Particularities of reproduction and oogenesis in teleost fish compared to mammals

Compared to mammals, teleost reproduction presents many original features. Reproductive strategies of species are diversified into numerous adaptations to a large variety of aquatic environments. This diversity may concern sexuality, spawning and parental behaviour, sensitivity to environmental factors, and specific features of gametogenesis such as the duration of vitellogenesis, and egg morphology. Sexuality presents a variety of natural modalities, from gonochorism to hermaphrodism. The absence of definitive arrest of body growth in the adult of most species gives a particular interest to the practical control of growth-reproduction interactions. Vitellogenesis, which represents an important metabolic effort for the maternal organism, involves the synthesis of vitellogenin, a specific glycolipo-phosphoprotein produced in the liver under estradiol stimulation, and its incorporation into oocytes by a receptor mediated process. Both estradiol synthesis in follicle cells and vtg uptake by vitellogenic follicles appear to be mainly controlled by FSH. Oocyte maturation is directly triggered by a progestin, or MIS (maturation inducing steroid) synthesised in follicle cells mainly under LH control, and acting through the non-genomic activation of a membrane receptor. Practical applications of some of these particularities result mainly from the external character of the fertilisation process and of embryonic development, which allows manipulating respectively egg chromosome stocks and sex differentiation. Moreover, the sensitivity of sex differentiation to exogenous factors favours the development of practical methods to control the sex of farmed populations. Finally, the sensitivity of reproductive mechanisms to xenobiotics has led to various kinds of bioassays for putative pollutants.     

A novel rod-like opsin isolated from the extra-retinal photoreceptors of teleost fish

We have isolated a novel opsin from the pineal complex of Atlantic salmon (Salmo salar) and from the brain of the puffer fish (Fugu rubripes). These extra-retinal opsins share approximately 74% identity at the nucleotide and amino acid level with rod-opsins from the retina of these species. By PCR, we have determined that the novel rod-like opsin is not expressed in the salmon retina, and the retinal rod-opsin is not expressed in the salmon pineal. Phylogenetic analysis suggests that the rod-like opsins arose from a gene duplication event approximately 205 million years ago, a time of considerable adaptive radiation of the bony fish. In view of the large differences in the coding sequences of the pineal/brain rod-like opsins, their extra-retinal sites of expression, and phylogenetic position we have termed these novel opsins 'extra-retinal rod-like opsins' (ERrod-like opsins). We speculate that the differences between retinal rod-opsins and ERrod-like opsins have arisen from their differing photosensory roles and/or genetic drift after the gene duplication event in the Triassic.     

The unobtrusive majority: mononucleated bone resorbing cells in teleost fish and mammals

The canonical view of a mammalian (usually shown as human) bone resorbing cell is that of a giant macrophage‐like cell (osteoclast) that dissolves bone minerals and digests bone matrix proteins. The cells’ presence and activity is easily recognised based on three distinct morphological features: (i) multinuclearity, (ii) a multiply folded apical cell membrane (ruffled border), and (iii) deep lacunae (Howship’s lacunae) that the cells eroded into the bone surface. Mononucleated osteoclasts without these features are considered to be inactive precursors. We challenge the view that bone resorbing cells must be multinucleated giant cells, based on our comparative studies on the teleost skeleton, on what is currently known – but often disregarded – about mononucleated mammalian osteoclasts, and on what is know about osteocytic osteolysis.     

Calcium and light adaptation in retinal photoreceptors.

In the past several years there has been great progress in the understanding of the phototransduction process in retinal photoreceptors. Recently, this knowledge has expanded and we now understand the mechanism of background light adaptation in these cells and the role that calcium has to play.     

Aestivation in the teleost fish Lepidogalaxias salamandroides (Mees)

• 1.1. Lepidogalaxias salamandroides does not lose water during the first 43 days of aestivation even though it is burrowed in extremely dry soil.
• 2.2. Little urea was accumulated in the body, which suggests that urea production is greatly diminished and/or urea is eliminated in the urine.
• 3.3. Theoretical considerations predict that water fluxes will, in the initial stages of aestivation, be positive until the soil moisture tension is equivalent to the plasma water potential.

