Cytogenetics of Xenopus laevis. I. G-banding pattern of Xenopus laevis chromosomes

Chromosome Banding in Amphibia. XXXII. The Genus Xenopus (Anura, Pipidae)

Mitotic chromosomes of 16 species of the frog genus Xenopus were prepared from kidney and lung cell cultures. In the chromosomes of 7 species, high-resolution replication banding patterns could be induced by treating the cultures with 5-bromodeoxyuridine (BrdU) and deoxythymidine (dT) in succession, and in 6 of these species the BrdU/dT-banded chromosomes could be arranged into karyotypes. In the 3 species of the clade with 2n = 20 and 4n = 40 chromosomes (X. tropicalis, X. epitropicalis, X. new tetraploid 1), as well as in the 3 species with 4n = 36 chromosomes (X. laevis, X. borealis, X. muelleri), the BrdU/dT-banded karyotypes show a high degree of homoeology, though differences were detected between these groups. Translocations, inversions, insertions or sex-specific replication bands were not observed. Minor replication asynchronies found between chromosomes probably involve heterochromatic regions. BrdU/dT replication banding of Xenopus chromosomes provides the landmarks necessary for the exact physical mapping of genes and repetitive sequences. FISH with an X. laevis 5S rDNA probe detected multiple hybridization sites at or near the long-arm telomeric regions in most chromosomes of X. laevis and X. borealis, whereas in X. muelleri, the 5S rDNA sequences are located exclusively at the long-arm telomeres of a single chromosome pair. Staining with the AT base pair-specific fluorochrome quinacrine mustard revealed brightly fluorescing heterochromatic regions in the majority of X. borealis chromosomes which are absent in other Xenopus species.

Xenopus Cytogenetics and Chromosomal Evolution

The genus Xenopus represents important model organisms in the field of developmental biology and chromosomal evolution. Developmental processes are tightly coupled with the analysis of gene function via genetic linkage and mapping. Cytogenetic techniques such as chromosome banding or FISH are essential tools for the determination of gene position and subsequently for the construction of linkage and physical maps. Here, we present a summary of key achievements in X. tropicalis and X. laevis cytogenetics with emphasis on the gene localization to chromosomes. The second part of this review is focused on the chromosomal evolution regarding both above-mentioned species. With respect to methodology, hybridization techniques such as FISH and chromosome-specific painting FISH are highlighted.

G-banding patterns of the chromosomes of amphioxus Branchiostoma belcheri tsingtauense

Chromosome banding study remains lacking in amphioxus heretofore. This paper reports the G-band patterns of the metaphase chromosomes of amphioxus Branchiostoma belcheri tsingtauense, the first description and numbering of the karyotyped bands in a cephalochordate. G-banding results show that four out of the ten second pair chromosomes examined are heteromorphic, while the remaining ones (6/10) are homomorphic. In contrast, the other 17 pair chromosomes are all homomorphic. These broadly support our previous suggestion that the chromosomes No. 2 are a pair of sex-chromosomes. G-banding method stains positively 65.5% of the chromosome surface, and a total of 149 G-bands (77 positive, 65 negative and 7 variable) has been recorded in the 17 autosomes and the dimorphic chromosomes 2A' and 2A.

Chromosome mapping of Xenopus tropicalis using the G- and Ag-bands: tandem duplication and polyploidization of larvae heads

Developmental cytogenetic analyses of Xenopus tropicalis larvae from two origins were performed on stage 27-34 heads treated with colchicine. Standard G-band karyotyping using trypsin and chromosome mapping of 184 bands were examined. Although the main karyotype was 2n = 20, polyploidy (3n = 30 or 4n = 40) and aneuploidy were detected in each individual treated with colchicine, even those treated for only 1 h. The percentage of polyploid karyotypes was 10-20% across the total of measured metaphases. The mean mitotic index was 0.10. Chromosomal breaks and exchanges were detected at the secondary constriction of chromosomes 5 or 6. Ag-band detection showed clearly positive staining at the secondary constriction of chromosome 5, which corresponds to the nucleolar organizer region. Tandem duplication of negative G-bands at the secondary constriction of chromosome 6 and the short arm of chromosome 10 was suggested by this study. X. tropicalis thus provides a good model to study the mechanism and effects of chromosomal abnormalities, gene mapping and tissue specific gene expression in the developmental process.

Patterns of DNase sensitivity in the chromosomes of Rana perezi (Amphibia: Anura)

We have analyzed the patterns of DNase I/nick translation in the chromosomes of Rana perezi. The results show a nonuniform DNase sensitivity in different chromosome domains; the hypersensitivity appears to be concentrated at both the NOR and the distal regions. The resemblance to the situation in mammals, where active genes are DNase I hypersensitive, is discussed.Key words: DNase sensitivity, chromosomes, Rana perezi.

Chromosome banding in Amphibia. XVI. High-resolution replication banding patterns in Xenopus laevis

High-resolution replication banding patterns were induced in prometaphase and prophase chromosomes of Xenopus laevis by treating kidney cell lines with 5-bromodeoxyuridine (BrdU) and deoxythymidine (dT) in succession. Up to 650 early and late replicating bands per haploid karyotype were demonstrated in the very long prophase chromosomes. This permits an exact identification of all chromosome pairs of X. laevis. Late replicating heterochromatin was located by analysing the time sequence of replication throughout the second half of S-phase. Neither heteromorphic sex chromosomes nor sex chromosome-specific replication bands were demonstrated in the heterogametic ZW females of X. laevis. A detailed examination of the BrdU/dT-labelled prometaphases and prophases revealed that the X. laevis chromosomes can be arranged in groups of four (quartets), most of which show conspicuous similarities in length, centromere position, and replication pattern. This is interpreted as further evidence for an ancient allotetraploid origin of X. laevis.

