Comparative and Phylogenetic Analysis Based on the Chloroplast Genome of Coleanthus subtilis (Tratt.) Seidel, a Protected Rare Species of Monotypic Genus

Coleanthus subtilis (Tratt.) Seidel (Poaceae) is an ephemeral grass from the monotypic genus Coleanthus Seidl, which grows on wet muddy areas such as fishponds or reservoirs. As a rare species with strict habitat requirements, it is protected at international and national levels. In this study, we sequenced its whole chloroplast genome for the first time using the next-generation sequencing (NGS) technology on the Illumina platform, and performed a comparative and phylogenetic analysis with the related species in Poaceae. The complete chloroplast genome of C. subtilis is 135,915 bp in length, with a quadripartite structure having two 21,529 bp inverted repeat regions (IRs) dividing the entire circular genome into a large single copy region (LSC) of 80,100 bp and a small single copy region (SSC) of 12,757 bp. The overall GC content is 38.3%, while the GC contents in LSC, SSC, and IR regions are 36.3%, 32.4%, and 43.9%, respectively. A total of 129 genes were annotated in the chloroplast genome, including 83 protein-coding genes, 38 tRNA genes, and 8 rRNA genes. The accD gene and the introns of both clpP and rpoC1 genes were missing. In addition, the ycf1, ycf2, ycf15, and ycf68 were pseudogenes. Although the chloroplast genome structure of C. subtilis was found to be conserved and stable in general, 26 SSRs and 13 highly variable loci were detected, these regions have the potential to be developed as important molecular markers for the subfamily Pooideae. Phylogenetic analysis with species in Poaceae indicated that Coleanthus and Phippsia were sister groups, and provided new insights into the relationship between Coleanthus, Zingeria, and Colpodium. This study presents the initial chloroplast genome report of C. subtilis, which provides an essential data reference for further research on its origin.

Interstitial Arabidopsis-Type Telomeric Repeats in Asteraceae

Tandem repeats of telomeric-like motifs at intra-chromosomal regions, known as interstitial telomeric repeats (ITR), have drawn attention as potential markers of structural changes, which might convey information about evolutionary relationships if preserved through time. Building on our previous work that reported outstanding ITR polymorphisms in the genus Anacyclus, we undertook a survey across 132 Asteraceae species, focusing on the six most speciose subfamilies and considering all the ITR data published to date. The goal was to assess whether the presence, site number, and chromosomal location of ITRs convey any phylogenetic signal. We conducted fluorescent in situ hybridization (FISH) using an Arabidopsis-type telomeric sequence as a probe on karyotypes obtained from mitotic chromosomes. FISH signals of ITR sites were detected in species of subfamilies Asteroideae, Carduoideae, Cichorioideae, Gymnarhenoideae, and Mutisioideae, but not in Barnadesioideae. Although six small subfamilies have not yet been sampled, altogether, our results suggest that the dynamics of ITR formation in Asteraceae cannot accurately trace the complex karyological evolution that occurred since the early diversification of this family. Thus, ITRs do not convey a reliable signal at deep or shallow phylogenetic levels and cannot help to delimitate taxonomic categories, a conclusion that might also hold for other important families such as Fabaceae.

Interstitial Telomeric-like Repeats (ITR) in Seed Plants as Assessed by Molecular Cytogenetic Techniques: A Review

The discovery of telomeric repeats in interstitial regions of plant chromosomes (ITRs) through molecular cytogenetic techniques was achieved several decades ago. However, the information is scattered and has not been critically evaluated from an evolutionary perspective. Based on the analysis of currently available data, it is shown that ITRs are widespread in major evolutionary lineages sampled. However, their presence has been detected in only 45.6% of the analysed families, 26.7% of the sampled genera, and in 23.8% of the studied species. The number of ITR sites greatly varies among congeneric species and higher taxonomic units, and range from one to 72 signals. ITR signals mostly occurs as homozygous loci in most species, however, odd numbers of ITR sites reflecting a hemizygous state have been reported in both gymnosperm and angiosperm groups. Overall, the presence of ITRs appears to be poor predictors of phylogenetic and taxonomic relatedness at most hierarchical levels. The presence of ITRs and the number of sites are not significantly associated to the number of chromosomes. The longitudinal distribution of ITR sites along the chromosome arms indicates that more than half of the ITR presences are between proximal and terminal locations (49.5%), followed by proximal (29.0%) and centromeric (21.5%) arm regions. Intraspecific variation concerning ITR site number, chromosomal locations, and the differential presence on homologous chromosome pairs has been reported in unrelated groups, even at the population level. This hypervariability and dynamism may have likely been overlooked in many lineages due to the very low sample sizes often used in cytogenetic studies.

