Formamide-Free Genomic in situ Hybridization Allows Unambiguous Discrimination of Highly Similar Parental Genomes in Diploid Hybrids and Allopolyploids

Formamide-Free Genomic In Situ Hybridization (ff-GISH)

Fluorescence in situ hybridization allows for the mapping of various sequence types in the genomes and is thus widely used in structural, functional, and evolutionary studies. One particular type of in situ hybridization that specifically allows to map whole parental genomes in diploid and polyploid hybrids is genomic in situ hybridization (GISH). The efficiency of GISH, i.e., the specificity of hybridization of genomic DNA probes to the parental subgenomes in hybrids depends, among others, on the age of the polyploids and the similarity of the parental genomes, specifically their repetitive DNA fractions. Typically, high levels of overall repeat similarity between the parental genomes result in lower efficiency of GISH. Here, we present the formamide-free GISH (ff-GISH) protocol that can be applied to diploid and polyploid hybrids of both monocots and dicots. ff-GISH allows higher efficiency of the labeling of the putative parental genomes compared to the standard GISH protocol and allows discrimination of parental chromosome sets that share up to 80-90% repeat similarity. This modified method is nontoxic, is simple, and lends itself to modifications. It can also be used for standard FISH and mapping of individual sequence types in chromosomes/genomes.

Comparative Analyses of Complete Chloroplast Genomes and Karyotypes of Allotetraploid Iris koreana and Its Putative Diploid Parental Species (Iris Series Chinenses, Iridaceae)

The Iris series Chinenses in Korea comprises four species (I. minutoaurea, I. odaesanensis, I. koreana, and I. rossii), and the group includes some endangered species, owing to their high ornamental, economic, and conservation values. Among them, the putative allotetraploid, Iris koreana (2n = 4x = 50), is hypothesized to have originated from the hybridization of the diploids I. minutoaurea (2n = 2x = 22) and I. odaesanensis (2n = 2x = 28) based on morphological characters, chromosome numbers, and genome size additivity. Despite extensive morphological and molecular phylogenetical studies on the genus Iris, little is known about Korean irises in terms of their complete chloroplast (cp) genomes and molecular cytogenetics that involve rDNA loci evolution based on fluorescence in situ hybridization (FISH). This study reports comparative analyses of the karyotypes of the three Iris species (I. koreana, I. odaesanensis, and I. minutoaurea), with an emphasis on the 5S and 35S rDNA loci number and localization using FISH together with the genome size and chromosome number. Moreover, the cp genomes of the same individuals were sequenced and assembled for comparative analysis. The rDNA loci numbers, which were localized consistently at the same position in all species, and the chromosome numbers and genome size values of tetraploid Iris koreana (four 5S and 35S loci; 2n = 50; 1C = 7.35 pg) were additively compared to its putative diploid progenitors, I. minutoaurea (two 5S and 35S loci; 2n = 22; 1C = 3.71 pg) and I. odaesanensis (two 5S and 35S loci; 2n = 28; 1C = 3.68 pg). The chloroplast genomes were 152,259–155,145 bp in length, and exhibited a conserved quadripartite structure. The Iris cp genomes were highly conserved and similar to other Iridaceae cp genomes. Nucleotide diversity analysis indicated that all three species had similar levels of genetic variation, but the cp genomes of I. koreana and I. minutoaurea were more similar to each other than to I. odaesanensis. Positive selection was inferred for psbK and ycf2 genes of the three Iris species. Phylogenetic analyses consistently recovered I. odaesanensis as a sister to a clade containing I. koreana and I. minutoaurea. Although the phylogenetic relationship, rDNA loci number, and localization, together with the genome size and chromosome number of the three species, allowed for the inference of I. minutoaurea as a putative maternal taxon and I. odaesanensis as a paternal taxon, further analyses involving species-specific molecular cytogenetic markers and genomic in situ hybridization are required to interpret the mechanisms involved in the origin of the chromosomal variation in Iris series Chinenses. This study contributes towards the genomic and chromosomal evolution of the genus Iris.

Anchoring alien chromosome segment substitutions bearing gene(s) for resistance to mustard aphid in Brassica juncea-B. fruticulosa introgression lines and their possible disruption through gamma irradiation