Variation within and between nucleolar organizer regions in Australian hylid frogs (Anura) shown by 18S + 28S in-situ hybridization

Five distinct classes of secondary constriction are found in the hylid frogs from the genera Litoria and Cyclorana, each of which is defined by its C-banding pattern and morphology (King, 1980, 1987). In-situ hybridization experiments utilizing 18S + 28S copy RNA probes derived from Xenopus and Drosophila rDNA templates, were made on nine species of frogs possessing the major constriction types. Types 1, 2, 4, and 5 are confirmed as being NORs. These results also indicate that type 1 and 2 constriction types are not differentially despiralized as previously suggested, but show absolute differences in the quantity of ribosomal DNA present. This variation took two forms, deletion polymorphism and amplification polymorphism. These differences were observed between homologues within cells and between cells within individuals. Animals possessing these 'despiralized' constrictions are therefore mosaics for both deletion and amplification polymorphisms. Polymorphism frequencies vary greatly between constriction types. Some specimens have a higher level of presence/absence heterozygosity, (L. moorei, type 2, L. nannotis type 5, L. raniformis (animal A, pair 8 type 2), than do others (L. peronii, L. rothii, L. caerulea). The above species also vary markedly in the degree and frequency of amplification of the NORs. The type 4 constrictions analysed (L. coplandi, L. lesueuri and C. novaehollandiae) have a particularly low frequency of presence/absence heterozygosity, and they have fewer size heteromorphisms between homologues. The type 3 ephemeral constrictions did not hybridize to cRNA probes at any stage. In all but one of the species studied, a single pair of chromosomes possessed an NOR. However, in L. raniformis these occurred on two pairs of chromosomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Evolutionary diversity of reverse (R) fluorescent chromosome bands in vertebrates

Mitotic chromosomes, interphase cell nuclei, and male meiosis of 41 species representing all vertebrate classes were analyzed with distamycin A/mithramycin counterstaining. The purpose of the study was to recognize differences and common characteristics in the reverse (R) fluorescent banding patterns in the chromosomes of vertebrate species at various stages of evolution. In contrast to the warm-blooded mammals and birds, the euchromatic segments in the chromosomes of most reptiles, amphibians, and fishes contain no multiple fluorescent R-bands. This is thought to be due to the absence of the long homogeneous regions (isochores) in the DNA of the cold-blooded vertebrates. Distamycin A/mithramycin banding specifically reveals the GC-rich constitutive heterochromatin in all vertebrates. In most of the vertebrate chromosomes examined, the heterochromatic regions have opposite staining properties with mithramycin and quinacrine. Mithramycin labels the nucleolus organizer regions very brightly in the karyotypes of fishes, amphibians, reptiles and birds, but not of mammals. The lack of mithramycin fluorescence at the nucleolus organizer regions of mammals is attributed to the relatively low level of redundancy of the GC-rich ribosomal DNA in their genomes. Studies on the various meiotic stages of the cold-blooded vertebrates show that the mithramycin labeling of the nucleolus organizers is independent of their state of activity. This can be confirmed by mithramycin fluorescence at the nucleoli of actinomycin-treated cells.

Chromosome banding in amphibia. XI. Constitutive heterochromatin, nucleolus organizers, 18S + 28S and 5S ribosomal RNA genes in Ascaphidae, Pipidae, Discoglossidae and Pelobatidae

The karyotypes of 14 species of Anura from 9 genera of the suborders Amphicoela, Aglossa, Opisthocoela and Anomocoela were analysed with various banding techniques and conventional cytogenetic methods. The 18S + 28S and 5S ribosomal RNA genes were localized by means of in situ hybridization. No Q-, R- and G-banding patterns in the euchromatic segments of the metaphase chromosomes could be demonstrated in any of the species; this does not seem to be caused by a higher degree of spiralization of the amphibian chromosomes, but by the special DNA organization in these organisms. In most karyotypes, constitutive heterochromatin is present at centromeres, telomeres and nucleolus organizer regions (NORs), but rarely in interstitial positions. The heterochromatic regions are either quinacrine positive and mithramycin negative or vice versa. All species examined possess only one homologous pair of NORs: these display the brightest mithramycin fluorescence in the karyotypes. Many specimens exhibited unequal labelling of the two NORs both after silver and mithramycin staining as well as after in situ hybridization with 3H-18S + 28S rRNA. In four species, between one and six chromosome pairs with homologous 5S rRNA sites could be identified. The 5S rRNA genes and the 18S + 28S rRNA genes are closely linked in two species. In the male meiosis of the Amphicoela and Opisthocoela, there are intersitial, subterminal and terminal chiasmata in the bivalents, whereas only terminal chiasmata are observed in the bivalents of the Aglossa and Anomocoela. No heteromorphic sex-specific chromosomes could be demonstrated in any of the species. The differential staining techniques revealed that the chromosomal structure in these four suborders is largely the same as in the highly evolved anuran suborders Procoela and Diplasiocoela.