5S rDNA Sites Research Articles

ZZ/ZW sex chromosome system in the endangered fish Lignobrycon myersi Miranda-Ribeiro, 1956 (Teleostei, Characiformes, Triportheidae).

Lignobrycon myersi is an endemic fish species from a few coastal rivers in northeastern Brazil. Based on molecular evidence, Lignobrycon myersi and genera Triportheus Cope, 1872, Agoniates Müller & Troschel, 1845, Clupeacharax Pearson, 1924 and Engraulisoma Castro, 1981 were placed in the family Triportheidae. In the present work, we report the first cytogenetic data for Lignobrycon myersi to test the hypothesis that Lignobrycon and Triportheus are closely related. Studied specimens presented 2n=52 with 28 metacentric (m), 18 submetacentric (sm) and six subtelocentric (st) chromosomes for males and 27 m, 19 sm and 6 st for females, characterizing a ZZ/ZW sex chromosome system. The Z chromosome corresponds to the largest chromosome in karyotype while the W is about 50% smaller than the Z and largely heterochromatic. Terminal nucleolus organizer regions, GC-rich sites and 18S rDNA signals were detected on pair 14. However, additional 18S rDNA sites were observed in the W chromosome. The 5S rDNA was mainly detected on long arms of pair 7. The apparent synapomorphic chromosomal traits of Triportheus and Lignobrycon myersi reinforce their close phylogenetic relationship, suggesting that the ZZ/ZW chromosome system in both genera has arisen before cladogenic events.

Fragile sites of 45S rDNA of Lolium multiflorum are not hotspots for chromosomal breakages induced by X-ray.

Sites of 45S rDNA of Lolium are regions denominated fragile sites (FSs), constituting regions slightly stained with DAPI due to increased DNA unpacking in metaphasic chromosomes. Considered to be fragile regions in the genome, the FSs might be more responsive to induced breaks and result in chromosomal fragments and rearrangements, unless repairing mechanisms such as recombination or de novo telomere formation play a role at the break site of the DNA. Thus, this study aimed at investigating if SFs from Lolium are hotspots for the occurrence of breakages induced by X-ray and if they are regions favorable to synthesize new telomeres, using Hordeum vulgare as a comparative model. Lolium multiflorum and H. vulgare seedlings were irradiated with 20 and 50Gy X-ray and evaluated oneday following the irradiation and at 7-days intervals for a 28-days period, using FISH technique with 45S rDNA and Arabidopsis-type telomere probes in order to investigate the presence of chromosomal breakages and new telomere formation. H. vulgare did not survive after a few days of irradiation due to the increased rate of abnormalities. L. multiflorum also exhibited chromosomal abnormalities following the exposure, yet over the 28-days trial it had a decrease in the chromosomal damage rate and formation of de novo telomere has not been detected along this time. Despite being considered to be fragile regions in the genome, the 45S rDNA sites of Lolium are not hotspots to chromosomal breakages after the induction of breakages.

Comparative chromosomal localization of 45S and 5S rDNAs and implications for genome evolution in Cucumis.

Ribosomal DNAs are useful cytogenetic markers for chromosome analysis. Studies investigating site numbers and distributions of rDNAs have provided important information for elucidating genome organization and chromosomal relationships of many species by fluorescence in situ hybridization. But relevant studies are scarce for species of the genus Cucumis, especially in wild species. In the present study, FISH was conducted to investigate the organization of 45S and 5S rDNA among 20 Cucumis accessions, including cultivars and wild accessions. Our results showed that the number of 45S rDNA sites varied from one to five pairs in different accessions, and most of these sites are located at the terminal regions of chromosomes. Interestingly, up to five pairs of 45S rDNA sites were observed in C. sativus var. sativus, the species which has the lowest chromosome number, i.e., 2n = 14. Only one pair of 5S rDNA sites was detected in all accessions, except for C. heptadactylus, C. sp, and C. spp that had two pairs of 5S rDNA sites. The distributions of 5S rDNA sites showed more variation than 45S rDNA sites. The phylogenetic analysis in this study showed that 45S and 5S rDNA have contrasting evolutionary patterns. We find that 5S rDNA has a polyploidization-related tendency towards the terminal location from an interstitial location but maintains a conserved site number, whereas the 45S rDNA showed a trend of increasing site number but a relatively conserved location.

