Medaka dmY/dmrt1Y is not the universal primary sex-determining gene in fish

An outstanding candidate for a primary male-determining gene equivalent to Sry of mammals has been recently described from a non-mammalian vertebrate, the medaka fish (Oryzias latipes). However, the universality of dmY/dmrt1Y as the master sex-determining gene in fish is questionable. Phylogenetic analysis shows that dmY/dmrt1Y is an evolutionarily young Y chromosome-specific duplicate of a gene involved in testis development in vertebrates, and that this duplicate cannot be the primary sex-determining gene in most other fish species. Study of alternative fish models will probably uncover new genetic strategies controlling sexual dimorphism in vertebrates.     

A comparative view on sex determination in medaka

In fish, an amazing variety of sex determination mechanisms are known, ranging from hermaphroditism to gonochorism and from environmental to genetic sex determination. This makes fish especially suited for studying sex determination from the evolutionary point of view. In several fish groups, different sex determination mechanisms are found in closely related species, and evolution of this process is still ongoing in recent organisms. The medaka (Oryzias latipes) has an XY-XX genetic sex determination system. The Y-chromosome in this species is at an early stage of evolution. The molecular differences between X and Y are only very subtle and the Y-specific segment is very small. The sex-determining region has accumulated duplicated sequences from elsewhere in the genome, leading to recombinational isolation. The region contains a candidate for the male sex-determining gene named dmrt1bY. This gene arose through duplication of an autosomal chromosome fragment of linkage group 9. While all other genes degenerated, dmrt1bY is the only functional gene in the Y-specific region. The duplication leading to dmrt1bY occurred recently during evolution of the genus Oryzias. This suggests that different genes might be the master sex-determining gene in other fish.     

Xmrk‐induced melanoma progression is affected by Sdf1 signals through Cxcr7

Chemokine signals mediated by Sdf1/Cxcl12 through the chemokine receptor Cxcr4 are thought to play an instructive role in tumor migration and organ‐specific metastasis. We have used a small aquarium fish model to contribute to a better understanding of how the course of melanoma development is influenced by Sdf1 signals in vivo. We studied oncogene‐induced skin tumor appearance and progression in the transgenic medaka (Oryzias latipes) melanoma model. Similar to humans, invasive medaka melanomas show increased levels of sdf1, cxcr4, and cxcr7 gene expression. Stable transgenic fish lines overexpressing sdf1 exclusively in pigment cells showed a reduction in melanoma appearance and progression. Remarkably, diminished levels of functional Cxcr7, but not of Cxcr4b, resulted in strongly reduced melanoma invasiveness and a repression of melanoma. Our results thereby indicate that Sdf1 signals via Cxcr7 are able to constrain melanoma growth in vivo and that these signals influence tumor outcome.     

Derivation of stable zebrafish ES-like cells in feeder-free culture

Zebrafish offers an excellent opportunity to combine embryological, genetic and molecular analyses of vertebrate development in vivo. Embryonic stem (ES) cells have enormous potential to study developmental potency and differentiation in vitro and thus to complement in vivo approaches. Zebrafish ES-like cells have been produced on a feeder cell layer. Here, we report the derivation of Z428, a zebrafish ES-like cell line, from blastula embryos in feeder-free culture. Fetal bovine serum, fish serum, fish embryo extract, basic fibroblast growth factor, non-essential amino acids and 2-mercaptoethanol were found to be important for Z428 growth. After more than 120 passages and many freezing/thawing cycles over a period of 20 years, Z428 exhibits stable growth and manifests many ES cell features including an ES cell phenotype, high alkaline phosphatase activity and spontaneous differentiation in culture. Most importantly, Z428 was transplantable to blastula hosts and capable of contributing to embryonic tissues and organ systems of the three germ layers. Therefore, Z428 is a stable cell line and contains ES-like cells with pluripotency in vitro and in vivo, and a feeder layer is dispensable for ES-like cell derivation in zebrafish. The derivation and easy maintenance of zebrafish ES-like cells under feeder-free conditions provide a useful extension of the present toolbox for studying development and differentiation in the zebrafish model.     

Limia vittata as host species for the Amazon molly: no evidence for sexual reproduction

DNA fingerprinting demonstrated no sexual reproduction of the gynogenetic Amazon molly Poecilia formosa with Limia vittata as host species, in contrast with a single report that claims to have found sexual reproduction with such matings.     

Brain-Derived Neurotrophic Factor Is More Highly Conserved in Structure and Function than Nerve Growth Factor During Vertebrate Evolution

Mammalian nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are members of a protein family with perfectly conserved domains arranged around the cysteine residues thought to stabilize an invariant three-dimensional scaffold in addition to distinct sequence motifs that convey different neuronal functions. To study their structural and functional conservation during evolution, we have compared NGF and BDNF from a lower vertebrate, the teleost fish Xiphophorus, with the mammalian homologues. Genomic clones encoding fish NGF and BDNF were isolated by cross-hybridization using probes from the cloned mammalian factors. Fish NGF and BDNF were expressed by means of recombinant vaccinia viruses, purified, and their neuronal survival specificities for different classes of neurons were found to mirror those of the mammalian factors. The half-maximal survival concentration for chick sensory neurons was 60 pg/ml for both fish and mammalian purified recombinant BDNF. However, the activity of recombinant fish NGF on both chick sensory and sympathetic neurons was 6 ng/ml, 75-fold lower than that of mouse NGF. The different functional conservation of NGF and BDNF is also reflected in their structures. The DNA-deduced amino acid sequences of processed mature fish NGF and BDNF showed, compared to mouse, 63% and 90% identity, respectively, indicating that NGF had reached an optimized structure later than BDNF. The retrograde extrapolation of these data indicates that NGF and BDNF evolved at strikingly different rates from a common ancestral gene about 600 million years ago. By RNA gel blot analysis NGF mRNA was detected during late embryonic development; BDNF was present in adult brain.     

Multiple origins of tetraploid taxa in the Eurasian Bufo viridis subgroup

We used Q-banding and analyzed nucleolar organizing regions (NORs) to study the cytogenetic evolution of tetraploids within the Palearctic Bufo viridis subgroup, the only known amphibian complex comprising di-, tri- and tetraploid bisexually reproducing taxa. We examined three diploid (2n) nominal taxa (Bufo viridis viridis, B. v. turanensis, B. v. kermanensis) from five Eurasian localities and six tetraploid (4n) nominal taxa (B. oblongus, B. o. danatensis, B. pewzowi pewzowi, B. p. taxkorensis, B. p. unicolor, B. p. strauchi) from eight Central Asian localities. Homeologous chromosomes of 2n and 4n toads exhibit a similar morphology. Silver-staining and in situ hybridization revealed terminal NORs in the long arms of chromosomes 6 in all 2n but in only two out of four chromosomes 6 in all 4n taxa. Q-banding and a rapidly evolving mitochondrial marker suggest at least two origination events for Asian 4n toads: “Western Central Asian tetraploids’’ (B. oblongus Nikolsky, 1896) exhibit distinct differences within some chromosome quartets, which are divisible into pairs of chromosomes and may be allopolyploid. In contrast, “Central Asian tetraploids” (B. pewzowi Bedriaga, 1898) showed homogenous Q-banding patterns within each quartet, suggesting autopolyploidy. In Northeastern Iran, we discovered a zone of either common ancestry or hybridization of 2n and Western Central Asian 4n toads. This raises intriguing questions about how diploid and tetraploid taxa may evolve by exchanging genetic material.