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In this study, we examined the feasibility of germ cell‐less hybrids, produced by a cross between female blue mackerel (Scomber australasicus) and male chub mackerel (S. japonicus), as recipients in order to develop a surrogate broodstock technology for mackerels, which is attracting attention as a new aquaculture target. Donor testicular cells, isolated from the testes of a chub mackerel, were transplanted into the peritoneal cavity of 500 hybrid mackerel larvae. Of the 42 male recipients that survived to 1 year of age, 35 matured and produced donor‐derived chub mackerel sperm. Amongst them, 22 produced only donor‐derived sperm, with none of the recipient's own sperm detected. Two of the six females that survived to 1‐year‐old produced donor‐derived eggs, and one of two matured females produced only donor‐derived eggs. Notably, when the mature recipients were housed in a single fish tank, some spontaneously mated, and normal hatchlings arising from fertilization of donor‐derived chub mackerel eggs and sperm were successfully obtained. Although the efficiency of this method needs to be improved in the future, hybrid mackerel can be used as surrogates for mackerel species breeding. This technology is expected to make a significant contribution to the breeding of mackerels. [ABSTRACT FROM AUTHOR]

Chub mackerel Scomber japonicus and blue mackerel S. australasicus have become important aquaculture target species in Japan. For an efficient and stable aquaculture production of mackerels, it is desirable to work with a female-biased broodstock and thus to be able to manage precisely their sex ratio. As these two mackerel species do not have any external sexual dimorphism, direct biopsy of the gonads or measurement of sex hormone plasma levels is used in research to determine their phenotypic sex. However, these methods are associated with handling stress that results in gonadal atresia. Besides, it is difficult to identify sex using these methods in immature individuals. Therefore, genetic sexing would be desirable in these species to overcome these problems. In this study, we developed molecular sex identification tools for both chub and blue mackerels using sex-specific single-nucleotide variations (SNVs) identified by pool-sequencing (pool-seq) analyses. Genomic DNA was isolated from 30 males and females, in each species, and sequenced as pooled samples. Results of these pool-seq analyses identified a clear XX/XY male heterogametic sex-determination system in blue mackerel and a ZZ/ZW female heterogametic system in chub mackerel. Using this pool-seq information, we designed primer sets for genetic sex identification in both species. Results demonstrated that these primer sets are robust and accurate in assessing the genotypic sex of both blue and chub mackerels (accuracy > 99.5% in the chub, and > 94.5% in the blue mackerel) in various geographical samples of different ages. This method can now be applied for the efficient management of sex ratios in both mackerel aquaculture populations and wild-caught animals for fisheries management. [ABSTRACT FROM AUTHOR]

Frozen sashimi can be stored for a long time and transported over long distances, giving it a distinct advantage over fresh sashimi, and a stable supply of material for frozen sashimi products can be obtained using cultured fish rather than natural resources fish. Hybrids of blue mackerel Scomber australasicus and chub mackerel Scomber japonicus possess unique characteristics that are advantageous in aquaculture, such as a high temperature tolerance. To evaluate the quality of the meat in these hybrids after freezing, we compared the muscle properties of hybrid and chub mackerel that had been reared in a land-based tank. The hybrid mackerel had a lower gonadosomatic index and a higher crude lipid content than the chub mackerel. However, there were no significant differences in the breaking strengths of unfrozen and frozen–thawed muscles between the hybrid and chub mackerel. These findings suggest that the hybrid mackerel may be suitable for use in frozen sashimi products, and that it has unique characteristics that will give it added value as a branded fish. [ABSTRACT FROM AUTHOR]

The albino mutation has been used as a visible genetic marker to evaluate offspring resulting from transplanting donor germ cells into recipients to produce donor-derived gametes in surrogates. Using albino mutants as either recipients or donors makes it possible to easily identify the donor-derived offspring based on body color. The chub mackerel (Scomber japonicus) is a promising recipient species for xenogeneic gamete production of commercially valuable scombrids such as bluefin tuna; however, no body-color mutants of this species have been reported. Using the CRISPR/Cas9 system, we produced F1 albino chub mackerel harboring a biallelic null mutation in the solute carrier family 45a2 (slc45a2) gene. However, albino larvae ingested significantly less rotifers than wild-type larvae when reared with a normal feeding density of rotifers, resulting in zero albino larvae survival beyond 12 days post hatch. To overcome this lethality, we developed a novel "spotlight method" of larviculture, in which positive phototaxis is used to attract both rotifers and larvae to create an ultra-high-density feeding area in a larval rearing tank. Under these conditions, albino larvae ingested as much rotifers as wild-type larvae, thus improving albino larvae survival. Finally, we confirmed albino chub mackerel completely lack melanin pigmentation, indicating that we have developed a potentially useful recipient for germ cell transplantation in Scombridae species. • We successfully produced F1 albino chub mackerel by biallelic knockout of the slc45a2 loci. • Albino larvae die because of failure to ingest rotifers. • Using phototaxis ("spotlight method"), rotifers and larvae increase their encounter frequency. • The spotlight method markedly increased survival of albino larvae. • Albino mackerels would be useful recipients carrying a visible marker for germ cell transplantation. [ABSTRACT FROM AUTHOR]

The original article has been corrected. [ABSTRACT FROM AUTHOR]

• Hybrid mackerel showed germ cell-less sterility. • Germ cell-less gonads retained Sertoli and Leydig cells for nursing donor germ cells. • Gonads of hybrid mackerel attracted and incorporated intraperitoneally transplanted bluefin tuna germ cells. • Hybrid mackerel are promising recipients for production of bluefin tuna gametes. 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. [ABSTRACT FROM AUTHOR]