Blocks Of Constitutive Heterochromatin Research Articles

Comparative Cytogenetics in Four Leptodactylus Species (Amphibia, Anura, Leptodactylidae): Evidence of Inner Chromosomal Diversification in Highly Conserved Karyotypes

With 82 species currently described, the genus Leptodactylus is the most diverse and representative one in the family Leptodactylidae. Concerning chromosomal organization, this genus represents an interesting and underexplored group since data from molecular cytogenetics are incipient, and little is known about the organization and distribution of repetitive DNA elements in the karyotypes. In this sense, this study aimed at providing a comparative analysis in 4 Leptodactylus species (L. macrosternum, L. pentadactylus, L. fuscus, and Leptodactylus cf. podicipinus), combining conventional cytogenetics (Giemsa staining, C-banding, and AgNOR staining) and mapping of molecular markers (18S rDNA, telomeric and microsatellite probes), to investigate mechanisms underlying their karyotype differentiation process. The results showed that all species had karyotypes with 2n = 22 and FN = 44, except for Leptodactylus cf. podicipinus which presented FN = 36. The 18S rDNA was observed in pair 8 of all analyzed species (corresponding to pair 4 in L. pentadactylus), coinciding with the secondary constrictions and AgNOR staining. FISH with microsatellite DNA probes demonstrated species-specific patterns, as well as an association of these repetitive sequences with constitutive heterochromatin blocks and ribosomal DNA clusters, revealing the dynamics of microsatellites in the genome of the analyzed species. In summary, our data demonstrate an ongoing process of genomic divergence inside species with almost similar karyotype, driven most likely by a series of pericentric inversions, followed by differential accumulation of repetitive sequences.

Chromosome puzzle in the southernmost populations of the medically important scorpion Tityus bahiensis (Perty 1833) (Buthidae), a polymorphic species with striking structural rearrangements

The scorpion Tityus bahiensis is the most common scorpion species of medical importance with sexual reproduction in South America. Many of the populations cytogenetically studied so far are polymorphic both for chromosome number and meiotic configuration in males. In this work, we perform cytogenetic analysis of three of the most meridional populations of T. bahiensis occurring in north-eastern Argentina (Misiones province), which showed numerical and structural variations with respect to previously studied populations. We describe four new diploid numbers in males for this species, from 2n = 12 to 2n = 15, seven different cytotypes, and we analyze the chromosome rearrangements involved in the different multivalent associations observed during meiosis (II, III, V, VI, VII and IX). Ribosomal DNA is invariably present at one terminal region on each of two chromosomes. Blocks of constitutive heterochromatin of different sizes are found at the terminal regions of every chromosome in the different cytotypes, along with subterminal and interstitial blocks in some chromosomes. Based on the rDNA and constitutive heterochromatin localization, we analyzed chromosome evolution within these populations. We propose that all the cytotypes could have originated from a hypothetical ancestral karyotype with 2n = 18 and 9 bivalents at meiosis I but followed two pathways with different sequences of mainly fusions and some translocations from the ancestral karyotype. Furthermore, we review all the cytogenetically studied populations of this species and their distribution to discuss the chromosomal evolution of this polymorphic species.

Taxonomic Diversity Not Associated with Gross Karyotype Differentiation: The Case of Bighead Carps, Genus Hypophthalmichthys (Teleostei, Cypriniformes, Xenocyprididae).

