Localization of NORs in cetacean karyotypes

Differential hypomethylation of the repetitive Tol2/Alu-rich sequences in the genome of Bodianus species (Labriformes, Labridae)

Representatives of the order Labriformes show karyotypes of extreme conservatism together with others with high chromosomal diversification. However, the cytological characterization of epigenetic modifications remains unknown for the majority of the species. In the family Labridae, the most abundant fishes on tropical reefs, the genomes of the genus Bodianus Bloch, 1790 have been characterized by the occurrence of a peculiar chromosomal region, here denominated BOD. This region is exceptionally decondensed, heterochromatic, argentophilic, GC-neutral and, in contrast to classical secondary constrictions, shows no signals of hybridization with 18S rDNA probes. In order to characterize the BOD region, the methylation pattern, the distribution of Alu and Tol2 retrotransposons and of 18S and 5S rDNA sites, respectively, were analyzed by Fluorescence In Situ Hybridization (FISH) on metaphase chromosomes of two Bodianus species, B. insularis Gomon & Lubbock, 1980 and B. pulchellus (Poey, 1860). Immunolocalization of the 5-methylcytosine revealed hypermethylated chromosomal regions, dispersed along the entire length of the chromosomes of both species, while the BOD regions exhibited a hypomethylated pattern. Hypomethylation of the BOD region is associated with the precise co-location of Tol2 and Alu elements, suggesting their active participation in the regulatory epigenetic process. This evidence underscores a probable differential methylation action during the cell cycle, as well as the role of Tol2/Alu elements in functional processes of fish genomes.

Description of the karyotypes of Stejneger's beaked whale (Mesoplodon stejnegeri) and Hubbs' beaked whale (M. carlhubbsi)

The genus Mesoplodon (Cetacea: Odontoceti: Ziphiidae) is one of the few cetacean genera with the karyotype 2n = 42. The 2n = 42 karyotype of M. europaeus and M. carlhubbsi is largely consistent with the general cetacean karyotype 2n = 44, although other 2n = 42 karyotypes do not exhibit clear homologies with the general cetacean karyotype. Therefore, the chromosomes of Mesoplodon species may be the key to understanding cetacean karyological evolution. In the present study, the male karyotypes of M. stejnegeri and M. carlhubbsi were examined. In both species, the diploid number of the male karyotype was 42. Both species had the following characteristics: 1) a huge subtelocentric X chromosome with a large C-block; 2) a small metacentric Y chromosome; 3) nucleolus organizer regions (NORs) in the terminal regions of a large autosome and one or two small metacentric autosomes; 4) small metacentric autosomes; 5) large submetacentric and subtelocentric autosomes; 6) less accumulated C-heterochromatin in the centromeric region; and 7) heteromorphism in C-heterochromatin accumulation between homologues. Characteristics 1 and 3 are peculiar to only the karyotypes of Mesoplodon species, whereas characteristics 4, 5, 6, and 7 are also found in the species with the general cetacean karyotype 2n = 44.

Molecular cytogenetic characterization of the Amazon River dolphin Inia geoffrensis

Classical and molecular cytogenetic (18S rDNA, telomeric sequence, and LINE-1 retrotransposon probes) studies were carried out to contribute to an understanding of the organization of repeated DNA elements in the Amazon River dolphin (boto, Inia geoffrensis). Twenty-seven specimens were examined, each presenting 2n = 44 chromosomes, the karyotype formula 12m + 14sm + 6st + 10t + XX/XY, and fundamental number (FN) = 74. C-positive heterochromatin was observed in terminal and interstitial positions, with the occurrence of polymorphism. Interstitial telomeric sequences were not observed. The nucleolar organizer region (NOR) was located at a single site on a smallest autosomal pair. LINE-1 was preferentially distributed in the euchromatin regions, with the greatest accumulation on the X chromosome. Although the karyotype structure in cetaceans is considered to be conserved, the boto karyotype demonstrated significant variations in its formula, heterochromatin distribution, and the location of the NOR compared to other cetacean species. These results contribute to knowledge of the chromosome organization in boto and to a better understanding of karyoevolution in cetaceans.

Composition and chromosomal localization of cetacean highly repetitive DNA with special reference to the blue whale, Balaenoptera musculus

Three highly repetitive DNA components--the common cetacean component, the heavy (GC-rich) satellite and the light (AT-rich) satellite--were were studied in the blue whale. Consensus sequences of the common component and the heavy satellite were determined on the basis of three repeats of the common component and eight repeats of the heavy satellite. The tandemly organized common cetacean component, which comprises a large portion of all cetacean--both odontocete (toothed whale) and mysticete (whalebone whale)--genomes has a repeat length of 1,760 bp and the three clones analysed showed a high degree of conformity. The repeat contains a 72 bp sequence with dyad symmetry and striking intrastrand complementarity. The rest of the repeat comprises a unique sequence. The repeat unit of the heavy satellite of the blue whale is 422 bp. Also this component is tandemly organized. About half the length of the repeat constitutes a unique sequence and the other half is made up of subrepeats with TTAGGG as a frequent motif. The light satellite has not been sequenced and its basic repeat unit has not yet been identified. The chromosomal localization of the three components was determined by in situ hybridization using 3H-labelled cloned fragments as probes. The common cetacean component was located in most interstitial and terminal C-bands. The heavy satellite occurred primarily in terminal C-bands. When the two components hybridized to the same terminal C-bands, the localization of the heavy satellite was distal to that of the common cetacean component. Neither component shared localization with the light satellite which is located in centromeric C-bands in just a few chromosome pairs.

Limited MHC polymorphism in whales

Little is known about disease and genetic variation in aquatic mammalian species such as whales. In this paper human HLA class I and class II probes were used to study major histocompatibility complex (MHC) genes from two species of whale: Fin (Balaenoptera physalus) and Sei (B. borealis). Stronger signals were obtained on whale than on equivalent concentrations of mouse DNA. Evidence was obtained for several DRB-related genes, a DNA genes, one DQA gene, and multiple class I genes in whales. Interestingly, the whale genes, from the small panel studied, were less polymorphic than those of humans or mice. The aquatic environment of this mammalian species may be a unique factor in shaping its immune response through the MHC.

Localization of NORs in spermatogonial metaphase chromosomes of six species of grasshoppers

The silver staining technique was employed to locate Nucleolar Organiser Regions (NORs) in six species of grasshoppers viz. Aiolopus thalassinus F. (Tryxalinae); Oeodaleus abruptus Thunb., Gastrimargus transversus Thunb., Heteropternis respondens Walk. (Oedipodinae); Parahieroglyphus biliniatus Bol. and Spathosternum prasiniferum Walk. (Catantopinae). Usually the NORs were located on the larger elements of the chromosomal complement. However, in O. abruptus NORs were found on autosomes S8 and S9. The salient observations were: (1) NORs were seen in only a few of the several spermatogonial metaphases examined; (2) Active NORs were mostly located either on one chromatid of the homologues or on the homologue depicting heteromorphism; (3) NORs showed either proximal, subproximal or interstitial locations. However, in O. abruptus and P. bilineatus NORs were located at two positions. Distribution of NORs in different species and their probable role in tracing the evolutionary pathways in Acridoidea are discussed.