KEY MESSAGE

Heavy doses of gamma irradiation can reduce linkage drag by disrupting large sized alien translocations and promoting exchanges between crop and wild genomes. Resistance to mustard aphid (Lipaphis erysimi) infestation was significantly improved in Brassica juncea through B. juncea-B. fruticulosa introgression. However, linkage drag caused by introgressed chromatin fragments has so far prevented the deployment of this resistance source in commercial cultivars. We investigated the patterns of donor chromatin segment substitutions in the introgression lines (ILs) through genomic in situ hybridization (GISH) coupled with B. juncea chromosome-specific oligonucleotide probes. These allowed identification of large chromosome translocations from B. fruticulosa in the terminal regions of chromosomes A05, B02, B03 and B04 in three founder ILs (AD-64, 101 and 104). Only AD-101 carried an additional translocation at the sub-terminal to intercalary position in both homologues of chromosome A01. We validated these translocations with a reciprocal blast hit analysis using shotgun sequencing of three ILs and species-specific contigs/scaffolds (kb sized) from a de novo assembly of B. fruticulosa. Alien segment substitution on chromosome A05 could not be validated. Current studies also endeavoured to break linkage drag by exposing seeds to a heavy dose (200kR) of gamma radiation. Reduction in the size of introgressed chromatin fragments was observed in many M3 plants. There was a complete loss of the alien chromosome fragment in one instance. A few M3 plants with novel patterns of chromosome segment substitutions displayed improved agronomic performance coupled with resistance to mustard aphid. SNPs in such genomic spaces should aid the development of markers to track introgressed DNA and allow application in plant breeding.

A simple non-toxic ethylene carbonate fluorescence in situ hybridization (EC-FISH) for simultaneous detection of repetitive DNA sequences and fluorescent bands in plants

The major drawbacks of standard plant fluorescence in situ hybridization (FISH) designed for double-stranded DNA probes include requirement for experimentally determined heat denaturation of chromosomes at high temperatures and at least overnight hybridization. Consequently, processing with chromosomal preparations may easily result in heat-induced deterioration of chromosomal structural details, is time-consuming, and involves the use of toxic formamide and formaldehyde. Here, I have described a simple and appealing non-toxic procedure with ethylene carbonate (EC)-a formamide-substituting solvent and double-stranded repetitive DNA probes. Applying EC as a component of the hybridization solution at 46 °C not only allowed successful overnight hybridization but also gave a possibility to reduce the hybridization time to 3 h, hence converting the technique into a 1-day procedure. Importantly, the EC-FISH tended to preserve well chromosome structural details, e.g., DAPI-positive bands, thus facilitating simultaneous FISH mapping and chromosome banding on the same slide. The procedure requires no formaldehyde and RNA-se treatment of chromosomes, and no heat denaturation of chromosomal DNA. The key condition is to obtain high-quality cytoplasm-free preparations. The method was reproducible in all the plants studied (Allium, Nigella, Tradescantia, Vicia), giving a species-specific signal pattern together with clear DAPI bands on chromosomes. The procedure described here is expected to give a positive stimulus for improving gene-mapping approaches in plants.

Dating the Species Network: Allopolyploidy and Repetitive DNA Evolution in American Daisies (Melampodium sect. Melampodium, Asteraceae)

Allopolyploidy has played an important role in the evolution of the flowering plants. Genome mergers are often accompanied by significant and rapid alterations of genome size and structure via chromosomal rearrangements and altered dynamics of tandem and dispersed repetitive DNA families. Recent developments in sequencing technologies and bioinformatic methods allow for a comprehensive investigation of the repetitive component of plant genomes. Interpretation of evolutionary dynamics following allopolyploidization requires both the knowledge of parentage and the age of origin of an allopolyploid. Whereas parentage is typically inferred from cytogenetic and phylogenetic data, age inference is hampered by the reticulate nature of the phylogenetic relationships. Treating subgenomes of allopolyploids as if they belonged to different species (i.e., no recombination among subgenomes) and applying cross-bracing (i.e., putting a constraint on the age difference of nodes pertaining to the same event), we can infer the age of allopolyploids within the framework of the multispecies coalescent within BEAST2. Together with a comprehensive characterization of the repetitive DNA fraction using the RepeatExplorer pipeline, we apply the dating approach in a group of closely related allopolyploids and their progenitor species in the plant genus Melampodium (Asteraceae). We dated the origin of both the allotetraploid, Melampodium strigosum, and its two allohexaploid derivatives, Melampodium pringlei and Melampodium sericeum, which share both parentage and the direction of the cross, to the Pleistocene ($<$1.4 Ma). Thus, Pleistocene climatic fluctuations may have triggered formation of allopolyploids possibly in short intervals, contributing to difficulties in inferring the precise temporal order of allopolyploid species divergence of M. sericeum and M. pringlei. The relatively recent origin of the allopolyploids likely played a role in the near-absence of major changes in the repetitive fraction of the polyploids' genomes. The repetitive elements most affected by the postpolyploidization changes represented retrotransposons of the Ty1-copia lineage Maximus and, to a lesser extent, also Athila elements of Ty3-gypsy family.

Euchromatic Supernumerary Chromosomal Segments-Remnants of Ongoing Karyotype Restructuring in the Prospero autumnale Complex?