Comparative Cytotaxonomy of Two Species of Fish from the Genus Satanoperca Reveals the Presence of a B Chromosome.

The taxonomy of Satanoperca spp. is still unresolved, especially because coloring, one of the main diagnostic characters, is variable among species of this genus. Thus, the aim of this study was to elucidate the relationship between the genome and the organization of the chromosome in two Satanoperca species. Our main goal was to develop a method to better differentiate taxa and understand the evolution of Satanoperca jurupari and Satanoperca lilith karyotypes, which we analyzed with classical and molecular cytogenetics. Both species have the same diploid number (2n) of 48 and location of 5S rDNA sites on pair 5. Nonetheless, the distribution of heterochromatin and 18S rDNA sites followed a species-specific pattern. The interstitial telomeric sites were not highlighted in either species. Regardless, a single B chromosome was identified in some metaphases of S. lilith. These data show that Satanoperca species harbor chromosomal features that can be used to identify the two species of Satanoperca studied here, allowing for the use of cytogenetic markers to make taxonomic inferences within the genus.

Cytogenetic characterization of the Passiflora edulis Sims × Passiflora cincinnata Mast. interspecific hybrid and its parents

The genus Passiflora L. consists of approximately 530 widely distributed species, including Passiflora edulis, which has drawn interest because of its commercial and agronomic value. Passiflora cincinnata is another important species owing to its long flowering period and resistance or tolerance to diseases and pests. In the present study, the meiotic segregation and pollen viability of an interspecific hybrid (P. edulis × P. cincinnata) and its parents were analyzed. The genomic contents were characterized using chromomycin A3 (CMA3)/4′-6′-diamidino-2-phenylindole (DAPI) staining, fluorescent in situ hybridization with 5S/45S ribosomal DNA (rDNA), genomic in situ hybridization (GISH), and inter-simple sequence repeat (ISSR) markers. The results indicated the diploid chromosome number for the parents and interspecific hybrid was 2n = 18. We also observed regular meiosis, one pair of 5S rDNA sites, and two pairs of 45S rDNA sites that co-localized with two pairs of CMA3 +/DAPI− bands. The GISH data revealed three distinct chromosomal groups in the hybrid. The genetic origins of the interspecific hybrid, and its relationship with its parents were also confirmed using ISSR markers.

Genomic Organization Under Different Environmental Conditions: Hoplosternum Littorale as a Model.

The Amazon has abundant rivers, streams, and floodplains in both polluted and nonpolluted environments, which show great adaptability. Thus, the goal of this study was to map repetitive DNA sequences in both mitotic chromosomes and erythrocyte micronuclei of tamoatás from polluted and nonpolluted environments and to assess the possible genotoxic effects of these environments. Individuals were collected in Manaus, Amazonas (AM), and submitted to classical and molecular cytogenetic techniques, as well as to a blood micronucleus test. Diploid number equal to 60 chromosomes are present in all individuals, with 18S ribosomal DNA sites present in one chromosome pair and no interstitial telomeric sites on chromosomes. The micronucleus test showed no significant differences in pairwise comparisons between environments or collection sites, but the Rex3 retroelement was dispersed on the chromosomes of individuals from unpolluted environments and compartmentalized in individuals from polluted environments. Divergent numbers of 5S rDNA sites are present in individuals from unpolluted and polluted environments. The mapping of repetitive sequences revealed that micronuclei have different compositions both intra- and interindividually that suggests different regions are lost in the formation of micronuclei, and no single fragile region undergoes breaks, although repetitive DNA elements are involved in this process.