Intragenomic Polymorphism of the ITS 1 Region of 35S rRNA Gene in the Group of Grasses with Two-Chromosome Species: Different Genome Composition in Closely Related Zingeria Species

Zingeria (Poaceae) is a small genus that includes Z. biebersteiniana, a diploid species with the lowest chromosome number known in plants (2n = 4) as well as hexaploid Z. kochii and tetraploid Z. pisidica, and/or Z. trichopoda species. The relationship between these species and the other low-chromosomes species Colpodium versicolor are unclear. To explore the intragenomic polymorphism and genome composition of these species we examined the sequences of the internal transcribed spacer 1 of the 35S rRNA gene via NGS approach. Our study revealed six groups of ribotypes in Zingeria species. Their distribution confirmed the allopolyploid nature of Z. kochii, whose probable ancestors were Colpodium versicolor and Z. pisidica. Z. pisidica has 98% of rDNA characteristic only for this species, and about 0.3% of rDNA related to that of Z. biebersteiniana. We assume that hexaploid Z. kochii is either an old allopolyploid or a homodiploid that has lost most of the rRNA genes obtained from Z. biebersteiniana. In Z. trichopoda about 81% of rDNA is related to rDNA of Z. biebersteiniana and 19% of rDNA is derived from Poa diaphora sensu lato. The composition of the ribotypes of the two plants determined by a taxonomy specialist as Z. pisidica and Z. trichopoda is very different. Two singleton species are proposed on this base with ribotypes as discriminative characters. So, in all four studied Zingeria species, even if the morphological difference among the studied species was modest, the genomic constitution was significantly different, which suggests that these are allopolyploids that obtained genomes from different ancestors.

Are diversification rates and chromosome evolution in the temperate grasses (Pooideae) associated with major environmental changes in the Oligocene-Miocene?

The Pooideae are a highly diverse C3 grass subfamily that includes some of the most economically important crops, nested within the highly speciose core-pooid clade. Here, we build and explore the phylogeny of the Pooideae within a temporal framework, assessing its patterns of diversification and its chromosomal evolutionary changes in the light of past environmental transformations. We sequenced five plastid DNA loci, two coding (ndhF, matk) and three non-coding (trnH-psbA, trnT-L and trnL-F), in 163 Poaceae taxa, including representatives for all subfamilies of the grasses and all but four ingroup Pooideae tribes. Parsimony and Bayesian phylogenetic analyses were conducted and divergence times were inferred in BEAST using a relaxed molecular clock. Diversification rates were assessed using the MEDUSA approach, and chromosome evolution was analyzed using the chromEvol software. Diversification of the Pooideae started in the Late-Eocene and was especially intense during the Oligocene-Miocene. The background diversification rate increased significantly at the time of the origin of the Poodae + Triticodae clade. This shift in diversification occurred in a context of falling temperatures that potentially increased ecological opportunities for grasses adapted to open areas around the world. The base haploid chromosome number n = 7 has remained stable throughout the phylogenetic history of the core pooids and we found no link between chromosome transitions and major diversification events in the Pooideae.

Karyotype structure and chromosome fragility in the grass Phleum echinatum Host

Phleum echinatum Host (2n = 2x = 10) is an annual Mediterranean species which differs from other representatives of the genus Phleum by reduced chromosome number, asymmetric karyotype and unusually high amount of DNA in the genome. Chromosomes of this plant were studied using conventional acetic-orcein staining and fluorescence in situ hybridization (FISH). FISH showed the major 35S ribosomal DNA (rDNA) site at the secondary constriction of satellite chromosome (3) and the minor 35S rDNA site near 5S rDNA cluster in the monobrachial chromosome 5. Telomeric repeats were detected at all chromosome ends within secondary constriction in satellited chromosome 3 and at the centromeric regions of chromosomes 1 and 2. Intrachromosomally located telomeric repeats are probably traces of chromosomal rearrangements that have shaped P.echinatum genome; they were prone to breakage which was manifested in chromosome fragmentation. The most distinct telomeric signals, suggesting massive amplification of interstitial telomeric sequences (ITRs), were observed at the nucleolar organizer region (NOR) of the third chromosome pair. Double FISH confirmed co-localization of telomeric and 35S rDNA repeats in this locus characterized by the biggest fragility in the karyotype. Fragile sites of P.echinatum, composed of amplified telomeric repeats, may bear a resemblance to metazoan rare fragile sites enriched in microsatellite repeats.

The intergenic spacer region of the rDNA in Haplopappus gracilis (Nutt.) Gray

In this paper, we provide further information on the genome organisation of Haplopappus gracilis, one of the six angiosperms showing the lowest chromosome number, i.e. 2n = 4, by determining the nucleotide sequence of the intergenic spacer region of the ribosomal RNA genes and its cytological localization on metaphase chromosomes. DNA sequence analysis reveals the occurring of a product of 4,382 bp in length, characterised by the presence of four blocks of different repeated sequences. Our analysis also evidenced putative promoter regions with three transcription initiation sites for polymerase I, as previously reported in Artemisia absinthium, belonging to the same Asteraceae family. A fluorescent in situ hybridization with the intergenic spacer probe indicates the presence of rDNA genes only in the satellited chromosomes of H. gracilis; besides, differences in the signal intensity between homologous chromosomes were frequently observed, thus suggesting for these chromosome sites the presence of a variable number of rDNA gene copies, even if a divergent chromatin organisation in corresponding regions cannot be ruled out.