Effects of cryopreservation at various temperatures on the survival of kelp grouper ( Epinephelus moara ) embryos from fertilization with cryopreserved sperm

Use of melatonin as an inhibitor of apoptotic process for cryopreservation of zebrafish (Danio rerio) embryos

This study investigated the use of melatonin to arrest the effects of apoptosis in vitrified zebrafish (D. rerio) embryos. Dechorionated embryos at 22-24 somite-stage were divided (n = 60/treatment) into a non-vitrified (Control Group, 0 M melatonin) and vitrified treatments with 0 M (T1), 1 µM (T2) and 1 mM of melatonin (T3). For vitrified treatments, a solution methanol/propylene glycol based was used and the embryos stored in -196 °C for a week. After thaw, survival rate, scanning electron microscopy, expression of anti (bcl-2) and pro-apoptotic (bax/caspase-3) genes, reactive oxygen species (ROS) formation and DNA fragmentation analyses were performed. No live embryos were obtained from vitrified treatments, observing a rapid degeneration immediately after thawing, with the vitelline layer rupture and leakage of its content, followed by breakdown of epithelial cells and melanisation of the tissue. Regarding the apoptotic process, T3 had the highest relative gene expression, for the three genes (P < 0.05) furthermore, T2 had similar expression of pro-apoptotic genes to CG (P < 0.05). ROS formation revealed that CG presented lower percentage of embryo surface area affected (3.80 ± 0.40%) (P < 0.05), in contrast, no differences were found among the other groups. T1 was most significantly (P < 0.05) damaged by DNA fragmentation. The vitrified groups with melatonin had similar damage levels of CG (P > 0.05). The inclusion of 1 µM of melatonin in the vitrifying solution, countered the effects of apoptotic process in post-thaw embryos, suggesting its utility in cryopreserving fish embryos.

Cooling of Siamese fighting fish Betta splendens (Teleostei, Osphronemidae) embryos at low temperatures

The Siamese fighting fish (Betta splendens) has great importance as an ornamental aquarium fish as well as laboratory model species. Due to its rapid development, a cooling-embryo protocol could provide some advantages in their transportation, embryonic synchronization, and optimization of hatcheries. In this context, this work aimed to develop a protocol to storage B. splendens embryos at two temperatures (5 and 14 °C), testing three cryoprotective solutions (S1: 0.5 M sucrose, 1.5 M methanol; S2: 0.25 M sucrose, 0.75 M methanol; and S3: 0.125 M sucrose, 0.375 M methanol) and evaluating the quality of the larvae obtained. Moreover, a method to isolate the embryos from the bubble nest constructed by the male and to incubate them without parental care was applied in this study. The cooling assays were done using embryos of 24-h-post-fertilization at 26 °C and the results demonstrated that it is possible to store these embryos deprived of cryoprotectants at 5 °C for at least 6-h without negative effects. Meanwhile, S2 and S3 were the most suitable solutions for its storage for 9-h at 5 °C or 24-h at 14 °C, obtaining 77% hatching and 52% normal larvae in the first case or 88% hatching and 81% larvae with mild abnormalities in the second one. Indeed, type and frequency of larval abnormalities were evaluated and, remarkably, a partial recovery was described on malformed larvae from embryo cooled at 14 °C. Finally, this work is the first report about the cooling of B. splendens embryos and establishes the conditions for further studies on this field with this species.

Gene expression changes in response to low temperatures in embryos of the kelp grouper, Epinephelus moara

The kelp grouper Epinephelus moara has high economic value and is popular in fisheries and aquaculture in China. In the previous study, we treated the embryos at 16-22 somite stage at 4 °C, -25.7 °C, -140 °C and -196 °C, and successfully obtained surviving embryos in each group. To better understand the molecular changes affected by the low temperatures, we conducted a comparative transcriptome analysis among embryos exposed at 4 °C for 30 min, embryos exposed at -25.7 °C for 30 min and the control group. qPCR assays were conducted for the validation. Signal transduction pathways were highly enriched for the differentially expressed genes. c-Fos, c-Jun, JunD, GADD45, involved in MAPK signaling pathway, were upregulated when embryos were treated at low temperatures. As immediate early genes, Egr-1a and b, and IER2, that respond quickly to the environment stress, their expression increased as well. Hsp70 showed similar expression pattern as immediate early genes. Meanwhile, transcription factors Sox, HES, TFIID, muscle movement and protein synthesis-related genes were downregulated. Taken together, our findings suggest that cooling disrupts gene expression patterns in E. moara embryos. The differentially expressed genes may be involved in cellular resistance against low temperatures, possibly through neural activation, apoptosis, proliferation, differentiation, cellular recovery and heat shock regulation. This study also provides transcriptome dataset of E. moara embryos exposed to cold temperatures for future studies focusing on the molecular effects of cryopreservation.

Hybridization of tiger grouper (Epinephelus fuscoguttatus ♀) x giant grouper (Epinephelus lanceolatus ♂) using cryopreserved sperm

Using Ringer solution as extender, the present study examined the protective effect of dimethyl sulphoxide (Me₂SO; 8-12%, v/v) on the cryopreservation of giant grouper (Epinephelus lanceolatus) sperm. The cryopreserved sperm was then successfully applied in interspecific hybridization with tiger grouper (E. fuscoguttatus). Higher motility (90.56 ± 6.58%) and fertilization rate (69.61 ± 4.83%) was achieved in 10% Me₂SO with Ringer solution as extender (dilution ratio 1:1), which should no significant difference in comparison with fresh sperm (95.88 ± 1.64% and 73.10 ± 1.28%). There were no statistical differences in both fertilization and hatching rates between hybrid and non-hybrid tiger grouper by using cryopreserved sperm for fertilization, but malformation rate of the hybrid was higher than non-hybrid (17%) (P < 0.05). Survival rate of the hybrid was lower than that of the controls at 15 days post hatching (23% vs 48%). However, hybrids showed survival rate equal to the controls at the end of the 60-day study period. Hybridization of E. fuscoguttatus x E. lanceolatus was successfully achieved using cryopreserved sperm from giant grouper. The cryopreservation of giant grouper sperm and its application in hybridization provided a technical support for further grouper breeding work.