Nuclear rDNA instability inin vitro-generated plants is amplified after sexual reproduction with conspecific wild individuals

Using micropropagation through tissue culture has become the most used approach worldwide for mass production for the conservation of endangered species. However, the screening of somaclonal variations generated using in vitro culture is usually restricted to the first generation of micropropagated plants, when they have not yet been released in the field. Accordingly, the fate of genetically modified regenerants after sexual reproduction is usually not assessed and changes in the genetic structures of species are unknown. In this work, we assess the cytogenetic stability of two rDNA gene families in the offspring of experimental crosses between accessions generated after in vitro culture and wild individuals of Cistus heterophyllus (Cistaceae). The cytogenetic rDNA profiles (45S rDNA, 5S rDNA) of 118 accessions including wild and in vitro micropropagated individuals and bi-directional artificial crosses between wild and in vitro-generated plants were assessed by fluorescence in situ hybridization (FISH) and Ag-NOR staining. Plants regenerated by micropropagation showed a lower size of the FISH signals in a 45S rDNA site, but this condition was not present in the wild accessions. Three new cytogenetic and cytological variants were present in 36% of the experimental progeny, involving the amplification of one additional 45S rDNA site and the presence of heteromorphic nucleoli. rDNA-based genomic instability was present after sexual reproduction between wild and in vitro-generated plants. The results of this study discourage the use of micropropagated materials for plant conservation unless comprehensive surveys of the genetic integrity and stability of the regenerants are performed after crossing between wild and micropropagated plants.

Chromosomal features ofFosterellaspecies (Bromeliaceae, Pitcairnioideae)

Fosterella (Bromeliaceae) comprises 31 species with rosulate leaves and mostly small, whitish flowers. Previous karyological studies were restricted to chromosome counts. In the present study, chromosomal variation in Fosterella was analysed using CMA3/DAPI staining and/or fluorescence in situ hybridization (FISH) using 45S and 5S rDNA probes, generating data for nine taxa with either 2n = 50 or 100 chromosomes. A single chromosome pair containing one CMA+/DAPI− band was identified in all diploid species. In the tetraploid Fosterella hatschbachii one pair had a CMA+/DAPI− band, whereas the other tetraploid studied, Fosterella yuvinkae, had two pairs with proximal bands. The presence of two CMA+/DAPI− pairs in F. yuvinkae may indicate a recent polyploidization event. This paper also reports the application of FISH in Bromeliaceae. FISH using 45S rDNA as the probe revealed one pair of terminal sites in most species and a co-localization with CMA+/DAPI− bands in all analysed species. The 5S rDNA sites were terminal in the tetraploid F. hatschbachii and proximal in all other species studied. Our data indicate that Fosterella species have little heterochromatin and it is largely restricted to the vicinity of the nucleolus organizer region. The data also indicate that hybridization (sometimes associated with polyploidy) has probably played an important role in the evolution of Fosterella.

Repetitive DNAs highlight the role of chromosomal fusions in the karyotype evolution of Dascyllus species (Pomacentridae, Perciformes).

The Dascyllus genus consists of 11 species spread over vast regions of the Indo-Pacific, showing remarkable reductions in the diploid chromosome numbers (2n). The present study analyzed the karyotypes and other chromosomal characteristics of D. trimaculatus (2n = 48; 2st + 46a; NF = 50), D. carneus (2n = 48; 2st + 46a; NF = 50) and D. aruanus (2n = 30; 18m + 2st + 10a; NF = 50) from the Thailand Gulf (Pacific Ocean) and D. melanurus (2n = 48; 2st + 46a; NF = 50) from the Andaman Sea (Indian Ocean), employing conventional cytogenetic analyses and the chromosomal mapping of repetitive DNAs, using 18S and 5S rDNA, telomeric sequences and (CA)15, (GA)15, and (CAA)10 microsatellites as probes. The C-positive heterochromatin was found in the centromeric regions of most chromosomal pairs and 18S rDNA phenotypes were single in all species. However, in D. aruanus (2n = 30), which harbors nine metacentric pairs; the 5S rDNA sites were located in the centromeric region of the shortest one. The mapping of the telomeric sequences in D. aruanus revealed the presence of interstitial telomeric sites (ITS) in the centromeric region of four metacentric pairs, with one of these pairs also displaying an additional ITS in the long arms. Distinct chromosomal markers confirmed the reduction of the 2n by chromosomal fusions, highlighting the precise characterization of these rearrangements by the cytogenetic mapping of the repetitive DNAs.

