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Anadromous salmonids exhibit partial migration, where some individuals within a population migrate down to the ocean through complex interactions between body size and photoperiod. This study aimed to integrate the ontogenetic and seasonal patterns of smoltification, a series of changes for future marine life, in a strain of masu salmon (Oncorhynchus masou). Spring smoltification, as evidenced by the activation of gill Na + ,K + -ATPase (NKA), was induced during winter under an advanced photoperiod. In addition, juveniles showed an additional peak in gill NKA activity in August regardless of the photoperiod. When juvenile masu salmon were subjected to feeding manipulations during the first spring/summer, only fish exceeding a fork length of 12 cm exhibited an increased gill NKA activity. We tested whether size-driven smoltification required a long-day period by exposing juveniles to a constant short-day length (9-h light and 15-h dark) from January to November. Juveniles under short-day conditions exceeded 12 cm in June but showed no signs of smoltification. Thus, masu salmon undergo photoperiod-limited, size-driven smoltification during the first summer and size-limited, photoperiod-driven smoltification the following spring. The findings of the present study provide a framework for further elucidation of the physiological mechanisms underlying partial migration in salmonids., (© 2023. Springer Nature Limited.)

The tropical Celebes eel, Anguilla celebesensis , has a short migration between its spawning and growth habitats. Its spawning areas were hypothesized to be in Tomini Bay and the Celebes Sea after collecting their small leptocephali. However, there is no information about the silver eel oceanic spawning migration behavior of A. celebesensis . To better understand their short-distance spawning migration behavior, four large female silver eels (Eel 1-4) were equipped with pop-up satellite archival tags (PSATs) and released near the mouth of the Poso River in Tomini Bay of Sulawesi Island on 22 February (Eel 1-3) and 11 March 2010 (Eel 4). All PSATs ascended in Tomini Bay and transmitted their data. Eel 3 and 4 provided clear records of consistent diel vertical migration (DVM: eight days-Eel 3, 13 days-Eel 4) with daytime dives to mean depths of 444.7 m (Eel 3) and 539.0 m (Eel 4), where mean temperatures were 9.1°C (Eel 3) and 7.7°C (Eel 4), and nighttime ascents to mean depths of 132.8 m (Eel 3) and 112.4 m (Eel 4), where mean temperatures were 20.6°C (Eel 3) and 23.4°C (Eel 4). Eel 3 and 4 started to dive to deeper water around nautical dawn and swam up to shallower water around sunset. During nighttime, both eels swam in deeper and colder water during nights with moonlight than during nights without moonlight, and there was a negative linear relationship between experienced water temperatures with the moon in the sky and the lunar age for the eels. The A. celebesensis daily rhythm of DVM behaviors was similar to spawning-migration DVM behaviors of other anguillid species. Essential life history characteristics of A. celebesensis appear to be a short migration between freshwater growth habitat and ocean spawning habitat, and high GSI values with advanced gonadal development in downstream-migrating silver eels.

