Production of a maternal-zygotic medaka mutant using hybrid sterility

Suitability of hybrid mackerel (Scomber australasicus × S. japonicus) with germ cell-less sterile gonads as a recipient for transplantation of bluefin tuna germ cells

We aim to establish a small-bodied surrogate broodstock, such as mackerel, which produces functional bluefin tuna gametes by spermatogonial transplantation. When reproductively fertile fish are used as recipients, endogenous gametogenesis outcompetes donor-derived gametogenesis, and recipient fish predominantly produce their gametes. In this study, we assessed fertility of hybrid mackerel, Scomber australasicus × S. japonicus, and its suitability as a recipient for transplantation of bluefin tuna germ cells. Hybrid mackerel were produced by artificially inseminating S. australasicus eggs with S. japonicus spermatozoa. Cellular DNA content and PCR analyses revealed that F1 offspring were diploid carrying both paternal and maternal genomes. Surprisingly, histological observations found no germ cells in hybrid mackerel gonads at 120 days post-hatch (dph), although they were present in the gonad of 30- and 60-dph hybrid mackerel. The frequency of germ cell-less fish was 100% at 120-dph, 63.1% at 1-year-old, and 81.8% at 2-year-old. We also confirmed a lack of expression of germ cell marker (DEAD-box helicase 4, ddx4) in the germ cell-less gonads of hybrid mackerel. By contrast, expression of Sertoli cell marker (gonadal soma-derived growth factor, gsdf) and of Leydig cell marker (steroid 11-beta-hydroxlase, cyp11b1) were clearly detected in hybrid mackerel gonads. Together these results showed that most of the hybrid gonads were germ cell-less sterile, but still possessed supporting cells and steroidogenic cells, both of which are indispensable for nursing donor-derived germ cells. To determine whether hybrid gonads could attract and incorporate donor bluefin tuna germ cells, testicular cells labeled with PKH26 fluorescent dye were intraperitoneally transplanted. Fluorescence observation of hybrid recipients at 14 days post-transplantation revealed that donor cells had been incorporated into the recipient's gonads. This suggests that hybrid mackerel show significant promise for use as a recipient to produce bluefin tuna gametes.

Learning to Fish with Genetics: A Primer on the Vertebrate Model Danio rerio

In the last 30 years, the zebrafish has become a widely used model organism for research on vertebrate development and disease. Through a powerful combination of genetics and experimental embryology, significant inroads have been made into the regulation of embryonic axis formation, organogenesis, and the development of neural networks. Research with this model has also expanded into other areas, including the genetic regulation of aging, regeneration, and animal behavior. Zebrafish are a popular model because of the ease with which they can be maintained, their small size and low cost, the ability to obtain hundreds of embryos on a daily basis, and the accessibility, translucency, and rapidity of early developmental stages. This primer describes the swift progress of genetic approaches in zebrafish and highlights recent advances that have led to new insights into vertebrate biology.

The medaka dhc2 mutant reveals conserved and distinct mechanisms of Hedgehog signaling in teleosts

Background

Primary cilia are essential for Hedgehog (Hh) signal transduction in vertebrates. Although the core components of the Hh pathway are highly conserved, the dependency on cilia in Hh signaling is considered to be lower in fish than in mice, suggesting the presence of species-specific mechanisms for Hh signal transduction.

Results

To precisely understand the role of cilia in Hh signaling in fish and explore the evolution of Hh signaling, we have generated a maternal-zygotic medaka (Oryzias latipes) mutant that lacks cytoplasmic dynein heavy chain 2 (dhc2; MZdhc2), a component required for retrograde intraflagellar transport. We found that MZdhc2 exhibited the shortened cilia and partial defects in Hh signaling, although the Hh defects were milder than zebrafish mutants which completely lack cilia. This result suggests that Hh activity in fish depends on the length of cilium. However, the activity of Hh signaling in MZdhc2 appeared to be higher than that in mouse Dnchc2 mutants, suggesting a lower requirement for cilia in Hh signaling in fish. We confirmed that Ptch1 receptor is exclusively localized on the cilium in fish as in mammals. Subsequent analyses revealed that Fused, an essential mediator for Hh signaling in Drosophila and fish but not in mammals, augments the activity of Hh signaling in fish as a transcriptional target of Hh signaling.

Conclusions

Ciliary requirement for Hh signaling in fish is lower than that in mammals, possibly due to fused-mediated positive feedback in Hh signaling. The finding of this fish-specific augmentation provides a novel insight into the evolution of Hh signaling.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-015-0057-x) contains supplementary material, which is available to authorized users.

A Neural Mechanism Underlying Mating Preferences for Familiar Individuals in Medaka Fish

Social familiarity affects mating preference among various vertebrates. Here, we show that visual contact of a potential mating partner before mating (visual familiarization) enhances female preference for the familiarized male, but not for an unfamiliarized male, in medaka fish. Terminal-nerve gonadotropin-releasing hormone 3 (TN-GnRH3) neurons, an extrahypothalamic neuromodulatory system, function as a gate for activating mating preferences based on familiarity. Basal levels of TN-GnRH3 neuronal activity suppress female receptivity for any male (default mode). Visual familiarization facilitates TN-GnRH3 neuron activity (preference mode), which correlates with female preference for the familiarized male. GnRH3 peptides, which are synthesized specifically in TN-GnRH3 neurons, are required for the mode-switching via self-facilitation. Our study demonstrates the central neural mechanisms underlying the regulation of medaka female mating preference based on visual social familiarity.

The art of medaka genetics and genomics: what makes them so unique?

The medaka fish, Oryzias latipes, is an emerging vertebrate model and now has a high quality draft genome and a number of unique mutants. The long history of medaka research in Japan has provided medaka with unique features, which are complementary to other vertebrate models. A large collection of spontaneous mutants collected over a century, the presence of highly polymorphic inbred lines established over decades, and the recently completed genome sequence all give the medaka a big boost. This review focuses on the state of the art in medaka genetics and genomics, such as the first isolation of active transposons in vertebrates, the influence of chromatin structure on sequence variation, fine quantitative trait locus (QTL) analysis, and versatile mutants as human disease models.