Breaks in the 45S rDNA Lead to Recombination-Mediated Loss of Repeats.

rDNA repeats constitute the most heavily transcribed region in the human genome. Tumors frequently display elevated levels of recombination in rDNA, indicating that the repeats are a liability to the genomic integrity of a cell. However, little is known about how cells deal with DNA double-stranded breaks in rDNA. Using selective endonucleases, we show that human cells are highly sensitive to breaks in 45S but not the 5S rDNA repeats. We find that homologous recombination inhibits repair of breaks in 45S rDNA, and this results in repeat loss. We identify the structural maintenance of chromosomes protein 5 (SMC5) as contributing to recombination-mediated repair of rDNA breaks. Together, our data demonstrate that SMC5-mediated recombination can lead to error-prone repair of 45S rDNA repeats, resulting in their loss and thereby reducing cellular viability.

Interstitial telomeric sites and Robertsonian translocations in species of Ipheion and Nothoscordum (Amaryllidaceae).

The genera Nothoscordum and Ipheion (Allioideae, Amaryllidaceae) are cytologically characterized by a dysploid series with variable numbers of metacentric and acrocentric chromosomes typical of karyotypes rearranged by Robertsonian translocations (RT). Since they have large chromosomes, low diploid numbers, and possess two telomeric motifs [the vertebrate-type (TTAGGG) n and the Arabidopsis-type (TTTAGGG) n ] they are suitable for investigating the occurrence and possible role of interstitial telomeric sites (ITS) associated with RT. We analyzed the distributions of telomeric sites in 12 species of Nothoscordum and Ipheion and found that both telomeric probes colocalized in all chromosome termini. Cloning and sequencing PCR products obtained using both telomeric primers simultaneously revealed long stretches of (TTAGGG) n and (TTTAGGG) n sequences together with degenerated telomeric sequences. Most acrocentric chromosomes have a 45S rDNA site at the terminal region of the short arms adjacent to the most distal telomeric sites. Telomeric signals were found at all chromosome ends, but ITS were also detected in a few proximal and subterminal regions in some Nothoscordum species. Although RT are common in this group of plants, our findings suggest that proximal positioning of telomeric motifs are not necessarily related to that kind of rearrangement. Rather, transposition of telomeric sequences followed by amplification, could better explain the presence of (TTAGGG) n and (TTTAGGG) n repeats at those sites. Furthermore, a few small interstitial sites found in some Nothoscordum species indicate that dispersion of these sequences was not restricted to the proximal region.

CMA band variability and physical mapping of 5S and 45S rDNA sites in Brazilian Cactaceae: Pereskioideae and Opuntioideae

Representatives of the Cactaceae subfamilies Pereskioideae and Opuntioideae from northeastern Brazil were studied using banding with the fluorochromes, CMA3 and DAPI, as well as with fluorescent in situ hybridization using 45S and 5S rDNA probes to identify the distributions of their heterochromatin and rDNA sites. Pereskia aculeata, P. bahiensis, P. grandifolia (Pereskioideae), Brasilopuntia brasiliensis, Tacinga funalis, and T. palmadora showed 2n = 22, while Opuntia dillenii showed 2n = 44, and O. ficus-indica 2n = 88. The karyotypes of all of the species were symmetric, with average chromosome lengths varying from 1.94 µm in O. dillenii to 3.17 µm in P. aculeata. One pair of terminal CMA+ bands corresponding to NORs occurred in all of the diploid cytotypes (except O. ficus-indica, which has two pairs of terminal CMA+ bands) as well as in O. dillenii (tetraploid). CMA+ bands were also observed in the interstitial region of the long arm of a chromosome pair in B. brasiliensis, while a number of variable proximal bands were observed on three chromosome pairs in O. dillenii and on most of the chromosomes of O. ficus-indica. The 45S rDNA sites corresponded to the terminal CMA+ bands, while the 5S rDNA sites were located in the interstitial regions of the long arms of the chromosome pairs of P. aculeata, P. bahiensis, P. grandifolia, and B. brasiliensis. Our data, and earlier publications, suggest that the subfamily Opuntioideae can be characterized as having proximal/interstitial CMA+ heterochromatin in at least one chromosome pair (except in Tacinga). The absence of proximal heterochromatic bands, however, appears to be a synapomorphy of the basal lineages of Cactaceae (subfamily Pereskioideae + Maihuenioideae), suggesting that karyotypes with heterochromatin restricted to the terminal region of a chromosome pair (45S rDNA) represent a plesiomorphic character of the family.

