The number of cytogenetic studies of marine fish has increased in recent years. Fish groups, such as Perciformes, which comprises many of the extant marine teleosts of economic importance, show little divergence in chromosome number and most species display a diploid number of 48 acrocentric chromosomes. In the Serranidae, Sparidae, Sciaenidae (Perciformes) and Mugilidae (Mugiliformes) small chromosome variations are restricted to subtle heterochromatin or nucleolar organizer region (NOR) modifications. There appears to exist a strict relationship between both absence of geographic barriers throughout the marine environment and high mobility of these animals (eggs, larvae, or adults), with a rarity of chromosome rearrangement at the macrostructural level. Moreover, a cellular homeostasis might also be important to karyotype maintenance among these fishes, limiting changes in the chromosome complement to cryptic chromosome rearrangements. Other groups, such as Blenniidae, Gobiidae and Scorpaenidae, for instance, show more extensive chromosome diversity, which is probably related to limited mobility. Numerical and structural chromosome polymorphisms and several sexual chromosome systems are recurrent among these fishes. A wide karyotypic diversification also characterizes the Tetraodontiformes, an interesting fish group with peculiar morphological, physiological and ecological characteristics.

An overview of marine fish cytogenetics

Mitotic metaphase chromosomes of the scarab beetle Thorectes intermedius (Costa) (Coleoptera Scarabaeoidea: Geotrupidae) were analyzed using various banding methods and fluorescent in-situ hybridization (FISH) with a ribosomal probe. The results obtained indicate that silver and CMA3 staining are unable to localize the chromosome sites of nucleolar organizer regions (NORs). Such an inadequacy is a consequence of the extensive silver and CMA3 stainability of both constitutive heterochromatin and heterochromatin associated to the NORs.

Ribosomal DNA location in the scarab beetle Thorectes intermedius (Costa) (Coleoptera: Geotrupidae) using banding and fluorescent in-situ hybridization.

The number and distribution of the 18S-28S and 5S rRNA (rDNA) gene sequences were examined on mitotic chromosomes of six sturgeon species by two-colour in situ hybridization. Four of the six species, Huso huso, Acipenser stellatus, Acipenser sturio, and Acipenser ruthenus, with about 120 chromosomes, showed from six to eight 18S-28S rDNA signals, while 5S rDNA signals were on only one chromosome pair. The two species with 250-270 chromosomes, Acipenser baerii and Acipenser transmontanus, showed from 10 to 12 18S-28S sites and two chromosome pairs bearing 5S rDNA signals. In all examined species, the rather intense 5S rDNA signals apparently overlapped those of 18S-28S rDNA. These data support the diploid-tetraploid relationships between the two chromosome groups of sturgeons. The close association between the two rDNA families in species belonging to an ancestral fish order, such as Acipenseriformes, supports the hypothesis that the association represents a primitive condition.

Chromosomal mapping of 18S-28S and 5S rRNA genes by two-colour fluorescent in situ hybridization in six sturgeon species.

Molecular cytogenetic analysis of the karyotype of the European Atlantic sturgeon, Acipenser sturio

/pdf/molecular-cytogenetic-analysis-of-the-karyotype-of-the-1mnj646ylb.pdf

Molecular cytogenetic analysis of the karyotype of the European Atlantic sturgeon, Acipenser sturio.

The karyotype of Chondrichthyes is still the least investigated among vertebrates. Over the last 40 years, the karyotypes of 63 out of the 1100 known species (5.73%) have been described in literature, namely seven squalomorph, one squatinomorph, 20 galeomorph, 33 batoid and two holocephalian species. Generally, the diploid number ranges from a minimum of 28 to a maximum of 106 elements, with more frequent values observed between 50 and 100 chromosomes. None of the four superorders is characterized by a peculiar chromosome set or morphology; the number of uniarmed and biarmed elements is variable in all the karyotypes, and microchromosomes are often present. The general trend in all groups seems to be a progressive reduction of the telocentric chromosome number in the most specialized species, followed by the loss of the microchromosomes. Polyploidy, followed by diploidization events and Robertsonian rearrangements, might have played a key role in the karyological evolution of elasmobranch fish. Chondrichthyes have the largest genome sizes among vertebrates, with the exception of dipnoans and urodeles. In the whole class, the species examined vary greatly in size, from 3 to 34 pg/N: the lowest values have been observed in holocephalians, while galeoids and batoids have a DNA amount ranging from 5 to 15 pg/N. Squaloids show heterogeneous DNA amounts, ranging from 8 to 34 pg/N. In more recent years, karyological studies have provided new data on the characterization of selachian karyotypes by C-banding, NOR staining, restriction enzymes in situ digestion and FISH with specific DNA probes, such as telomeric and SINE sequences.

Selachian cytogenetics: a review.

A biotinylated 28S rDNA probe was prepared from the genomic DNA of the Antarctic ice-fishChampsocephalus gunnari and hybridized to metaphase chromosomes of the same species by fluorescencein situ hybridization (FISH). The hybridization signal appeared over the whole heterochromatic arm of the submetacentric chromosomes bearing the nucleolar organizer regions. The results of rDNA/FISH are compared with those coming from classical cytogenetic (C, Q, Ag-NOR, chromomycin A3) banding techniques. Thein situ detection of a specific DNA sequence offers a new more precise perspective for understanding the evolving process in chromosomes of Antarctic fish and will provide an interesting contribution to comparative cytogenetics of lower vertebrates.

Ribosomal RNA location in the Antarctic fish Champsocephalus gunnari (Notothenioidei, Channichthyidae) using banding and fluorescence in situ hybridization.

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Ribosomal RNA location in the Antarctic fish Champsocephalus gunnari (Notothenioidei, Channichthyidae) using banding and fluorescence in situ hybridization.