     

Ultrastructural diurnal changes of the retinal photoreceptors in the embryo of a viviparous teleost (Poecilia reticulata P.)

All diurnal changes studied - ellipsosome excepted - start at midgestation, following differentiation of photoreceptors and pigment-epithelium cells. These are: (1) shedding of the tips of the light-sensitive photoreceptor outer segments and subsequent phagocytosis by the pigment-epithelium; (2) retinomotor movements of pigment-epithelium processes, rods and cones; (3) changes of cone square-mosaics into row-mosaics at night. Newly-differentiated photoreceptors in the embryo are, therefore, already vulnerable to disruption of cyclical systems. Several inherited human retinal diseases, such as Retinitis pigmentosa, are thought not to affect differentiation of photoreceptors but their cyclical renewal pathways. The retina of the guppy-embryo is, therefore, a valuable model for such studies.     

Proteinase inhibitor(s) in seminal plasma of teleost fish

Anti-proteinase activity has been found in seminal plasma of three teleost fish: rainbow trout, whitefish and yellow perch. The activity was effective against trypsin (bovine and cod) and acrosin (boar), but not bovine chymotrypsin. Inhibitor activity against fish trypsin was nine- to ten-fold higher than against bovine trypsin. All anti-proteinase activity remained in the retentate after filtration through molecular filter with 30 kDa cut-off membrane and eluted from a column of Sephacryl S-200 HR at the volume characteristic for molecular weight of approximately 90 kDa (data for rainbow trout). Inhibitor(s) had low thermal stability (50–60% activity remained after 15 min at 60° C). The discovery of proteinase inhibitor(s) in the seminal plasma of teleosts raised the question of the regulatory function of this protein in the systematic group of fishes having anacrosomal spermatozoa.     

The hapten-carrier effect in teleost fish.

The sea robin (Prionotus evolans), a representative teleost fish, was found to undergo the hapten-carrier effect, suggestive of T-B cell cooperativity, in vivo with dinitrophenyl as the hapten and bovine gamma-globulin or bovine serum albumin as carriers. In contrast to mammalian systems, the secondary anti-hapten response in the sea robin was found to involve exclusively high molecular weight antibody.     

Suppression of osteoclastic activities by calcitonin in the scales of goldfish (freshwater teleost) and nibbler fish (seawater teleost)

Using a tartrate-resistant acid phosphatase (TRACP) activity, the effects of calcitonin (CT) and estradiol-17beta(E(2)) on osteoclastic activities in cultured scales of goldfish (freshwater fish) and nibbler fish (seawater fish) were examined. In mature male and female goldfish, scales were collected and incubated in Earle's minimun essential medium (MEM) supplemented with either CT (100 ng/ml) or E(2) (100 ng/ml). In both sexes, CT suppressed TRACP activities in the scales, whereas E(2) increased them. When CT (100 ng/ml) and E(2) (100 ng/ml) were added simultaneously to MEM, TRACP activities in the scales did not change as compared with the control. Similar results were obtained with the scales of female nibbler fish. In goldfish, furthermore, the activation of scale TRACP by E(2) was suppressed, depending on CT concentrations (100 pg/ml to 1 microg/ml). In fish reproduction, this function of CT may be useful to protect scales from excess degradation of calcium at vitellogenesis.     

Pituitary cytology of the teleost fish Tilapia mossambica (Peters)

In the pituitary of Tilapia mossambica, eight cell types were identified on the basis of their staining reactions. the RPD consists of erythrosinophils and PbH positive cells. PbH cells border the NH and give amphipilic reaction to tri- and tetrachrome dyes and Halmi's stain. Erythrosinophils are also stained with acid fuchsin and orange G. The PPD is made up of acidophils and cyanophils. The acidophils are stained with orange G, acid fuchsin and erythrosin. These cells form a palisade zone between the neurohypophysis and the cyanophils. The cyanophils are AB, AF, ATh, PAS and aniline blue positive. They include both thyrotrophs and gonadotrophs and cannot be differentiated. The gonadotrophs may be those which are degranulated and vacuolated in the breeding season. The PI is formed of PAS- and PbH positive cells which lack any definite pattern of arrangement. Apart from the chromophils, scattered chromophobes were seen throughout the adenohypophysis. Occasionally, cells resembling the cyanophils of PPD were noticed in the RPD and PI also.     