The Evolutionary Dynamics of Ribosomal Genes, Histone H3, and Transposable Rex Elements in the Genome of Atlantic Snappers.

Lutjanidae is a family of primarily marine and carnivorous fishes distributed in the Atlantic, Indian, and Pacific oceans, with enormous economic and ecological importance. In order to better clarify the conservative chromosomal evolution of Lutjanidae, we analyzed the evolutionary dynamics of 5 repetitive DNA classes in 5 Lutjanus and in 1 Ocyurus species from the Western Atlantic. The ribosomal 18S sites were generally located in a single chromosome pair, except for L. jocu and L. alexandrei where they are found in 2 pairs. In turn, the 5S rDNA sites are unique, terminal and nonsyntenic with the 18S rDNA sites. In 3 species analyzed, H3 hisDNA genes were found in 1 chromosomal pair. However, while L. jocu presented 2 H3 sites, O. chrysurus showed a noteworthy dispersion of this gene in almost all chromosomes of the karyotype. Retrotransposons Rex1 and Rex3 do not exhibit any association with the explosive distribution of H3 sequences in O. chrysurus. The low compartmentalization of Rex elements, in addition to the general nondynamic distribution of ribosomal and H3 genes, corroborate the karyotype conservatism in Lutjanidae species, also at the microstructural level. However, some "disturbing evolutionary waves" can break down this conservative scenario, as evidenced by the massive random dispersion of H3 hisDNA in the genome of O. chrysurus. The implication of the genomic expansion of H3 histone genes and their functionality remain unknown, although suggesting that they have higher evolutionary dynamics than previously thought.

Genomic organization of repetitive DNAs highlights chromosomal evolution in the genus Clarias (Clariidae, Siluriformes).

BackgroundThe genus Clarias (Clariidae, Siluriformes) contains at least 61 species naturally spread over vast regions of Asia, India and Africa. However, Clarias species have also been introduced in many different countries and represent the most widespread catfishes in the world. These fishes are also known as “walking catfishes” due to their ability to move over land. A large degree of chromosomal variation has been previously found in this family, mainly using conventional cytogenetic investigations, with diploid chromosome numbers ranging between 48 and 100. In this study, we analyzed the karyotype structure and distribution of four repetitive DNA sequences (5S and 18S rDNAs and (CA)15 and (GA)15 microsatellites) in three Clarias species (C. batrachus, C. gariepinus, C. macrocephalus), as well as in a probable natural hybrid of the two latter species from different Thailand river basins.ResultsClarias gariepinus and C. macrocephalus had 2n = 56 and 2n = 54, respectively, as well as karyotypes composed mainly by metacentric and submetacentric chromosomes. Their karyotypes differed in the number and location of 5S and 18S rDNA sites and in the degree of microsatellite accumulation. An intermediate chromosomal pattern incorporating those of the parental species was found in the probable hybrid, confirming its interspecific origin. Clarias batrachus had 2n = 104 chromosomes and its karyotype was dominated by mainly acrocentric elements, indicating that unusual multiple centric fissions were involved in its karyotype differentiation. The karyotype of this species presented an unexpected dispersion of ribosomal DNAs, possessing 54 and 12 sites of 5S and 18S rDNAs, respectively, as well as a high accumulation and differential distribution of both microsatellite repeats, representing ‘hot spots’ for chromosomal rearrangement.ConclusionBoth conventional and molecular cytogenetic markers were useful tools for demonstrating remarkable evolutionary dynamism and highlighting multiple chromosomal rearrangements and hybridization events correlated with the notable karyotypic diversity of these walking catfishes.

Decondensation of chromosomal 45S rDNA sites in Lolium and Festuca genotypes does not result in karyotype instability.