In early zebrafish development, the program for dorsal axis formation begins soon after fertilization. Previous studies suggested that dorsal determinants (DDs) localize to the vegetal pole, and are transported to the dorsal blastomeres in a microtubule-dependent manner. The DDs activate the canonical Wnt pathway and induce dorsal-specific genes that are required for dorsal axis formation. Among wnt-family genes, only the wnt8a mRNA is reported to localize to the vegetal pole in oocytes and to induce the dorsal axis, suggesting that Wnt8a is a candidate DD. Here, to reveal the roles of maternal wnt8a, we generated wnt8a mutants by transcription activator-like effector nucleases (TALENs), and established zygotic, maternal, and maternal zygotic wnt8a mutants by germ-line replacement. Zebrafish wnt8a has two open reading frames (ORF1 and ORF2) that are tandemly located in the genome. Although the zygotic ORF1 or ORF2 wnt8a mutants showed little or no axis-formation defects, the ORF1/2 compound mutants showed antero-dorsalized phenotypes, indicating that ORF1 and ORF2 have redundant roles in ventrolateral and posterior tissue formation. Unexpectedly, the maternal wnt8a ORF1/2 mutants showed no axis-formation defects. The maternal-zygotic wnt8a ORF1/2 mutants showed more severe antero-dorsalized phenotypes than the zygotic mutants. These results indicated that maternal wnt8a is dispensable for the initial dorsal determination, but cooperates with zygotic wnt8a for ventrolateral and posterior tissue formation. Finally, we re-examined the maternal wnt genes and found that Wnt6a is an alternative candidate DD., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Generation of clonal zebrafish will facilitate large-scale genetic screening and help us to overcome other biological and biotechnological challenges due to their isogenecity. However, protocols for the development of clonal lines have not been optimized. Here, we sought to develop a novel method for generation of clonal zebrafish by androgenesis induced by cold shock. Androgenetic zebrafish doubled haploids (DHs) were induced by cold shock of just-fertilized eggs, and the eggs were then heat shocked to double the chromosome set. The yield rate of putative DHs relative to the total number of eggs used was 1.10% ± 0.19%. Microsatellite genotyping of the putative DHs using 30 loci that covered all 25 linkage groups detected no heterozygous loci, confirming the homozygosity of the DHs. Thus, a clonal line was established from sperm of a DH through a second cycle of cold-shock androgenesis and heat-shock chromosome doubling, followed by genetic verification of the isogenic rate confirming the presence of identical DNA fingerprints by using amplified fragment length polymorphism markers. In addition, our data provided important insights into the cytological mechanisms of cold-shock-induced androgenesis.

As complete absence of germ cells leads to sterile males in zebrafish, we explored the relationship between primordial germ cell (PGC) number and sexual development. Our results revealed dimorphic proliferation of PGCs in the early zebrafish larvae, marking the beginning of sexual differentiation. We applied morpholino-based gene knockdown and cell transplantation strategies to demonstrate that a threshold number of PGCs is required for the stability of ovarian fate. Using histology and transcriptomic analyses, we determined that zebrafish gonads are in a meiotic ovarian stage at 14 days postfertilization and identified signaling pathways supporting meiotic oocyte differentiation and eventual female fate. The development of PGC-depleted gonads appears to be restrained and delayed, suggesting that PGC number may directly regulate the variability and length of gonadal transformation and testicular differentiation in zebrafish. We propose that gonadal transformation may function as a developmental buffering mechanism to ensure the reproductive outcome., (Copyright © 2015 The Authors. Published by Elsevier Inc. All rights reserved.)

The presence of germ cells in the early gonad is important for sexual fate determination and gonadal development in vertebrates. Recent studies in zebrafish and medaka have shown that a lack of germ cells in the early gonad induces sex reversal in favor of a male phenotype. However, it is uncertain whether the gonadal somatic cells or the germ cells are predominant in determining gonadal fate in other vertebrate. Here, we investigated the role of germ cells in gonadal differentiation in goldfish, a gonochoristic species that possesses an XX-XY genetic sex determination system. The primordial germ cells (PGCs) of the fish were eliminated during embryogenesis by injection of a morpholino oligonucleotide against the dead end gene. Fish without germ cells showed two types of gonadal morphology: one with an ovarian cavity; the other with seminiferous tubules. Next, we tested whether function could be restored to these empty gonads by transplantation of a single PGC into each embryo, and also determined the gonadal sex of the resulting germline chimeras. Transplantation of a single GFP-labeled PGC successfully produced a germline chimera in 42.7% of the embryos. Some of the adult germline chimeras had a developed gonad on one side that contained donor derived germ cells, while the contralateral gonad lacked any early germ cell stages. Female germline chimeras possessed a normal ovary and a germ-cell free ovary-like structure on the contralateral side; this structure was similar to those seen in female morphants. Male germline chimeras possessed a testis and a contralateral empty testis that contained some sperm in the tubular lumens. Analysis of aromatase, foxl2 and amh expression in gonads of morphants and germline chimeras suggested that somatic transdifferentiation did not occur. The offspring of fertile germline chimeras all had the donor-derived phenotype, indicating that germline replacement had occurred and that the transplanted PGC had rescued both female and male gonadal function. These findings suggest that the absence of germ cells did not affect the pathway for ovary or testis development and that phenotypic sex in goldfish is determined by somatic cells under genetic sex control rather than an interaction between the germ cells and somatic cells., (Copyright © 2012 Elsevier Inc. All rights reserved.)