Supernumerary chromosomal segments (SCSs) represent additional chromosomal material that, unlike B chromosomes, is attached to the standard chromosome complement. The Prospero autumnale complex (Hyacinthaceae) is polymorphic for euchromatic large terminal SCSs located on the short arm of chromosome 1 in diploid cytotypes AA and B⁷B⁷, and tetraploid AAB⁷B⁷ and B⁶B⁶B⁷B⁷, in addition to on the short arm of chromosome 4 in polyploid B⁷B⁷B⁷B⁷ and B⁷B⁷B⁷B⁷B⁷B⁷ cytotypes. The genomic composition and evolutionary relationships among these SCSs have been assessed using fluorescence in situ hybridisation (FISH) with 5S and 35S ribosomal DNAs (rDNAs), satellite DNA PaB6, and a vertebrate-type telomeric repeat TTAGGG. Neither of the rDNA repeats were detected in SCSs, but most contained PaB6 and telomeric repeats, although these never spanned whole SCSs. Genomic in situ hybridisation (GISH) using A, B⁶, and B⁷ diploid genomic parental DNAs as probes revealed the consistently higher genomic affinity of SCSs in diploid hybrid B⁶B⁷ and allopolyploids AAB⁷B⁷ and B⁶B⁶B⁷B⁷ to genomic DNA of the B⁷ diploid cytotype. GISH results suggest a possible early origin of SCSs, especially that on chromosome 1, as by-products of the extensive genome restructuring within a putative ancestral P. autumnale B⁷ genome, predating the complex diversification at the diploid level and perhaps linked to B-chromosome evolution.

Multiple Origins and Nested Cycles of Hybridization Result in High Tetraploid Diversity in the Monocot Prospero

Polyploidy is a major driving force in angiosperm evolution, but our understanding of establishment and early diversification processes following allo- vs. auto-polyploidy is limited. An excellent system to address such questions is the monocot plant Prospero autumnale, as it comprises several genomically and chromosomally distinct diploid cytotypes and their auto- and allotetraploid derivatives. To infer origins and evolutionary trajectories of the tetraploids, we use genome size data, in situ hybridization with parental genomic DNAs and specific probes (satDNA, rDNAs), as well as molecular-phylogenetic analyses. Thus, we demonstrate that an astounding range of allotetraploid lineages has been formed recurrently by chromosomal re-patterning, interactions of chromosomally variable parental genomes and nested cycles of extensive hybridization, whereas autotetraploids have originated at least twice and are cytologically stable. During the recurrent formation and establishment across wide geographic areas hybridization in some populations could have inhibited lineage diversification and nascent speciation of such a hybrid swarm. However, cytotypes that became fixed in populations enhanced the potential for species diversification, possibly exploiting the extended allelic base, and fixed heterozygosity that polyploidy confers. The time required for polyploid cytotype fixation may in part reflect the lag phase reported for polyploids between their formation and species diversification.

A safer, urea-based in situ hybridization method improves detection of gene expression in diverse animal species

In situ hybridization is a widely employed technique allowing spatial visualization of gene expression in fixed specimens. It has greatly advanced our understanding of biological processes, including developmental regulation. In situ protocols are today routinely followed in numerous laboratories, and although details might change, they all include a hybridization step, where specific antisense RNA or DNA probes anneal to the target nucleic acid sequence. This step is generally carried out at high temperatures and in a denaturing solution, called hybridization buffer, commonly containing 50% (v/v) formamide - a hazardous chemical. When applied to the soft-bodied hydrozoan medusa Clytia hemisphaerica, we found that this traditional hybridization approach was not fully satisfactory, causing extensive deterioration of morphology and tissue texture which compromised our observation and interpretation of results. We thus tested alternative solutions for in situ detection of gene expression and, inspired by optimized protocols for Northern and Southern blot analysis, we substituted the 50% formamide with an equal volume of 8M urea solution in the hybridization buffer. Our new protocol not only yielded better morphologies and tissue consistency, but also notably improved the resolution of the signal, allowing more precise localization of gene expression and reducing aspecific staining associated with problematic areas. Given the improved results and reduced manipulation risks, we tested the urea protocol on other metazoans, two brachiopod species (Novocrania anomala and Terebratalia transversa) and the priapulid worm Priapulus caudatus, obtaining a similar reduction of aspecific probe binding. Overall, substitution of formamide by urea during in situ hybridization offers a safer alternative, potentially of widespread use in research, medical and teaching contexts. We encourage other workers to test this approach on their study organisms, and hope that they will also obtain better sample preservation, more precise expression patterns and fewer problems due to aspecific staining, as we report here for Clytia medusae and Novocrania and Terebratalia developing larvae.