The bighead carps of the genus Hypophthalmichthys (H. molitrix and H. nobilis) are important aquaculture species. They were subjected to extensive multidisciplinary research, but with cytogenetics confined to conventional protocols only. Here, we employed Giemsa-/C-/CMA3- stainings and chromosomal mapping of multigene families and telomeric repeats. Both species shared (i) a diploid chromosome number 2n = 48 and the karyotype structure, (ii) low amount of constitutive heterochromatin, (iii) the absence of interstitial telomeric sites (ITSs), (iv) a single pair of 5S rDNA loci adjacent to one major rDNA cluster, and (v) a single pair of co-localized U1/U2 snDNA tandem repeats. Both species, on the other hand, differed in (i) the presence/absence of remarkable interstitial block of constitutive heterochromatin on the largest acrocentric pair 11 and (ii) the number of major (CMA3-positive) rDNA sites. Additionally, we applied here, for the first time, the conventional cytogenetics in H. harmandi, a species considered extinct in the wild and/or extensively cross-hybridized with H. molitrix. Its 2n and karyotype description match those found in the previous two species, while silver staining showed differences in distribution of major rDNA. The bighead carps thus represent another case of taxonomic diversity not associated with gross karyotype differentiation, where 2n and karyotype structure cannot help in distinguishing between genomes of closely related species. On the other hand, we demonstrated that two cytogenetic characters (distribution of constitutive heterochromatin and major rDNA) may be useful for diagnosis of pure species. The universality of these markers must be further verified by analyzing other pure populations of bighead carps.

Dynamics of Replication and Nuclear Localization of the B Chromosome in Kidney Tissue Cells in Astyanax scabripinnis (Teleostei: Characidae).

B chromosomes are extra genomic compounds found in different taxonomic groups, including plants and animals. Obtaining patterns of resolutive chromosomal bands is necessary to understand the nuclear organization, variability and nature of B chromosome chromatin and possible transcriptional regions. In this study, we analyzed 35 Astyanax scabripinnis specimens sampled from Fazenda Lavrinha, a stream in the Paraíba do Sul river basin, Brazil. Through the incorporation of the thymidine analog 5'-bromo-2'-deoxyuridine (5-BrdU) in vivo, it was possible to recognize the replicating regions of the B chromosome at the beginning of the S phase, differentially characterized in relationship to the regions of late replication. In this perspective, it is possible to suggest that the B chromosome of this species possesses a territory and the chromatin accessible for transcription, especially in the light (i.e., early replicating) bands (p1.1; p1.3; and p2.1 and q1.1, q1.3, q2.1, and q2.2). The late-replicating regions are corresponding to the blocks of constitutive heterochromatin. They show a preferential accumulation of satellite DNA As51. By the use of the fluorochrome chromomycin A3 (CMA3), it was possible to identify GC-rich chromosomal regions, corresponding to late-replicating parts of genome, confirming the revealed data by the replication banding and C-banding. In addition, the analysis by confocal microscopy in kidney cells indicates the location of a peripheral anchorage of this chromosome in the nuclear lamina, reinforcing the idea of downregulation of the associated regions.

Cytogenetics of the small-sized fish, Copeina guttata (Characiformes, Lebiasinidae): Novel insights into the karyotype differentiation of the family.

Lebiasinidae is a small fish family composed by miniature to small-sized fishes with few cytogenetic data (most of them limited to descriptions of diploid chromosome numbers), thus preventing any evolutionary comparative studies at the chromosomal level. In the present study, we are providing, the first cytogenetic data for the red spotted tetra, Copeina guttata, including the standard karyotype, C-banding, repetitive DNA mapping by fluorescence in situ hybridization (FISH) and comparative genomic hybridization (CGH), providing chromosomal patterns and novel insights into the karyotype differentiation of the family. Males and females share diploid chromosome number 2n = 42 and karyotype composed of 2 metacentric (m), 4 submetacentric (sm) and 36 subtelocentric to acrocentric (st-a) chromosomes. Blocks of constitutive heterochromatin were observed in the centromeric and interstitial regions of several chromosomes, in addition to a remarkably large distal block, heteromorphic in size, which fully corresponded with the 18S rDNA sites in the fourth chromosomal pair. This overlap was confirmed by 5S/18S rDNA dual-color FISH. On the other hand, 5S rDNA clusters were situated in the long and short arms of the 2nd and 15th pairs, respectively. No sex-linked karyotype differences were revealed by male/female CGH experiments. The genomic probes from other two lebiasinid species, Lebiasina melanoguttata and Pyrrhulina brevis, showed positive hybridization signals only in the NOR region in the genome of C. guttata. We demonstrated that karyotype diversification in lebiasinids was accompanied by a series of structural and numeric chromosome rearrangements of different types, including particularly fusions and fissions.

