Genome size, fluorochrome banding, and karyotype evolution in some Hypochoeris species

Karyotype characterization of two populations of Vernonia geminata (Asteraceae, Vernonieae) using banding and FISH techniques

In order to extend our knowledge concerning karyotypes of the genus Vernonia, we applied various techniques of chromosome banding, including AgNOR and triple staining with the fluorochromes CMA/DA/DAPI (CDD), and of fluorescent in situ hybridization (FISH) for the 45S rDNA probe to specimens of two populations of Vernonia geminata collected from an open-pasture area, in southern Brazil. B chromosomes were observed in one of the populations. Both populations of V. geminata presented a pair of CMA(3)(+) terminal bands and one pair of chromosomes with terminal AgNOR banding. The FISH evidenced, in one population, two pairs of small sites of 45S rDNA; these being two small terminal sites and two centromeric sites. In the other population, there was only one pair of small terminal sites and two sites in two B chromosomes, one in each chromosome. There was coincidence of localization between CMA(+) and NOR bands with one of the small terminal sites of 45S rDNA of one chromosome of the normal complement, but not in B chromosomes.

Ribosomal DNA, heterochromatin, and correlation with genome size in diploid and polyploid North American endemic sagebrushes (Artemisia, Asteraceae)

Subgenus Tridentatae ( Artemisia , Asteraceae) can be considered a polyploid complex. Both polyploidy and hybridization have been documented in the Tridentatae. Fluorescent in situ hybridization (FISH) and fluorochrome banding were used to detect and analyze ribosomal DNA changes linked to polyploidization in this group by studying four diploid-polyploid species pairs. In addition, genome sizes and heterochromatin patterns were compared between these populations. The linked 5S and 35S rRNA genes are confirmed as characteristic for Artemisia, and a pattern at the diploid level of three rDNA loci located at telomeric positions proved to be typical. Loss of rDNA loci was observed in some polyploids, whereas others showed additivity with respect to their diploid relatives. Genome downsizing was observed in all polyploids. Banding patterns differed depending on the pair of species analysed, but some polyploid populations showed an increased number of heterochromatic bands. FISH and fluorochrome banding were useful in determining the systematic position of Artemisia bigelovii , for which a differential pattern was found as compared with the rest of the group. Additionally, FISH was used to detect the presence of the Arabidopsis-type telomere repeat for the first time in Artemisia.

Karyotype diversification and evolution in diploid and polyploid South American Hypochaeris (Asteraceae) inferred from rDNA localization and genetic fingerprint data

BACKGROUND AND AIMS

Changes in chromosome structure and number play an important role in plant evolution. A system well-suited to studying different modes of chromosome evolution is the genus Hypochaeris (Asteraceae) with its centre of species' diversity in South America. All South American species uniformly have a chromosome base number of x = 4 combined with variation in rDNA number and distribution, and a high frequency of polyploidy. The aim of this paper is to assess directions and mechanisms of karyotype evolution in South American species by interpreting both newly obtained and previous data concerning rDNA localization in a phylogenetic context.

METHODS

Eleven Hypochaeris species from 18 populations were studied using fluorescence in situ hybridization (FISH) with 35S and 5S rDNA probes. A phylogenetic framework was established from neighbour-net analysis of amplified fragment length polymorphism (AFLP) fingerprint data.

KEY RESULTS

A single 5S rDNA locus is invariably found on the short arm of chromosome 2. Using 35S rDNA loci, based on number (one or two) and localization (interstitial on the long arm of chromosome 2, but sometimes lacking, and terminal or interstitial on the short arm of chromosome 3, only very rarely lacking), seven karyotype groups can be distinguished; five of these include polyploids. Karyotype groups with more than one species do not form monophyletic groups.

CONCLUSIONS

Early evolution of Hypochaeris in South America was characterized by considerable karyotype differentiation resulting from independent derivations from an ancestral karyotype. There was marked diversification with respect to the position and evolution of the 35S rDNA locus on chromosome 3, probably involving inversions and/or transpositions, and on chromosome 2 (rarely 3) concerning inactivation and loss. Among these different karyotype assemblages, the apargioides group and its derivatives constitute by far the majority of species.

