Satellite DNA in Paphiopedilum subgenus Parvisepalum as revealed by high-throughput sequencing and fluorescent in situ hybridization

Phylogenomics and genome size evolution in Amomum s. s. (Zingiberaceae): Comparison of traditional and modern sequencing methods

BACKGROUND AND AIMS

A targeted enrichment NGS approach was used to construct the phylogeny of Amomum Roxb. (Zingiberaceae). Phylogenies based on hundreds of nuclear genes, the whole plastome and the rDNA cistron were compared with an ITS-based phylogeny. Trends in genome size (GS) evolution were examined, chromosomes were counted and the geographical distribution of phylogenetic lineages was evaluated.

METHODS

In total, 92 accessions of 54 species were analysed. ITS was obtained for 79 accessions, 37 accessions were processed with Hyb-Seq and sequences from 449 nuclear genes, the whole cpDNA, and the rDNA cistron were analysed using concatenation, coalescence and supertree approaches. The evolution of absolute GS was analysed in a phylogenetic and geographical context. The chromosome numbers of 12 accessions were counted.

KEY RESULTS

Four groups were recognised in all datasets though their mutual relationships differ among datasets. While group A (A. subulatum and A. petaloideum) is basal to the remaining groups in the nuclear gene phylogeny, in the cpDNA topology it is sister to group B (A. repoeense and related species) and, in the ITS topology, it is sister to group D (the Elettariopsis lineage). The former Elettariopsis makes a monophyletic group. There is an increasing trend in GS during evolution. The largest GS values were found in group D in two tetraploid taxa, A. cinnamomeum and A. aff. biphyllum (both 2n = 96 chromosomes). The rest varied in GS (2C = 3.54-8.78 pg) with a constant chromosome number 2n = 48. There is a weak connection between phylogeny, GS and geography in Amomum.

CONCLUSIONS

Amomum consists of four groups, and the former Elettariopsis is monophyletic. Species in this group have the largest GS. Two polyploids were found and GS greatly varied in the rest of Amomum.

Study and Physical Mapping of the Species-Specific Tandem Repeat CS-237 Linked with 45S Ribosomal DNA Intergenic Spacer in Cannabis sativa L

Hemp (Cannabis sativa L.) is a valuable crop and model plant for studying sex chromosomes. The scientific interest in the plant has led to its whole genome sequencing and the determination of its cytogenetic characteristics. A range of cytogenetic markers (subtelomeric repeat CS-1, 5S rDNA, and 45S rDNA) has been mapped onto hemp's chromosomes by fluorescent in situ hybridization (FISH). In this study, another cytogenetic marker (the tandem repeat CS-237, with a 237 bp monomer) was found, studied, and localized on chromosomes by FISH. The signal distribution and karyotyping revealed that the CS-237 probe was localized in chromosome 6 with one hybridization site and in chromosome 8 with two hybridization sites, one of which colocalizes with the 45S rDNA probe (with which a nucleolus organizer region, NOR, was detected). A BLAST analysis of the genomic data and PCR experiments showed that the modified CS-237 monomers (delCS-237, 208 bp in size) were present in the intergenic spacers (IGSs) of hemp 45S rDNA monomers. Such a feature was firstly observed in Cannabaceae species. However, IGS-linked DNA repeats were found in several plant species of other families (Fabaceae, Solanaceae, and Asteraceae). This phenomenon is discussed in this article. The example of CS-237 may be useful for further studying the phenomenon as well as for the physical mapping of hemp chromosomes.

Molecular and Cytogenetic Analysis of rDNA Evolution in Crepis Sensu Lato

Although Crepis was the first model plant group in which chromosomal changes were considered to play an important role in speciation, their chromosome structure and evolution have been barely investigated using molecular cytogenetic methods. The aim of the study was to provide a better understanding of the patterns and directions of Crepis chromosome evolution, using comparative analyses of rDNA loci number and localisation. The chromosome base number and chromosomal organisation of 5S and 35S rDNA loci were analysed in the phylogenetic background for 39 species of Crepis, which represent the evolutionary lineages of Crepis sensu stricto and Lagoseris, including Lapsana communis. The phylogenetic relationships among all the species were inferred from nrITS and newly obtained 5S rDNA NTS sequences. Despite high variations in rDNA loci chromosomal organisation, most species had a chromosome with both rDNA loci within the same (usually short) chromosomal arm. The comparative analyses revealed several independent rDNA loci number gains and loci repositioning that accompanied diversification and speciation in Crepis. Some of the changes in rDNA loci patterns were reconstructed for the same evolutionary lineages as descending dysploidy.

