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

Patagonian toothfish (Dissostichus eleginoides) is a large notothenioid fish that supports valuable fisheries throughout the Southern Ocean D eleginoides are found on the southern shelves and slopes of South America and around the sub-Antarctic islands of the Southern Ocean Patagonian toothfish are a long-lived species (>50 years), which initially grow rapidly on the shallow shelf areas, before undertaking an ontogenetic migration into deeper water Although they are active predators and scavengers, there is no evidence of large-scale geographic migrations, and studies using genetics, biochemistry, parasite fauna and tagging indicate a high degree of isolation between populations in the Indian Ocean, South Georgia and the Patagonian Shelf Patagonian toothfish spawn in deep water (ca 1000 m) during the austral winter, producing pelagic eggs and larvae Larvae switch to a demersal habitat at around 100 mm (1-year-old) and inhabit relatively shallow water (<300 m) until 6-7 years of age, when they begin a gradual migration into deeper water As juveniles in shallow water, toothfish are primarily piscivorous, consuming the most abundant suitably sized local prey With increasing size and habitat depth, the diet diversifies and includes more scavenging Toothfish have weakly mineralised skeletons and a high fat content in muscle, which helps neutral buoyancy, but limits swimming capacity Toothfish generally swim with labriform motion, but are capable of more rapid sub-carangiform swimming when startled Toothfish were first caught as a by-catch (as juveniles) in shallow trawl fisheries, but following the development of deep water longlining, fisheries rapidly developed throughout the Southern Ocean The initial rapid expansion of the fishery, which led to a peak of over 40,000 tonnes in reported landings in 1995, was accompanied by problems of bird by-catch and overexploitation as a consequence of illegal, unreported and unregulated fishing (IUU) These problems have now largely been addressed, but continued vigilance is required to ensure that the species is sustainably exploited and the ecosystem effects of the fisheries are minimised

The Patagonian toothfish: biology, ecology and fishery.

Background The Southern Ocean is the coldest ocean on Earth but a hot spot of evolution. The bottom-dwelling Eocene ancestor of Antarctic notothenioid fishes survived polar marine glaciation and underwent adaptive radiation, forming >120 species that fill all water column niches today. Genome-wide changes enabling physiological adaptations and the rapid expansion of the Antarctic notothenioids remain poorly understood. Results We sequenced and compared 2 notothenioid genomes-the cold-adapted and neutrally buoyant Antarctic toothfish Dissostichus mawsoni and the basal Patagonian robalo Eleginops maclovinus, representing the temperate ancestor. We detected >200 protein gene families that had expanded and thousands of genes that had evolved faster in the toothfish, with diverse cold-relevant functions including stress response, lipid metabolism, protein homeostasis, and freeze resistance. Besides antifreeze glycoprotein, an eggshell protein had functionally diversified to aid in cellular freezing resistance. Genomic and transcriptomic comparisons revealed proliferation of selcys-transfer RNA genes and broad transcriptional upregulation across anti-oxidative selenoproteins, signifying their prominent role in mitigating oxidative stress in the oxygen-rich Southern Ocean. We found expansion of transposable elements, temporally correlated to Antarctic notothenioid diversification. Additionally, the toothfish exhibited remarkable shifts in genetic programs towards enhanced fat cell differentiation and lipid storage, and promotion of chondrogenesis while inhibiting osteogenesis in bone development, collectively contributing to the achievement of neutral buoyancy and pelagicism. Conclusions Our study revealed a comprehensive landscape of evolutionary changes essential for Antarctic notothenioid cold adaptation and ecological expansion. The 2 genomes are valuable resources for further exploration of mechanisms underlying the spectacular notothenioid radiation in the coldest marine environment.

The genomic basis for colonizing the freezing Southern Ocean revealed by Antarctic toothfish and Patagonian robalo genomes.

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.

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.

The two giant notothenioid species, the Patagonian toothfish Dissostichus eleginoides and the Antarctic toothfish D. mawsoni, are important components of the Antarctic ichthyofauna and heavily exploited commercially. They have similar appearance and size, both are piscivorous and bentho-pelagic, but differ in their geographic distribution and absence/presence of the antifreeze trait. We karyotyped these two sister species by analyzing specimens collected from multiple Antarctic and sub-Antarctic sites. Both species have a diploid number of 48, but differ in karyotypic formula, (2m + 2sm + 44a) for D. eleginoides and (2m + 4sm + 42a) for D. mawsoni, due to an extra pair of submetacentric chromosomes in the latter. Chromosomal fluorescence in situ hybridization with rDNA probes revealed unexpected species-specific organization of rRNA genes; D. mawsoni possesses two rDNA loci (versus one locus in D. eleginoides), with the second locus mapping to its additional submetacentric chromosome. The additional rRNA genes in D. mawsoni may be a cold-adaptive compensatory mechanism for growth and development of this large species in freezing seawater.

The two giant sister species of the Southern Ocean, Dissostichus eleginoides and Dissostichus mawsoni, differ in karyotype and chromosomal pattern of ribosomal RNA genes

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

The two giant sister species of the Southern Ocean, Dissostichus eleginoides and Dissostichus mawsoni, differ in karyotype and chromosomal pattern of ribosomal RNA genes