spp.) Reveals Differences in Chromosome Structure

Edible banana cultivars are diploid, triploid, or tetraploid hybrids, which originated by natural cross hybridization between subspecies of diploid Musa acuminata, or between M. acuminata and diploid Musa balbisiana. The participation of two other wild diploid species Musa schizocarpa and Musa textilis was also indicated by molecular studies. The fusion of gametes with structurally different chromosome sets may give rise to progenies with structural chromosome heterozygosity and reduced fertility due to aberrant chromosome pairing and unbalanced chromosome segregation. Only a few translocations have been classified on the genomic level so far, and a comprehensive molecular cytogenetic characterization of cultivars and species of the family Musaceae is still lacking. Fluorescence in situ hybridization (FISH) with chromosome-arm-specific oligo painting probes was used for comparative karyotype analysis in a set of wild Musa species and edible banana clones. The results revealed large differences in chromosome structure, discriminating individual accessions. These results permitted the identification of putative progenitors of cultivated clones and clarified the genomic constitution and evolution of aneuploid banana clones, which seem to be common among the polyploid banana accessions. New insights into the chromosome organization and structural chromosome changes will be a valuable asset in breeding programs, particularly in the selection of appropriate parents for cross hybridization.

The Evolution of Chromosome Numbers: Mechanistic Models and Experimental Approaches

Chromosome numbers have been widely used to describe the most fundamental genomic attribute of an organism or a lineage. Although providing strong phylogenetic signal, chromosome numbers vary remarkably among eukaryotes at all levels of taxonomic resolution. Changes in chromosome numbers regularly serve as indication of major genomic events, most notably polyploidy and dysploidy. Here, we review recent advancements in our ability to make inferences regarding historical events that led to alterations in the number of chromosomes of a lineage. We first describe the mechanistic processes underlying changes in chromosome numbers, focusing on structural chromosomal rearrangements. Then, we focus on experimental procedures, encompassing comparative cytogenomics and genomics approaches, and on computational methodologies that are based on explicit models of chromosome-number evolution. Together, these tools offer valuable predictions regarding historical events that have changed chromosome numbers and genome structures, as well as their phylogenetic and temporal placements.

Development of alien addition lines from Cucumis hystrix in Cucumis sativus: cytological and molecular marker analyses

Transferring desired genes from wild species to cultivars through alien addition lines (AALs) has been shown to be an effective method for genetic improvement. Cucumis hystrix Chakr. (HH, 2n = 24) is a wild species of Cucumis that possesses many resistant genes. A synthetic allotetraploid species, C. hytivus (HHCC, 2n = 38), was obtained from the cross between cultivated cucumber, C. sativus (CC, 2n = 14), and C. hystrix followed by chromosome doubling. Cucumis sativus - C. hystrix AALs were developed by continuous backcrossing to the cultivated cucumbers. In this study, 10 different types of AALs (CC-H01, CC-H06, CC-H08, CC-H10, CC-H12, CC-H06+H09, CC-H06+H10, CC-H06+H12, CC-H08+H10, CC-H01+H06+H10) were identified based on the analysis of fluorescence in situ hybridization (FISH) and molecular markers specific to C. hystrix chromosomes. And the behavior of the alien chromosomes in three AALs (CC-H01, CC-H06+H10, CC-H01+H06+H10) at meiosis was investigated. The results showed that alien chromosomes paired with C. sativus chromosome in few pollen mother cells (PMCs). Further, disomic alien addition lines (DAALs) carrying a pair of C. hystrix chromosome H10 were screened from the selfed progenies of CC-H10. Chromosome pairing between genomes provides cytological evidence for the possible introgression of alien chromosome segments. The development of AALs could serve as a key step for exploiting and utilizing valuable genes from C. hystrix.

Extraordinarily conserved chromosomal synteny of Citrus species revealed by chromosome-specific painting

Reliable identification of individual chromosomes in eukaryotic species is the foundation for comparative chromosome synteny and evolutionary studies. Unfortunately, chromosome identification has been a major challenge for plants with small chromosomes, such as the Citrus species. We developed oligonucleotide-based chromosome painting probes for all nine chromosomes in Citrus maxima (Pummelo). We were able to identify all C. maxima chromosomes in the same metaphase cells using multiple rounds of sequential fluorescence in situ hybridization with the painting probes. We conducted comparative chromosome painting analysis in six different Citrus and related species. We found that each painting probe hybridized to only a single chromosome in all other five species, suggesting that the six species have maintained a complete chromosomal synteny after more than 9 million years of divergence. No interchromosomal rearrangement was identified in any species. These results support the hypothesis that karyotypes of woody species are more stable than herbaceous plants because woody plants need a longer period to fix chromosome structural variants in natural populations.

A universal chromosome identification system for maize and wild Zea species

Maize was one of the first eukaryotic species in which individual chromosomes can be identified cytologically, which made maize one of the oldest models for genetics and cytogenetics research. Nevertheless, consistent identification of all 10 chromosomes from different maize lines as well as from wild Zea species remains a challenge. We developed a new technique for maize chromosome identification based on fluorescence in situ hybridization (FISH). We developed two oligonucleotide-based probes that hybridize to 24 chromosomal regions. Individual maize chromosomes show distinct FISH signal patterns, which allow universal identification of all chromosomes from different Zea species. We developed karyotypes from three Zea mays subspecies and two additional wild Zea species based on individually identified chromosomes. A paracentric inversion was discovered on the long arm of chromosome 4 in Z. nicaraguensis and Z. luxurians based on modifications of the FISH signal patterns. Chromosomes from these two species also showed distinct distribution patterns of terminal knobs compared with other Zea species. These results support that Z. nicaraguensis and Z. luxurians are closely related species.

Chromosome Painting Based on Bulked Oligonucleotides in Cotton

Chromosome painting is one of the key technologies in cytogenetic research, which can accurately identify chromosomes or chromosome regions. Oligonucleotide (oligo) probes designed based on genome sequences have both flexibility and specificity, which would be ideal probes for fluorescence in situ hybridization (FISH) analysis of genome structure. In this study, the bulked oligos of the two arms of chromosome seven of cotton were developed based on the genome sequence of Gossypium raimondii (DD, 2n = 2× = 26), and each arm contains 12,544 oligos. Chromosome seven was easily identified in both D genome and AD genome cotton species using the bulked chromosome-specific painting probes. Together with 45S ribosomal DNA (rDNA) probe, the chromosome-specific painting probe was also successfully used to correct the chromosomal localization of 45S rDNA in G. raimondii. The study reveals that bulked oligos specific to a chromosome is a useful tool for chromosome painting in cotton.