Cooling of pejerrey Odontesthes bonariensis (Teleostei, Atherinidae) embryos at sub-zero temperatures

Pejerrey fish (Odontesthes bonariensis) is a seasonal multiple spawner with great economic importance and an adequate species for Aquaculture. For these reasons, it is necessary to apply biotechnologies to optimize its reproduction in captivity. In this context, the aim of this work was to develop a cooling protocol for pejerrey embryos at sub-zero temperatures. Two cryoprotective solutions (CSs: S1 and S2), two cooling curves (a fast and a slow one) and two storage temperatures (-14 and -20 °C) were evaluated for 1 h. High percentages of embryo survival (80-100%) were obtained in all cases. In particular, for cooling at -14 °C, the most suitable protocol was the slow temperature decrease in combination with S1 (2.5 M methanol, 1.4 M Me2SO, 0.3 M sucrose, and 0.08 M NaCl). The hatching rate (86.67 ± 11.55%) and the larval survival observed did not differ from those of the control group, and about 30% of normal-looking larvae were obtained. Besides, the slow cooling was also the best way to reach -20 °C, obtaining a hatching rate of around 60%. However, all the larvae had different kind of malformations. Finally, in order to improve the results obtained at -20 °C, the CSs were incorporated into the embryos by microinjection. In this case, it was observed that the most convenient combination was the microinjection of S2 (same composition as S1 but without Me2SO) in the perivitelline space followed by rapid cooling. Although the hatching rate was not improved (67.93 ± 8.31%), the microinjection allowed to obtain at least 4.5% normal-looking larvae. These results showed that the cooling of pejerrey embryos at zub-zero temperatures was feasible. Moreover, the microinjection of cryoprotectants within the pejerrey O.bonariensis embryos was employed for the first time in this species.

Optimization of vitrification factors for embryo cryopreservation of kelp grouper (Epinephelus moara)

Cryopreservation of marine fish embryos causes to severe cryogenic damage, and to date, adults have not been reared from embryos that were cryopreserved. Here, we optimized vitrification factors to improve the survival and hatching rate of kelp grouper (Epinephelus moara) embryos after cryopreservation. We screened the effects of 11 vitrification solution concentrations (25-50%) on the survival rate of embryos at four developmental stages (16S, 18S, 22S, TB). We investigated the effects of different equilibration time (25-45min) on the survival rate and the influence of vitrification solutions on embryonic volume. In addition, we tested the effects of treating embryos at five different developmental stages (4-6S, 16S, 22S, TB, HB) with different vitrification solutions (35% PMG3S and 35% PMG3T), prechilling temperature (-5 °C and 4 °C) and prechilling time. In total, 9855 embryos were cryopreserved at 10 developmental stages, from optic capsule stage to pre-hatch stage. We found that kelp grouper embryos performed best at equilibration time of 30 min. Embryos at the tail-bud stage exhibited greater tolerance to vitrification than other stages. Vitrification solutions that contained sucrose showed better survival rates compared to embryos treated with vitrification solutions containing trehalose. Pre-chilling treatment improved viability before freezing, but did not improve viability after freezing. In the most optimal condition we identified in this study, the average survival, normal development and malformation rates of cryopreserved embryos were 6.32%, 2.36% and 3.49%, and 39.85% of the surviving embryos that were cryopreserved hatched. The hatched larvae gradually died at day 12 of cultivation, where the longest surviving individuals lived for 16 days. This study provides valuable data for improving survival and hatching rate of cryopreserved grouper embryos, and provides references for further exploring techniques in fish embryo cryopreservation.

Time-series transcriptomic analysis of the kelp grouper Epinephelus moara in response to low salinity stress

The Kelp grouper Epinephelus moara is one of the most widely consumed and economically important marine fish in China. The species can tolerate a wide range of salinity, but genomic resources are not available, and the molecular mechanisms underlying adaptation to salinity at the transcriptomic level remain largely unclear. In this study, the transcriptomic responses of the liver of E. moara under low salinity were investigated using the Illumina digital gene expression system. After de novo assembly, 499,356 transcripts were generated and contributed 445,068 unigenes. A total of 14, 19, 33 and 3101 genes were differentially expressed following exposure to low salinity stress for 2, 6, 24 and 48 h, respectively. Only two genes were differentially expressed in all groups. Four genes related to metabolism and ambient salinity adaption were randomly selected to validate the differentially expressed genes (DEGs) by real-time PCR. Gene Ontology (GO) and Kyoto Encyclopaedia of Genes and Genomes (KEGG) pathway enrichment analysis were used to analyse the functional significance of DEGs, including those responding to salinity through diverse biological processes, cellular components, molecular functions, and pathways associated with metabolic and osmotic responses. This work provides new insight into the response to salinity challenges in E. moara, and the findings expand our knowledge of the molecular basis of metabolic regulation mechanisms in this species. Additionally, the transcriptional data provide a valuable resource for future molecular and genetic studies on E. moara.