Brain gangliosides and cold-adaptation in high-antarctic fish

The concentration and composition of gangliosides from the brain of eight species of Antarctic Notothenioid fishes belonging to the class of perciformes and two species of boreal fishes (tropic cichlid fish Oreochromis mossambicus; Codfish Gadus morhua) were investigated. The concentration of whole brain gangliosides in Notothenioid fishes (between 1622 and 2183 μg NeuAc/g dry wt.) was slightly lower than that in the brains of fish species, which live in warm, temperate habitats (2483 μg NeuAc/g dry wt.). The composition of brain gangliosides was completely different from that of warm adapted fish species (e.g. the tropic cichlid fish Oreochromis mossambicus). The relative concentration of polysialogangliosides (GT1b-GH) is strongly increased in all the investigated Antarctic species. They were found to have the most complex and most polar brain ganglioside pattern (high degree of sialylation and alkali-lability) within the teleosts. This may be one of the mechanisms, beside antifreeze proteins, to keep the neuronal membranes functional even below the freezing point.     

Changes of brain gangliosides in chicken and mice during heterothermic development

Developmental profiles of brain gangliosides of chicken and mice were correlated to thermo-biological parameters. In parallel with an increasing body temperature and resistance to cold stress (within the first 11 postnatal days) the brain gangliosides change to a less polar pattern, indicated by a higher proportion of oligosialogangliosides.     

Context of diversification of the viviparous Gyrodactylidae (Platyhelminthes, Monogenoidea)

Using four criteria proposed a decade ago by Brooks & McLennan to identify a case of adaptive radiation indicates that the evolutionary history of the viviparous clade of the Gyrodactylidae is dominated by nonvicariant processes. The viviparous clade, with 446 species, has significantly more species than its sister clade (one species), and high species richness was shown to be an apomorphic trait of only the viviparous gyrodactylids within the Gyrodactylidae. Reconciliation of the phylogenetic tree of the viviparous Gyrodactylidae with that of its hosts showed a low probability for cospeciation suggesting that adaptive modes of speciation and not vicariance were predominant during the historical diversification of the clade. The proposed hypothesis suggests that the Gyrodactylidae originated on the South American continent about 60 Mya after geographical dispersal and host switching of its common ancestor to demersal freshwater catfishes by a marine ancestor. Development of hyperviviparity and the consequent loss of 'sticky' eggs in conjunction with other symplesiomorphic and apomorphic features allowed rapid diversification coupled with high dispersal to new host groups and geographical areas by viviparous members of the Gyrodactylidae.     

Characterization of Sialyltransferase-IV Activity and Its Involvement in the c-Pathway of Brain Ganglioside Metabolism

Abstract: To characterize the sialyltransferase-IV activity in brain tissues, the activities of GM1b-, GD1a-, GT1b-, and GQ1c-synthases in adult cichlid fish and rat brains were examined using GA1, GM1, GD1b, or a cod brain ganglioside mixture as the substrate. The GD1a-synthase activity in the total membrane fraction from cichlid fish brain required divalent cations such as Mg²⁺ or Mn²⁺ and Triton CF-54 for its full activity. The V_max value was 1,340 pmol/mg of protein/h at an optimal pH of 6.5, whereas the apparent K_m values for CMP-sialic acid and GM1 were 172 and 78 µM, respectively. Cichlid fish and rat brains also contained GM1b-, GT1b-, and GQ1c-synthase activities. The ratio of GM1b-, GD1a-, and GT1b-synthase activities in fish brain was 1.00:0.89:1.13, respectively, and in rat brain 1.00:0.60:0.63. Incubation of fish brain membranes with a cod brain ganglioside mixture, which contains GT1c, and [³H]CMP-sialic acid produced radiolabeled GQ1c. It is interesting that the adult rat brain also contains an appreciable level of GQ1c-synthase activity despite its very low concentrations of c-series gangliosides. The GD1a- or GQ1c-synthase activity in fish and rat brain was inhibited specifically by coincubation with the glycolipids that serve as the substrates for other sialyltransferase-IV reactions. Thus, the GD1a-synthase activity was inhibited by GA1 and GD1b, but not by LacCer, GM3, or GD3. In a similar manner, the synthesis of GQ1c was suppressed by GA1, GM1, and GD1b, but not by LacCer, GM3, or GD3. The GD1a-synthase activity directed toward endogenous GM1 was inhibited by GA1 or GT1b, whereas the endogenous GT1b-synthase activity was suppressed by GA1 or GM1. GA1, GM1, and GD1b did not affect the endogenous GM3- and GD3-synthase activities. These results clearly demonstrate that sialyltransferase-IV in brain tissues catalyzes the reaction for GQ1c synthesis in the c-pathway as well as the corresponding steps in the asialo-, a-, and b-pathway in ganglioside biosynthesis.     

Phylogenetic status and historical origins of the oviparous and viviparous gyrodactylids (Monogenoidea,Gyrodactylidea)

Phylogenetic analyses of sequences of the 18S rDNA and MT‐CO2 gene fragments indicated that the oviparous and viviparous gyrodactylid‐like monogenoids formed independent monophyletic clades within the Order Gyrodactylidea, supporting the reinstatement of the Oogyrodactylidae and limiting the Gyrodactylidae to the viviparous species. Analyses further indicated that the clade comprising the two families shared a common ancestor with the Udonellidae. Two clades, that of Aglaiogyrodactylus and that of Phanerothecium, were identified within the Oogyrodactylidae, while Onychogyrodactylus was shown to be polyphyletic and Oogyrodactylus basal within the family. One putative synapomorphy was identified for the Oogyrodactylidae, that is presence of a massive Mehlis’ gland. The Gyrodactylidae was limited to species having a viviparous mode of reproduction, although relationships within the family were generally poorly resolved. Several putative synapomorphies were found for the Gyrodactylidae, including viviparity and protogyny, a bulbous and armed MCO, absence of a vitellarium, and presence of a knob‐like deep anchor root (Fig. 3e). Ultrametric analyses suggested that the initial divergence of the clade of the gyrodactylid‐like monogenoids and Udonellidae occurred about 335 mya (based on the 18S rDNA fragment) and about 400 mya (based on the MT‐CO2 gene fragment). Using the 18S rDNA fragment and three calibration points, ultrametric analyses indicated that the Gyrodactylidae and Oogyrodactylidae diverged at approximately 278 mya, with initial diversification within the Gyrodactylidae (about 211 mya) occurring earlier than that of the Oogyrodactylidae (about 133 mya), the latter coinciding with the breakup of Gondwana and the initial diversification of the armoured catfishes (Loricariidae). Finally, diagnoses were provided for the Gyrodactylidae and Oogyrodactylidae along with a list of genera assigned to each family.