Genome-wide repeat dynamics reflect phylogenetic distance in closely related allotetraploid Nicotiana (Solanaceae)

Nicotiana sect. Repandae is a group of four allotetraploid species originating from a single allopolyploidisation event approximately 5 million years ago. Previous phylogenetic analyses support the hypothesis of N. nudicaulis as sister to the other three species. This is concordant with changes in genome size, separating those with genome downsizing (N. nudicaulis) from those with genome upsizing (N. repanda, N. nesophila, N. stocktonii). However, a recent analysis reflecting genome dynamics of different transposable element families reconstructed greater similarity between N. nudicaulis and the Revillagigedo Island taxa (N. nesophila and N. stocktonii), thereby placing N. repanda as sister to the rest of the group. This could reflect a different phylogenetic hypothesis or the unique evolutionary history of these particular elements. Here we re-examine relationships in this group and investigate genome-wide patterns in repetitive DNA, utilising high-throughput sequencing and a genome skimming approach. Repetitive DNA clusters provide support for N. nudicaulis as sister to the rest of the section, with N. repanda sister to the two Revillagigedo Island species. Clade-specific patterns in the occurrence and abundance of particular repeats confirm the original (N. nudicaulis (N. repanda (N. nesophila + N. stocktonii))) hypothesis. Furthermore, overall repeat dynamics in the island species N. nesophila and N. stocktonii confirm their similarity to N. repanda and the distinctive patterns between these three species and N. nudicaulis. Together these results suggest that broad-scale repeat dynamics do in fact reflect evolutionary history and could be predicted based on phylogenetic distance.

rDNA Loci Evolution in the Genus Glechoma (Lamiaceae)

Glechoma L. (Lamiaceae) is distributed in eastern Asia and Europe. Understanding chromosome evolution in Glechoma has been strongly hampered by its small chromosomes, constant karyotype and polyploidy. Here phylogenetic patterns and chromosomal variation in Glechoma species are considered, using genome sizes, chromosome mapping of 5S and 35S rDNAs by fluorescence in situ hybridisation (FISH), and phylogenetic analyses of internal transcribed spacers (nrITS) of 35S rDNA and 5S rDNA NTS sequences. Species and populations of Glechoma are tetraploid (2n = 36) with base chromosome number of x = 9. Four chromosomes carry pericentric 5S rDNA sites in their short arms in all the species. Two to four of these chromosomes also carry 35S rDNA in subterminal regions of the same arms. Two to four other chromosomes have 35S rDNA sites, all located subterminally within short arms; one individual possessed additional weak pericentric 35S rDNA signals on three other chromosomes. Five types of rDNA locus distribution have been defined on the basis of 35S rDNA variation, but none is species-specific, and most species have more than one type. Glechoma hederacea has four types. Genome size in Glechoma ranges from 0.80 to 0.94 pg (1C), with low levels of intrapopulational variation in all species. Phylogenetic analyses of ITS and NTS sequences distinguish three main clades coinciding with geographical distribution: European (G. hederacea–G. hirsuta), Chinese and Korean (G. longituba), and Japanese (G. grandis). The paper presents the first comparative cytogenetic analyses of Glechoma species including karyotype structure, rDNA location and number, and genome size interpreted in a phylogenetic context. The observed variation suggests that the genus is still in genomic flux. Genome size, but not rDNA loci number and distribution, provides a character for species delimitation which allows better inferences of interspecific relationships to be made, in the absence of well-defined morphological differentiation.

Structural polymorphisms and distinct genomic composition suggest recurrent origin and ongoing evolution of B chromosomes in the Prospero autumnale complex (Hyacinthaceae)

Supernumerary B chromosomes (Bs) are genomic parasitic components, originating from the A complement via chromosomal rearrangements, which follow their own evolutionary trajectories. They often contain repetitive DNAs, some shared with regular chromosomes and some newly evolved. Genomic composition, origin and evolution of Bs have been analysed in the chromosomally variable Prospero autumnale complex. Two rDNAs and a satellite DNA (PaB6) from regular chromosomes were mapped to Bs of 26 plants from three diploid cytotypes, their hybrids and polyploid derivatives. In homoploid diploid hybrids, genomic in situ hybridization (GISH) allowed B painting with the parental DNAs. Bs were structurally variable and highly enriched in 5S rDNA and satDNA PaB6, and rarely in 35S rDNA. Eleven combinations of rDNA and PaB6 localization were observed. The quantities of PaB6 in Bs and regular chromosomes were not correlated, suggesting amplification mechanisms other than recombination. PaB6 and 5S rDNA amounts increased with increasing ploidy level. GISH revealed two independent origins of Bs. The structural variation, repeat content, repeat-type fluctuations and differing genomic affinities of Bs in different cytotypes suggest that they represent young proto-B chromosomes. Bs in P. autumnale probably form recurrently as by-products of the extensive genome restructuring within this chromosomally variable species complex.