Identification of extraretinal photoreceptors in the teleost Phoxinus phoxinus

The existence of cells capable of detecting changes of the photoperiod within the deep brain, the so-called deep brain photoreceptors, was proposed in the early years of the twentieth century. By using immunocytochemistry with antisera against phototransductory proteins on paraffin and vibratome sections, we have localized several positive areas in the brain of the teleost Phoxinus phoxinus. These areas were restricted to two encephalic regions: the epithalamus and the hypothalamus. Immunopositive (rod-opsin- and a-transducin-like) pinealocytes and parapinealocytes, as well as some sparse neurons in the habenula, were seen in the epithalamus. The immunoreaction of the hypothalamus was represented by a-transducin-like positive (magnocellular and parvicellular) neurons of the Nucleus Preopticus, as well as by a-transducin- and arrestin-like positive fibers corresponding to the hypothalamic-hypophyseal tract and a few fibers running towards the basal telencephalon. These findings corroborate the data published on other teleost fish and fully support the hypothesis of the presence of photosensitive cells in the encephalon of lower vertebrates. The labelling with antisera against different components of the phototransductory cascade also strengthens the idea that such cells employ a biochemical mechanism similar to that in the retinal visual photoreceptor cells, rods and cones. Although the function is still unclear, the detection of the photoperiod seems to be the most likely role for these extraretinal photoreceptors.     

Filial cannibalism in teleost fish

This review summarizes information on filial cannibalism (the act of eating one's own offspring) in teleost fish. Cannibalistic parents can either consume their whole brood (total filial cannibalism), or eat only some of the eggs in the nest (partial filial cannibalism). Offspring consumption has been argued to be adaptive under the assumption that offspring survival is traded against feeding, and that offspring can act as an alternative food source for the parents. The evidence supporting the basic predictions formulated under these assumptions is summarized for both total and partial filial cannibalism. These two forms of cannibalism differ significantly since the former represents an investment only in future reproductive success, whereas the latter can affect both present and future reproductive success. Despite a few inconsistencies in the data from laboratory and field studies, the energy-based explanation appears valid for both forms of cannibalism. Alternative non-energy-based explanations are considered, but they are unable to account for the wide distribution of this behaviour in teleosts. The intersexual conflict arising from attempts of the non-cannibal sex to minimize the cost of filial cannibalism is also discussed, together with the potential effect of this behaviour on the operational sex ratio at a population level.     

Polyunsaturated fatty acid metabolism in fish cells: differential metabolism of (n-3) and (n-6) series acids by cultured cells originating from a freshwater teleost fish and from a marine teleost fish

1. The incorporation and metabolism of (n-3) and (n-6) polyunsaturated fatty acids (PUFA) supplemented to growing cultures were studied in rainbow trout (RTG-2) and turbot (TF) cell lines. 2. A fatty acid concentration of 20 microM considerably altered the fatty acid composition of the cells without affecting lipid class composition or the appearance of cytoplasmic lipid droplets. 3. Both cell lines exhibited considerable delta 6 desaturase activities. 4. Whereas delta 5 desaturase activity was expressed in RTG-2 cells, delta 4 desaturase activity was absent and, conversely, delta 4 desaturase activity was expressed in TF cells, but there was an apparent deficiency in the C18 to C20 elongase multi-enzyme complex. 5. The delta 6 desaturase activity in both cell lines showed little preference between 18:2(n-6) and 18:3(n-3) but the delta 5 desaturase activity of RTG-2 cells and the delta 4 desaturase activity of TF cells showed a preference for (n-3)PUFA. 6. Two fish oil concentrates were assessed for their ability to generate fatty acid compositions in the cell lines more closely resembling those of intact fish tissues.     