New Comparative Cytogenetic Data on Three Genera of Armored Catfishes of Ancistrini (Loricariidae: Hypostominae)

The karyotypes and other chromosomal markers of 4 catfish species, namely Lasiancistrus schomburgkii, Lasiancistrus sp., Araichthysloro, and Megalancistrus sp., members of a taxonomically complex and speciose tribe of catfishes Ancistrini, Hypostominae, were examined using conventional (Giemsa staining, Ag-NOR, and C-banding) and molecular cytogenetic protocols (FISH) and DNA barcoding. In L. schomburgkii, Lasiancistrus sp., and A.loro a diploid number 2n = 54 was observed, with karyotypes composed of 28m + 16sm + 10st, 36m + 12sm + 6st chromosomes, while Megalancistrus sp. had 2n = 52, with the karyotype composed of 28m + 16sm + 8st chromosomes. The Ag-NOR phenotypes were simple in all 4 species, which was confirmed by FISH with an 18S rDNA probe. However, the positive 5S rDNA sites varied among species: 2 chromosome pairs in L. schomburgkii, Lasiancistrus sp., and A. loro, and only 1 pair in Megalancistrus sp. The blocks of constitutive heterochromatin were poorly visible in the pericentromeric and telomeric regions of most chromosomes of the examined species by C-banding. The genetic distance analysis, based on mtDNA COI gene sequences (DNA barcoding), confirmed the species as 4 taxonomic units. Ours and other published data indicate that karyotype differentiation among Ancistrini is complex and divergent and indicates the occurrence of common chromosomal rearrangements, such as pericentric inversions conserving the diploid number, and other rearrangements that are more frequent in some genera, such as centric fusions in Ancistrus.

A Comparative Chromosome Mapping Study in Japanese Podismini Grasshoppers (Orthoptera: Acrididae: Melanoplinae)

In the present paper, karyotypes of 7 Japanese Podismini species, Anapodisma beybienkoi, Fruhstorferiola okinawaensis, Parapodisma caelestis, P. mikado, P. setouchiensis, P. tenryuensis, and Sinopodisma punctata (2n♂ = 21, all acrocentric), are described and compared on the basis of conventional (C-banding, DAPI/CMA₃-staining, Ag-NOR) and molecular (FISH with 18S rDNA and telomeric probes) cytogenetic staining methods. This is the first study to report karyotypes of A. beybienkoi and P. caelestis. Differential staining techniques showed karyotypic diversity in these species. The number of 18S rDNA signals ranged from 2 to 6, and the signals were located on the autosomes or sex chromosomes. In all species, clusters of rDNA coincided with Ag-NORs. Telomeric signals occurred at the chromosome ends at the pachytene stage and seldom at other stages of meiosis. Paracentromeric and some distal and interstitial blocks of constitutive heterochromatin were detected in the chromosomes of Anapodisma, Fruhstorferiola, and Parapodisma species. Staining with DAPI and CMA₃ revealed 2 groups of heterochromatin composition. In addition, intraspecific differences in the number of rDNA clusters and C-bands were observed within Parapodisma species. Based on the evidence of cytogenetic characteristics, the monophyly of Tonkinacridina cannot be supported.

Application of differential staining techniques in Vampyrum spectrum (Linnaeus, 1758) (Phyllostomidae, Chiroptera, Mammalia)

AbstractVampyrum spectrum is the largest bat species found in the New World. Few cytogenetic data are available on the species, although the diploid chromosome number is known from a specimen captured in Central America in 1970. The present study provides further data on the cytogenetics of V. spectrum, based on the techniques of C banding, impregnation with silver nitrate (AgNORs), staining with chromomycin A3 (CMA3) and fluorescent in situ hybridization (FISH) with probes for 18S rDNA and telomeric sequences. The species presented 2n = 30 chromosomes and 56 autosomal arms. The C banding revealed blocks of constitutive heterochromatin in the pericentromeric regions of all the chromosomes and subterminal blocks on the short arms of chromosome pairs 5, 6, 8 and 9. The AgNORs were found in the secondary constrictions of the long arm of chromosome pair 1. The CMA3 revealed multiple bands in the chromosomes, which permitted the accurate identification of the different chromosome pairs that constitute the kary...