Fragile sites (FSs) in plants have been described for species like Lolium and other grasses. Whereas in humans FSs were shown to be involved in genome instabilities; the consequences of FSs expression in plants are not known yet. To evaluate whether FSs cause karyotype instabilities, we assessed the frequency of micronuclei and lagging chromosomes in meristematic cells, the stability of the DNA content, and the occurrence of neocentromeres in the presumed chromosomal fragments of Lolium perenne, Lolium multiflorum, Festuca arrundinacea, and two Festulolium hybrids. The cell cycle analysis along with flow cytometric genome size measurements showed high stability in all genomes evaluated. Neocentromeric activity was neither observed in the presumed fragments nor in any other chromosomal region, then this is not the mechanism responsible by the stability. However, Fluorescence in situ hybridization (FISH) with a 45S ribosomal DNA (rDNA) probe in combination with YOYO staining of metaphasic chromosomes showed that many extended nucleolus organizing region (NOR) form very thin YOYO-positive chromatin fibers connecting the acentric 'fragment' with the centromere-containing chromosome region. The obtained data indicate that the expression of FSs does not result in genome instabilities or neocentromere formation. The FS-containing 45S rDNA carrying chromatin fibers undergo a cell cycle and gene activity-dependent dynamic decondensation process.

Comparative karyotype investigations in the white-clawed crayfish Austropotamobius pallipes (Lereboullet, 1858) species complex and stone crayfish A. torrentium (Schrank, 1803) (Decapoda: Astacidae)

We report for the first time the chromosome number and karyological characteristics of the endangered European freshwater crayfish genus Austropotamobius (Decapoda, Astacidae). The karyotypes of the white-clawed crayfish ( Austropotamobius pallipes species complex) and of the stone crayfish ( A. torrentium ) consisted of 2 n = 176 chromosomes arranged into 76 metacentric, 11 metacentric-submetacentric, and one heteromorphic acrocentric/metacentric chromosome pair. Fluorochrome staining with 4,6-diamino-2-phenylindole (DAPI) revealed distinctive centromeric/pericentromeric as well as interstitial AT-rich blocks in A. pallipes and A. torrentium . Centromeric/pericentromeric AT-rich blocks were observed on all chromosomes, whereas interstitial AT-rich blocks were observed on some chromosomes, mostly on the larger ones. Fluorescence in situ hybridization (FISH) with the 45S rDNA probe on mitotic metaphases of A. pallipes and A. torrentium have revealed four major Texas-red signals, corresponding to four sites of 45S rDNA. One of the two 45S rDNA-bearing chromosome pairs was highly heteromorphic, considering the length of chromosomes as well as the size and intensity of 45S rDNA FISH signal. We speculate that the heteromorphic chromosome pair might represents X and Y sex chromosomes, suggesting the presence of XX-XY sex determination system in the A. pallipes species complex and A. torrentium .

Microstructural chromosome reorganization in the genus Trichomycterus (Siluriformes: Trichomycteridae)

Trichomycterus is a specious fish genus within Trichomycterinae and displays remarkable karyotype diversity. However, knowledge about their genomic structure and location of repetitive sequence is still limited. In order to better understand the karyotype diversification, we analyzed nine species of Trichomycterus using classical and molecular cytogenetic techniques. Results revealed a conserved diploid chromosome number of 2n=54 chromosomes in all analyzed species, although remarkable differences on the constitutive heterochromatin distribution were observed. In addition, while the 18S rDNA showed a conserved distribution pattern, the 5S rDNA sites showed a quite diverse location considering the analyzed species. Remarkably, both ribosomal genes were co-located in all species, except in T . iheringi , suggesting that co-localization is probably an ancestral condition in Trichomycterus . Finally, three analyzed species showed heterochromatic B chromosomes, reinforcing the intense genomic reorganization occurring in Trichomycterus . Our results showed that chromosomal variations are not restricted to differences in karyotype formula as previously proposed, but also to modifications on the microstructural level of resolution.