Satellite DNA probes of Alstroemeria longistaminea (Alstroemeriaceae) paint the heterochromatin and the B chromosome, reveal a G-like banding pattern, and point to a strong structural karyotype conservation

Alstroemeria species present a well-conserved and asymmetric karyotype. The genus is divided into a Chilean clade, rich in heterochromatin, and a Brazilian clade, poor in heterochromatin. We investigated the distribution of the main repetitive sequences in the chromosomes of the Brazilian species A. longistaminea (2n = 16 + 0-6B) aiming to evaluate the role played by these sequences on the structural organization of the karyotype. In situ hybridization of the three most abundant retrotransposons, corresponding to ~ 45% of the genome, was uniformly distributed. Three satellite DNA sequences, representing near half of the whole satellite fraction (1.93% of the genome), were mainly concentrated on the heterochromatin and one of them painted the whole B chromosome. Noteworthy, some satellites were located on euchromatin, either dispersed or concentrated in clusters along the chromosomes, revealing a G-band-like pattern. The two satellites that presented more C-band- and G-band-like labeling were also hybridized in situ in two other Alstroemeria species. They revealed astonishing similar patterns of distribution, indicating an unusually structural karyotype conservation among Brazilian species.

Genome Size Doubling Arises From the Differential Repetitive DNA Dynamics in the Genus Heloniopsis (Melanthiaceae)

Plant genomes are highly diverse in size and repetitive DNA composition. In the absence of polyploidy, the dynamics of repetitive elements, which make up the bulk of the genome in many species, are the main drivers underpinning changes in genome size and the overall evolution of the genomic landscape. The advent of high-throughput sequencing technologies has enabled investigation of genome evolutionary dynamics beyond model plants to provide exciting new insights in species across the biodiversity of life. Here we analyze the evolution of repetitive DNA in two closely related species of Heloniopsis (Melanthiaceae), which despite having the same chromosome number differ nearly twofold in genome size [i.e., H. umbellata (1C = 4,680 Mb), and H. koreana (1C = 2,480 Mb)]. Low-coverage genome skimming and the RepeatExplorer2 pipeline were used to identify the main repeat families responsible for the significant differences in genome sizes. Patterns of repeat evolution were found to correlate with genome size with the main classes of transposable elements identified being twice as abundant in the larger genome of H. umbellata compared with H. koreana. In addition, among the satellite DNA families recovered, a single shared satellite (HeloSAT) was shown to have contributed significantly to the genome expansion of H. umbellata. Evolutionary changes in repetitive DNA composition and genome size indicate that the differences in genome size between these species have been underpinned by the activity of several distinct repeat lineages.

The Genomics of Plant Satellite DNA

The twenty-first century began with a certain indifference to the research of satellite DNA (satDNA). Neither genome sequencing projects were able to accurately encompass the study of satDNA nor classic methodologies were able to go further in undertaking a better comprehensive study of the whole set of satDNA sequences of a genome. Nonetheless, knowledge of satDNA has progressively advanced during this century with the advent of new analytical techniques. The enormous advantages that genome-wide approaches have brought to its analysis have now stimulated a renewed interest in the study of satDNA. At this point, we can look back and try to assess more accurately many of the key questions that were left unsolved in the past about this enigmatic and important component of the genome. I review here the understanding gathered on plant satDNAs over the last few decades with an eye on the near future.

Large-scale comparative analysis of cytogenetic markers across Lepidoptera

Fluorescence in situ hybridization (FISH) allows identification of particular chromosomes and their rearrangements. Using FISH with signal enhancement via antibody amplification and enzymatically catalysed reporter deposition, we evaluated applicability of universal cytogenetic markers, namely 18S and 5S rDNA genes, U1 and U2 snRNA genes, and histone H3 genes, in the study of the karyotype evolution in moths and butterflies. Major rDNA underwent rather erratic evolution, which does not always reflect chromosomal changes. In contrast, the hybridization pattern of histone H3 genes was well conserved, reflecting the stable organisation of lepidopteran genomes. Unlike 5S rDNA and U1 and U2 snRNA genes which we failed to detect, except for 5S rDNA in a few representatives of early diverging lepidopteran lineages. To explain the negative FISH results, we used quantitative PCR and Southern hybridization to estimate the copy number and organization of the studied genes in selected species. The results suggested that their detection was hampered by long spacers between the genes and/or their scattered distribution. Our results question homology of 5S rDNA and U1 and U2 snRNA loci in comparative studies. We recommend the use of histone H3 in studies of karyotype evolution.