Cloning and characterization of glucose transporter in teleost fish rainbow trout (Oncorhynchus mykiss)

The facilitated diffusion of monosaccharides across the plasma membrane is mediated by glucose transporters (GLUTs). In contrast to mammals, the glucose transport system of lower vertebrates remains unexplored. We detected glucose transport activity in rainbow trout embryos. Two GLUTs sharing 83% amino acid identity were cloned from juvenile fish, these have been denoted OnmyGLUT1A and OnmyGLUT1B. In adult trout OnmyGLUT1A is predominantly expressed in the heart with low expression in other tissues. An inverse terminal repeat of a Tc1-like transposable element was found in the 3'-untranslated region of OnmyGLUT1B. Phylogenetic analysis suggested that rainbow trout genes share a common ancestor with higher vertebrate GLUT1. We also found GLUT genes in several salmonid species.     

Genome evolution and biodiversity in teleost fish

Teleost fish, which roughly make up half of the extant vertebrate species, exhibit an amazing level of biodiversity affecting their morphology, ecology and behaviour as well as many other aspects of their biology. This huge variability makes fish extremely attractive for the study of many biological questions, particularly of those related to evolution. New insights gained from different teleost species and sequencing projects have recently revealed several peculiar features of fish genomes that might have played a role in fish evolution and speciation. There is now substantial evidence that a round of tetraploidization/rediploidization has taken place during the early evolution of the ray-finned fish lineage, and that hundreds of duplicate pairs generated by this event have been maintained over hundreds of millions of years of evolution. Differential loss or subfunction partitioning of such gene duplicates might have been involved in the generation of fish variability. In contrast to mammalian genomes, teleost genomes also contain multiple families of active transposable elements, which might have played a role in speciation by affecting hybrid sterility and viability. Finally, the amazing diversity of sex determination systems and the plasticity of sex chromosomes observed in teleost might have been involved in both pre- and postmating reproductive isolation. Comparison of data generated by current and future genome projects as well as complementary studies in other species will allow one to approach the molecular and evolutionary mechanisms underlying genome diversity in fish, and will certainly significantly contribute to our understanding of gene evolution and function in humans and other vertebrates.     

Sexual determination and differentiation in teleost fish

The present work reviews the latest information on the cellular, molecular and physiological aspects of sexual determination and differentiation in teleost fish. The group exhibits a large variety of mechanisms of sexual determination. These may be genetic, or depend on environmental conditions such as temperature, pH, and social factors, all of which can influence the proportion of the sexes. Additionally, sex steroids play an important role in the regulation of sexual differentiation. The patterns of gonadal sexual differentiation are diverse, and species may be hermaphroditic or gonochoristic, some of the latter displaying juvenile hermaphroditism. In recent years, several genes involved in the sexual determination and differentiation pathways in vertebrates, particularly in mammals, have also been characterized in teleosts. Conserved as well as diversified functions have been proposed.     

Ovarian germline stem cells in the teleost fish, medaka (Oryzias latipes)

In the mammalian testis germline stem cells keep producing many sperms, while there is no direct evidence for the presence of germline stem cells in the ovary. It is widely accepted in mammals that the mature oocytes are supplied from a pool of primordial follicles in the adult ovary. In other vertebrates, such as fish, however, there has been no investigation on the mechanism underlying the high egg-producing ability. In this review, we introduce the recently identified ovarian germline stem cells and the surrounding unique structure in teleost fish, medaka (Oryzias latipes) [Nakamura S et al. Science. 2010; 328: 1561-1563]. We also discuss about the expression and function of sox9 that characterizes this unique structure.     

Different aspects of tubulin polymerization in spermatids of cyprinodontidae (fish,teleost)

Our study concerns 10 genera and 26 species of cyprinodontid fish. In the cytoplasm of spermatids tubulin polymerizes in various forms according to species. We have demonstrated the presence of classic microtubules with a diameter of 24 nm and also of tubules of smaller diameter (15 run) and greater diameter (30 to 50 nm). Microtubules are very numerous in the cytoplasm of certain species. They may be arranged without any order or form bundles which may contain several hundred parallel elements. They never form a manchette. In two species (Aplocheilichthys normani and Epiplatys fasciolatus) only spermatids that degenerate show this peculiarity. The microtubules present two kinds of decorations. The first type are small elements composed of MAP which enable two or more microtubules to link up. The second type are curved tubulin elements that give the microtubule that bear them the appearance of an incomplete doublet. Doublets and triplets may also be formed. Cyprinodontidae spermatocytes and spermatids probably synthesize a very large quantity of tubulin which polymerizes in certain species.