Karyotype structure of Hypostomus cf. plecostomus (Linnaeus, 1758) from Tapajós River basin, Southern Amazon: occurrence of sex chromosomes (ZZ/ZW) and their evolutionary implications.

Hypostomus is a group of fish with numerical and struc-tural karyotypic variability. Among them, only six species, three of which belong to the Amazon basin, show a sex chromosome. In this study, we present the karyotype structure of Hypostomus cf. plecos-tomus from the Teles Pires river basin in the municipality of Alta Flo-resta, MT. The species has 2n = 68 and the karyotype formula 14m+ 24sm+ 14st+ 16a [fundamental number (FN) = 120] in males and 15m+ 24sm+14st+15a (FN = 121) in females and sex chromosomes ZZ/ZW. Argyrophilic nucleolar organizer regions (AgNORs) were identified in two pairs of chromosomes at different positions: short arm of the pair 21and long arm of the pair 27, matching the signals displayed by 18S FISH and indicating multiple NORs. Analysis of band C detected few blocks of constitutive heterochromatin in the pericentromeric regions of most chromosomes and the telomeric regions of some pairs, includ-ing the nucleolar pair 21. However, large blocks on the long arm of the nucleolar pair 27 still stood out. GC-rich heterochromatin (CMA3) was visualized only coincidently with nucleolar sites. Mapping of 5S rDNA sites with FISH revealed markings in eight chromosomes, demonstrat-ing synteny between the 18S and 5S sites. The data obtained for H. cf. plecostomus are important for taxonomic studies of this Amazon com-plex "H. plecostomus group". The occurrence of sex chromosomes in Amazon species of Hypostomus suggests an evolutionary event that is independent of other species in the group.

Karyotypic variation in the Andean rodent Phyllotis xanthopygus (Waterhouse, 1837) (Rodentia, Cricetidae, Sigmodontinae)

Phyllotis xanthopygus (Waterhouse, 1837) is an Andean rodent endemic to South America. Despite its wide geographical distribution in Argentina, few individuals have been studied on the cytogenetic level and only through conventional staining. In this work, chromosome characterization of Argentine samples of this species was performed using solid staining, C-banding and base-specific fluorochromes. Twenty two specimens were analyzed, collected in the provinces of Jujuy, Catamarca, and the north and south of Mendoza. All studied specimens showed 2n=38, having mostly the bi-armed autosomes, metacentric or submetacentric. Fundamental Number varied between 70 and 72. These changes were due to the presence of chromosome heteromorphisms in individuals from southern Mendoza and Jujuy. C-banding revealed pericentromeric blocks of constitutive heterochromatin in most chromosomes. Acrocentric chromosomes involved in heteromorphisms showed high variation in the amount of heterochromatin within and among populations. Additionally, banding with fluorochromes (DAPI and chromomycin A3) revealed homologous localization of AT and GC rich regions among chromosomes of the different populations analyzed. Comparisons among heteromorphic pairs suggested, however, that the variation might be the result of complex chromosome rearrangements, involving possibly amplifications and/or deletions of heterochromatic segments. These results are in accordance with molecular studies that indicate genetic variability within and among the populations of this taxon.

Heterochromatin characterization through differential fluorophore binding pattern in some species of Vigna Savi.