Identification and characterization of satellite DNAs in Poa L

BACKGROUND

Poa L. is a large genus of grass in Gramineae, among which P. pratensis is widely cultivated as turf and forage. Satellite DNA is the main components of the plant genome. Information of satellites will helpful for dissection the genome composition and definition of the phylogeny relationship of these species. However, the knowledge about the satellites in genus Poa is still limited.

RESULTS

Four satellite DNAs were identified using the Repeat Explorer pipeline in HiSeq Illumina reads from diploid plants in P. malaca (2n = 26). Two satellites showed high similarity with the previously identified PpTr-1 and PpTr-3, whereas two others are newly identified with the monomer of 326 bp (Poa-326) and 353 bp (Poa-353) respectively. The clone DNAs of PpTr-1 and PpTr-3, and oligonucleotides designed representing satellites Poa-326 and Poa-353 were probed to test on chromosomes across 13 Poa speceis with different polyploidy level by fluorescent in situ hybridization (FISH). PpTr-1, PpTr-3, and Poa-362 were stably positioned in the subtelomeric regions in nearly all species with the variation of hybridization sites number. However, Poa-353 showed different FISH patterns of multiple regions with the variation of hybridization intensity and distribution sites across species. In addition, 5S rDNA and 45S rDNA were used to characterize the genome of the Poa species. Four rDNA FISH patterns were revealed in the tested species.

CONCLUSION

Four identified satellite were high conservable across Poa species. Genome distribution of these satellites can be characterized by FISH. The variation of satellite DNAs and rDNA chromosomal distributions between species provide useful information for phylogenetic analysis in genus Poa.

How diverse is heterochromatin in the Caesalpinia group? Cytogenomic characterization of Erythrostemon hughesii Gagnon & G.P. Lewis (Leguminosae: Caesalpinioideae)

Cytogenomic characterization of Erythrostemon hughesii reveals a heterogeneity of repeats in its subtelomeric heterochromatin. Comparative analyses with other Caesalpinia group species reveal a significant reduction in the abundance of Ty3-gypsy/Chromovirus Tekay retrotransposons during its evolution. In numerically stable karyotypes, repetitive DNA variability is one of the main causes of genome and chromosome variation and evolution. Species from the Caesalpinia group (Leguminosae) are karyotypically characterized by 2n = 24, with small chromosomes and highly variable CMA⁺ heterochromatin banding patterns that correlate with environmental variables. Erythrostemon hughesii differs from other species of the group examined to date for having subtelomeric CMA⁺ bands; this contrasts with most species in the group which have proximal bands. Here we analyse the repeatome of E. hughesii using genome skimming and chromosomal mapping approaches to characterize the identity of the most abundant repetitive elements and their physical location. The repetitive fraction of E. hughesii comprises 28.73% of the genome. The most abundant elements were retrotransposons (RT) with long terminal repeats (LTR-RT; 9.76%) and satellite DNAs (7.83%). Within the LTR-RTs, the most abundant lineages were: Ty1/copia-Ale (1%), Ty3/gypsy CRM (0.88%) and Ty3/gypsy Athila (0.75%). Using fluorescent in situ hybridization four satellite DNAs and several LTR-RT elements were shown to be present in most subtelomeric CMA⁺ bands. These results highlight how the repeatome in E. hughesii, a species from Oaxaca state in Mexico, is clearly distinct from Northeast Brazilian species of the Caesalpinia group, mainly due to its high diversity of repeats in its subtelomeric heterochromatic bands and low amount of LTR-RT Ty3/gypsy-Tekay elements. Comparative sequence analysis of Tekay elements from different species is congruent with a clade-specific origin of this LTR-RT after the divergence of the Caesalpinia group. We hypothesize that repeat-rich heterochromatin may play a role in leading to faster genomic divergence between individuals, increasing speciation and diversification.