Molecular cytogenetics of Xeranthemum L. and related genera (Asteraceae, Cardueae)

Seven representatives of the genera Amphoricarpus, Chardinia, Siebera, and Xeranthemum, all of them closely related as demonstraded by molecular phylogeny, have been studied from a cytogenetic perspective. Morphometrical karyotype parameters were calculated and idiograms obtained. Fluorochrome banding was performed with chromomycin A (3) to identify GC-rich regions in the chromosomes. Fluorescence in situ hybridization allowed us to locate the sites of 18S-5.8S-26S and 5S rDNA. Silver nitrate staining was used to count the number of nucleoli and to detect the active nucleolar organizing regions. Systematic and evolutionary issues are addressed in the light of these data.

Evolutionary DNA variation in the genus hypochaeris

The genome size and the base composition (GC%) of eight Hypochaeris species were determined by flow cytometry in order to establish the pattern of nuclear DNA variation within the genus. The species analysed showed an almost fivefold range of variation from 1.68 pg in H. cretensis to 8.10 pg in H. uniflora. This variation in DNA content is greater between taxonomic sections of Hypochaeris species than within a section. There was no correlation between 2C DNA content and GC% indicating that neither the GC fraction nor the AT fraction were preferentially associated with variation in genome size. Because there is little heterochromatin, these results show that it is interspersed repeated sequences that are most probably implicated in this variation. From phylogenetic analysis, it is likely that genome size has evolved by loss of DNA content in some lineages and by gain in one lineage from an ancestral genome which was probably similar to genomes of intermediate size in Hypochaeris.

Hypochaeris angustifolia (Asteraceae): Distribución, ecología y fitosociología

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The chromosomes of Diprion pini and D. similis (Hymenoptera: Diprionidae): implications for karyotype evolution

Earlier cytological investigations characterize the family Diprionidae with a modal chromosome number of seven. This study shows that Diprion pini and D. similis have 14 acrocentric chromosomes (n = 14 for haploid males and 2n = 28 for diploid females). In D. pini, rRNA genes are located on the satellite and the round short arm of the chromosome carrying this segment. We studied six populations with no evidence of chromosomal polymorphism at the species level. Our results disagree with those published previously and reopen the question of karyotype evolution. Chromosome morphology, ISH and C-banding results support the hypothesis of chromosome number doubling by centric fission and not by polyploidization, followed by the growth of short arms by means of pericentromeric DNA amplification.

Analysis of the nucleolar organizing regions in the ant Tapinoma nigerrimum (Hymenoptera, Formicidae)

This study analyses the NORs of Tapinoma nigerrimum, a species that, as known from previous studies, has various chromosomes which carry a NOR site. The analysis was made by a combination of three methods: silver nitrate staining, in situ hybridization with fluorescein-or digoxigenin-labelled probes, and staining with the CG-specific fluorochrome chromomycin A3. The silver staining technique showed an Ag-positive region on chromosome 6 and on various other chromosomes. However, the application of in situ hybridization techniques showed only one positive signal in the proximal region of the short arm of chromosome 6 of T. nigerrimum. Similar results were observed by CMA banding. The absence of rDNA genes or the presence of only a small number of these, not detectable with the above probes, might explain the absence of hybridization signal in the remaining chromosomes.

Cytogenetic studies in the genus Artemisia L. (Asteraceae): fluorochrome-banded karyotypes of five taxa, including the Iberian endemic species Artemisia barrelieri Besser

Fluorochrome-banded karyotypes of eight populations belonging to five taxa of the genus Artemisia from different European origins are presented. The most common basic number x = 9 is found in six populations of two diploid and two tetraploid species, whereas two populations of one diploid species have the less frequent basic number x = 8. The data on chromosome morphology and fluorochrome banding lead to some karyosystematic and evolutionary considerations, among others the postulation of descendent dysploidy to explain the occurrence of the two basic numbers in the genus. Key words: Asteraceae, Anthemideae, Artemisia, karyotypes, fluorochrome banding, cytotaxonomy, evolution.