Heterochromatin regions are the most intensively studied and best known chromosome markers in plants. In Vigna species, blocks of constitutive heterochromatin were found either in the terminal or interstitial region of the chromosomes. The number and distribution of CMA(+) and DAPI(+) binding sites exhibit high chromosomal variability with characteristic unique banding patterns in all the eight taxa. A predominant feature was observed, i.e., most of the CMA(+) binding sites were in the terminal region of the short arm of some chromosomes while DAPI(+) binding sites were found mostly in the intercalary region of the chromosomes. The higher divergence in the heterochromatin blocks, as revealed by chromomycin A3 (CMA) binding pattern, in a few taxa, viz. Vigna glabrescens, Vigna khandalensis, and Vigna mungo, suggests that the processes of divergent evolution of repetitive sequences in genomic DNA involve a guanine-cytosine (GC)-rich region. On the contrary, Vigna dalzelliana had shown a prominent adenine-thymine (AT)-rich repetitive DNA sequence in terminal regions in the short arm of chromosomes while Vigna umbellata had shown in interstitial regions. The presence of prominent heterochromatic-rich regions, either GC- or AT-rich regions, does facilitate the rate of chromosomal rearrangements leading to restructuring of the karyotypes and thereby helping the species to attempt structural alterations as means of speciation.

Repetitive sequences associated with differentiation of W chromosome in Semaprochilodus taeniurus

The possible origins and differentiation of a ZZ/ZW sex chromosome system in Semaprochilodus taeniurus, the only species of the family Prochilodontidae known to possess heteromorphic sex chromosomes, were examined by conventional (C-banding) and molecular (cross-species hybridization of W-specific WCP, Fluorescence in situ hybridization (FISH) with telomere (TTAGGG)n, and Rex1 probes) cytogenetic protocols. Several segments obtained by W-specific probe were cloned, and the sequences localized on the W chromosome were identified by DNA sequencing and search of nucleotide collections of the NCBI and GIRI using BLAST and CENSOR, respectively. Blocks of constitutive heterochromatin in chromosomes of S. taeniurus were observed in the centromere of all autosomal chromosomes and in the terminal, interstitial, and pericentromeric regions of the W chromosome, which did not demonstrate interstitial telomeric sites with FISH of the telomere probe. The Rex1 probe displayed a compartmentalized distribution pattern in some chromosomes and showed signs of invasion of the pericentromeric region in the W chromosome. Chromosomal painting with the W-specific WCP of S. taeniurus onto its own chromosomes showed complete staining of the W chromosome, centromeric sites, and the ends of the Z chromosome, as well as other autosomes. However, cross-species painting using this WCP on chromosomes of S. insignis, Prochilodus lineatus, and P. nigricans did not reveal a proto-W element, but instead demonstrated scattered positive signals of repetitive DNAs. Identification of the W-specific repetitive sequences showed high similarity to microsatellites and transposable elements. Classes of repetitive DNA identified in the W chromosome suggested that the genetic degeneration of this chromosome in S. taeniurus occurred through accumulation of these repetitive DNAs.

Y-SNP haplogroups related to the Yqh+ heteromorphism in the Mexican northwestern population

Morphological variation of the Y chromosome has been observed in different populations. This variation is mostly related to the heteromorphic Yq12 band, which is composed of a variable block of constitutive heterochromatin. The Yqh+ heteromorphism has a worldwide frequency of 2.85% and is considered clinically innocuous. The aim of this study was to identify the ancestry of the Yqh+ heteromorphism present in individuals from western Mexico. For this purpose, 17 Y-chromosome single nucleotide polymorphisms were analysed by multiplex polymerase chain reaction and SNaPshot assays. In 28 Yqh+ males, only five haplogroups were observed; with a haplogroup diversity of 0.4841 ± 0.1094, which was less than that observed in a study of unselected Mexican mestizo population. Differences were specifically conferred by the high frequencies of haplogroups R1b1 and P*(xQ,R), and by the absence of the Amerindian haplogroup Q (Q*(xQ1a3a) plus Q1a3a) from the Yqh+ group. This study suggests a post-1492 incorporation for Yqh+ chromosomes into the Mexican northwestern population.