The chromosomal distribution of repetitive DNA sequences in Chrysanthemum boreale revealed a characterization in its genome

The sequencing of Chrysanthemum boreale genome is now in progress. However, the repetitive DNA sequences that dominates greatly in the genome have not yet been elucidated in Chrysanthemum. Therefore, we performed fluorescence in situ hybridization (FISH) by using the mitotic metaphase chromosome of C. boreale with well-identified repeats and C0t DNA as probes. The most conserved repeats, rDNAs, as well as telomeric repeats were first observed. Three pairs of 45S rDNA sites were located at the termini of the short arms of chromosomes 3, 7, and 8, while one pair of 5S rDNA signals was observed in the middle of the long arm of chromosome 4. Arabidopsis -type telomeric repeats colocalizing with 45S rDNA sites, were detected at the ends of both chromosome arms. Next, on the basis of DNA reassociation kinetics, three types of C0t DNA that exhibited different degrees of repetitive nature were fractionated and used as FISH probes to map the repetitive sequences. Signals from all C0t DNAs were successfully observed, but their coverage on the chromosomes was different for C0t-1, C0t-10, and C0t-100. C0t-1 FISH signals showed intensity in the telomeric region and were dispersed on both chromosome arms, except for distal regions. C0t-10 signals were observed almost in all parts of the chromosome, with greater intensity around pericentromeric regions. Bright C0t-100 signals were observed throughout the chromosome excluding some distal parts. C0t FISH signals covered the rDNAs and telomeric repeats, thus signifying their repetitive attributes. The relative proportions of repetitive DNA sequences in C. boreale genome is discussed.

Cryptic Chromosomal Diversity in the Complex "Geophagus" brasiliensis (Perciformes, Cichlidae).

Among American cichlids, Geophagus stands out as a species-rich genus widespread over neotropical region. Despite their diversity and confusing taxonomy, only few and basic chromosomal reports are available in populations/species along Atlantic coast, hindering our understanding about evolutionary trends in this genus. Therefore, detailed chromosomal studies were performed in "Geophagus" brasiliensis complex from coastal rivers in northeastern Brazil, totalizing seven populations of Geophagus brasiliensis and one of Geophagus itapicuruensis. All samples shared a diploid number (2n) of 48 divided into 2 submetacentric and 46 subtelocentric/acrocentric chromosomes with (peri)centromeric heterochromatin, hypothesized as a symplesiomorphy for Geophagini. Although G. itapicuruensis and two populations of G. brasiliensis presented single NORs on short arms, multiple GC-rich rDNA sites were observed in the remaining G. brasiliensis samples, ranging from three to six NOR-bearing chromosomes. Inversely, 5S rDNA sites were invariably located at interstitial region on a st/a pair, nonsyntenic to NORs. A compilation of the data set in Geophagus shows that their chromosomal evolution has been driven by pericentric inversions and microstructural changes. Besides, the divergence found in northeastern Brazil places this region as a biodiversity hotspot. A taxonomic revision in the complex "Geophagus" brasiliensis is recommended with the support of cytogenetic analyses.

Distribution of various types of repetitive DNAs in Allium cepa L. based on dual color FISH

Fluorescence in situ hybridization (FISH) is a powerful tool for the detection of DNA sequences in a specific region of a chromosome as well as for integrated physical mapping. The detailed karyotypes of two onion cultivars (‘Eumjinara’ and ‘Sinseonhwang’), which are resources for the onion genome sequencing project were constructed based on dual color FISH using 5S and 45S rDNAs, telomeric tandem repeats, and Cot-1 DNA. All materials showed 2n = 2x = 16. Four loci of 5S rDNAs were located on the interstitial regions of the short arms of one pair of chromosomes in both onion cultivars. One loci of 45S rDNA signal was distally detected on each short arm of the two pairs of chromosomes in ‘Eumjinara’ and ‘Sinseonhwang’, but the latter possessed another locus of 45S rDNA on the distal part of the long arm in one homolog of a chromosome pair. Co-localization of telomeric tandem repeats and 45S rDNA signals was observed in ‘Eumjinara’ and ‘Sinseonhwang’. A difference in the distribution of 45S rDNA sites and the co-localization of signals observed between the two cultivars are indicators of recent activities in the nuclear genome that may involve homologous recombination or transposition of certain repeats. Cot-1 DNA signals are distributed throughout the chromosomes and show stronger signals in the terminal regions. The elucidation of Cot-1 DNA through in-situ hybridization would only show a large amount of tandemly repeating, non-coding and dispersedly repetitive DNAs in onion genome.