Asymmetrical canina meiosis is accompanied by the expansion of a pericentromeric satellite in non-recombining univalent chromosomes in the genus Rosa

BACKGROUND AND AIMS

Despite their abundant odd-ploidy (2n = 5x = 35), dogroses (Rosa sect. Caninae) are capable of sexual reproduction due to their unique meiosis. During canina meiosis, two sets of chromosomes form bivalents and are transmitted by male and female gametes, whereas the remaining chromosomes form univalents and are exclusively transmitted by the egg cells. Thus, the evolution of chromosomes is expected to be driven by their behaviour during meiosis.

METHODS

To gain insight into differential chromosome evolution, fluorescence in situ hybridization was conducted for mitotic and meiotic chromosomes in four dogroses (two subsections) using satellite and ribosomal DNA probes. By exploiting high-throughput sequencing data, we determined the abundance and diversity of the satellite repeats in the genus Rosa by analysing 20 pentaploid, tetraploid and diploid species in total.

KEY RESULTS

A pericentromeric satellite repeat, CANR4, was found in all members of the genus Rosa, including the basal subgenera Hulthemia and Hesperhodos. The satellite was distributed across multiple chromosomes (5-20 sites per mitotic cell), and its genomic abundance was higher in pentaploid dogroses (2.3 %) than in non-dogrose species (1.3 %). In dogrose meiosis, univalent chromosomes were markedly enriched in CANR4 repeats based on both the number and the intensity of the signals compared to bivalent-forming chromosomes. Single-nucleotide polymorphisms and cluster analysis revealed high intragenomic homogeneity of the satellite in dogrose genomes.

CONCLUSIONS

The CANR4 satellite arose early in the evolution of the genus Rosa. Its high content and extraordinary homogeneity in dogrose genomes is explained by its recent amplification in non-recombining chromosomes. We hypothesize that satellite DNA expansion may contribute to the divergence of univalent chromosomes in Rosa species with non-symmetrical meiosis.

Evolutionary convergence or homology? Comparative cytogenomics of Caesalpinia group species (Leguminosae) reveals diversification in the pericentromeric heterochromatic composition

We demonstrated by cytogenomic analysis that the proximal heterochromatin of the Northeast Brazilian species of Caesalpinia group is enriched with phylogenetically conserved Ty3/Gypsy-Tekay RT, but diverge in the presence of Ty3/Gypsy-Athila RT and satDNA. The Caesalpinia Group includes 225 species and 27 monophyletic genera of which four occur in Northeastern Brazil: Erythrostemon (1 sp.), Cenostigma (7 spp.), Libidibia (1 sp.), and Paubrasilia (1 sp.). The last three genera are placed in different clades in the Caesalpinia Group phylogeny, and yet they are characterized by having a numerically stable karyotype 2n = 24 (16 M+8A) and GC-rich heterochromatic bands (chromomycin A₃ positive/CMA⁺ bands) in the proximal chromosome regions. To characterize the composition of their heterochromatin and test for the homology of these chromosomal regions, genomic DNA was extracted from Cenostigma microphyllum, Libidibia ferrea, and Paubrasilia echinata, and sequenced at low coverage using the Illumina platform. The genomic repetitive fractions were characterized using a Galaxy/RepeatExplorer-Elixir platform. The most abundant elements of each genome were chromosomally located by fluorescent in situ hybridization (FISH) and compared to the CMA⁺ heterochromatin distribution. The repetitive fraction of the genomes of C. microphyllum, L. ferrea, and P. echinata were estimated to be 41.70%, 38.44%, and 72.51%, respectively. Ty3/Gypsy retrotransposons (RT), specifically the Tekay lineage, were the most abundant repeats in each of the three genomes. FISH mapping revealed species-specific patterns for the Tekay elements in the proximal regions of the chromosomes, co-localized with CMA⁺ bands. Other species-specific patterns were observed, e.g., for the Ty3/Gypsy RT Athila elements which were found in all the proximal heterochromatin of L. ferrea or restricted to the acrocentric chromosomes of C. microphyllum. This Athila labeling co-localized with satellite DNAs (satDNAs). Although the Caesalpinia Group diverged around 55 Mya, our results suggest an ancestral colonization of Tekay RT in the proximal heterochromatin. Thus, the present-day composition of the pericentromeric heterochromatin in these Northeast Brazilian species is a combination of the maintenance of an ancestral Tekay distribution with a species-specific accumulation of other repeats.