In situ hybridization analysis of gibbon chromosomes suggests that amplification of alpha satellite DNA in the telomere region is confined to two of the four genera

The siamang (Symphalangus syndactylus), a species of the family Hylobatidae (gibbons), carries large blocks of constitutive heterochromatin in the telomere region of chromosomes. We recently found that alpha satellite DNA constitutes these heterochromatin blocks as a main component. Alpha satellite DNA, tandem repeat sequences of 171-bp repeat units, is a major component of centromeres in primates. In addition to the siamang, the white-cheeked gibbon (Nomascus leucogenys) was previously found to carry the alpha satellite DNA in the telomere region, although not as large a scale as the siamang. Gibbons comprise four genera: Hoolock, Hylobates, Nomascus, and Symphalangus. Here, we report that the amplification of alpha satellite DNA in the telomere region is probably confined to two genera: Nomascus and Symphalangus. We examined one species of Hoolock and four species of Hylobates and obtained evidence against such an amplification event in these species. The phylogenetic relationship of the four gibbon genera remains unclear. One simple explanation for the current distribution of the telomere region alpha satellite DNA would be that Nomascus and Symphalangus are relatively closely related and the amplification occurred in their common ancestor.

Cytogenetics of the darkling beetles Zophobas aff. confusus and Nyctobates gigas (Coleoptera, Tenebrionidae)

Males of Zophobas aff. confusus and Nyctobates gigas (Tenebrionidae) collected in the State of Pernambuco, Brazil, were studied through conventional staining, C-banding, silver nitrate impregnation (AgNO₃), and the base specific fluorochromes CMA₃ and DAPI. Z. aff. confusus was found to have 2n = 20 (9+Xyp) while N. gigas exhibited 2n = 18 (8+neoXY). Large pericentromeric blocks of constitutive heterochromatin (CH) were detected throughout the autosomal complement of the two species, except in one autosomal pair of N. gigas in which no heterochromatic block was observed. The sex chromosomes of both species were almost totally heterochromatic. Double staining with CMA₃/DA (distamycin) and DAPI/DA marked CH in Z. aff. confusus. However, DAPI staining was more intense. N. gigas was found to possess blocks of CH-positive CMA₃ and homogeneous DAPI. AgNO₃ staining also revealed differences between the two species. In Z. confusus an NOR was observed in the sexual bivalent Xyp and N. gigas was found to have an autosomal NOR.

Ommexecha virens (Thunberg, 1824) and Descampsacris serrulatum (Serville, 1831) (Orthoptera, Ommexechidae): karyotypes, constitutive heterochromatin and nucleolar organizing regions

Chromosomes of Ommexecha virens and Descampsacris serrulatum (Ommexechidae) were analyzed through conventional staining, C-banding, base specific fluorochromes, silver nitrate impregnation (AgNO3), and fluorescent in situ hybridization (FISH) with probe for 45S rDNA. The two species presented diploid number 2n= 23,X0 in males and acrocentric autosomes, except the pair one that presented submetacentric morphology. The X chromosome has distinct morphology in the two analyzed species, being a medium acrocentric in Ommexecha virens and large submetacentric in Descampsacris serrulatum. The C-banding revealed pericentromeric blocks of constitutive heterochromatin (CH) in all the chromosomes of Descampsacris serrulatum. For Ommexecha virens it was evidenced that the blocks of CH are preferentially located in the pericentromeric area (however some bivalents presents additional blocks) or in different positions. The staining with CMA3/DA/DAPI showed GC rich CH blocks (CMA3+) in some chromosomes of the two species. The nucleolar organizer regions (NORs) were located in the bivalents L2, S9, S10 of Ommexecha virens and M5, M6, M7, S11 of Descampsacris serrulatum. The FISH for rDNA showed coincident results with the pattern of active NORs revealed by AgNO3. This work presents the first chromosomal data, obtained through differential cytogenetics techniques in Ommexechidae, contributing to a better characterization of karyotypic evolution for this grasshopper family.

Cytogenetic Study on Microtus guentheri (Danford and Alston, 1880) (Mammalia: Rodentia) from Turkey: Constitutive Heterochromatin Distribution and Nucleolar Organizer Regions

Conventionally stained, C- and Ag-NOR banded karyotypes of Guenther's vole, Microtus guentheri were studied from Turkey. The species possesses a karyotype of 2n = 54, NFa = 52 and NF = 54 in specimens from Kahramanmaraş and Gaziantep provinces, whereas NF = 56 in females and NF = 55 in males were found in individuals from Kirikkale and Nevşehir provinces. The X chromosome was a large acrocentric (NF = 54) or submetacentric (NF = 55, 56) while the Y chromosome was a small telocentric in all specimens examined. Blocks of constitutive heterochromatin were located in the pericentromeric areas of autosomes including the X chromosome. Nucleolar organizer regions (NORs) were located at the telomeric regions of the short arms of five acrocentric pairs and centromeric regions of two telocentric pairs in the Nevşehir and Kirikkale specimens.

Comparative cytogenetic analysis of two grasshopper species of the tribe Abracrini (Ommatolampinae, Acrididae)

The grasshopper species Orthoscapheus rufipes and Eujivarus fusiformis were analyzed using several cytogenetic techniques. The karyotype of O. rufipes, described here for the first time, had a diploid number of 2n = 23, whereas E. fusiformis had a karyotype with 2n = 21. The two species showed the same mechanism of sex determination (XO type) but differed in chromosome morphology. Pericentromeric blocks of constitutive heterochromatin (CH) were detected in the chromosome complement of both species. CMA3/DA/DAPI staining revealed CMA3-positive blocks in CH regions in four autosomal bivalents of O. rufipes and in two of E. fusiformis. The location of active NORs differed between the two species, occurring in bivalents M6 and S9 of O. rufipes and M6 and M7 of E. fusiformsi. The rDNA sites revealed by FISH coincided with the number and position of the active NORs detected by AgNO3 staining. The variability in chromosomal markers accounted for the karyotype differentiation observed in the tribe Abracrini.

Comparative Cytogenetic Analysis of the Genus Symphysodon (Discus Fishes, Cichlidae): Chromosomal Characteristics of Retrotransposons and Minor Ribosomal DNA

As part of a genetic screening program for wild Discus fishes, we analyzed karyotypes and cytogenetic characteristics of Symphysodon aequifasciatus, S. discus and S. haraldi using C-banding and fluorescent in situ hybridization (FISH) with the Rex3 retrotransposon and 5S rDNA probes in mitotic and meiotic chromosomes. In the 3 species, diploid chromosome number was 2n = 60 and karyotypes contained predominantly meta-submetacentric chromosomes. C-banding showed blocks of constitutive heterochromatin mainly in the pericentromeric region. Physical mapping of repetitive 5S rDNA sequences and Rex3 retrotransposons in mitotic and meiotic chromosomes showed partial colocalization of constitutive heterochromatin and repetitive elements. Correlations among the accumulation of repetitive elements, heterochromatinization and chromosome rearrangements have been hypothesized to explain the karyotype differentiation in the Symphysodon genus. The role of repetitive elements in adaptation to highly diverse habitats, as well as in the generation of the phenotypic and genetic variability found in wild Discus populations, needs to be further investigated.

Polymorphic Heterochromatic Segments in Japanese Quail Microchromosomes

Using highly extended lampbrush chromosomes from diplotene oocytes, we have examined the distribution of heterochromatin protein 1 β (HP1β) and histone H3 modifications on chicken (Gallus gallus) and Japanese quail (Coturnix japonica) (2n = 78) microchromosomes. Acrocentric microchromosomes of chicken and submetacentric microchromosomes of quail differ in several morphological features. In addition to pericentromeric and subtelomeric blocks of constitutive heterochromatin, which are enriched in HP1β protein and repressive histone modifications, not completely condensed but heterochromatic segments were found to be an attribute of the short arms of submetacentric microchromosomes in Japanese quail. These heterochromatic regions are variable in length and do not form chiasmata in female germ cells. Dissimilarity in the centromere positions in chicken and Japanese quail microchromosomes is proposed to be due to the accumulation of repetitive sequences on the short arms of quail microchromosomes. Transcriptional activation of polymorphic heterochromatic segments of quail microchromosomes during the lampbrush stage is demonstrated.