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Long-term storage of oogonia and germ-line stem cells provides an alternative to the limitations associated with cryopreserving eggs of important fish species. These cell types are less vulnerable to the stresses of freezing. Cryopreservation has enormous potential for aquaculture advancement, but protocols must be developed for each species and cell type since its success hinges on various input factors. Blue Catfish Ictalurus furcatus were selected as the test species in this study because of the need to improve fry production of Blue Catfish ♂ × Channel Catfish I. punctatus ♀ hybrids, which can be facilitated by storing oogonia in gene banks. Our objective was to develop a freezing protocol for oogonia of this species. We tested different permeating and nonpermeating cryoprotectants, concentrations of these agents, and freezing rates. We proved that all three factors influenced postthaw recovery of oogonia. Of the permeating cryoprotectants, 1.0 M dimethyl sulfoxide resulted in the most live cells with the highest viability percentages, and adding 0.2 M lactose with 10% egg yolk further improved the results. There were also specific interactions in which the effects of concentration and freezing rate varied among the cryoprotectant treatments. The most effective freezing rate was −1.0°C/min, and cell viability was reduced at −2.5°C/min and −5.0°C/min. From these results, we propose adding 1.0 M dimethyl sulfoxide with 0.2 M lactose and 10% egg yolk to cryomedia and freezing it at a rate of −1.0°C/min. By developing a cryopreservation protocol for a commonly cultured catfish, this work may guide the development of protocols for other species of interest.

Sustainability of channel catfish, Ictalurus punctatus ♀ × blue catfish, Ictalurus furcatus ♂ hybrid aquaculture relies on new innovative technologies to maximize fry output. Transplanting spermatogonial stem cells (SSCs) from blue catfish into channel catfish hosts has the potential to greatly increase gamete availability and improve hybrid catfish fry outputs. Cryopreservation would make these cells readily accessible for xenogenesis, but a freezing protocol for blue catfish testicular tissues has not yet been fully developed. Therefore, the objectives of this experiment were to identify the best permeating [dimethyl sulfoxide (DMSO), ethylene glycol (EG), glycerol, methanol] and non-permeating (lactose or trehalose with egg yolk or BSA) cryoprotectants, their optimal concentrations, and the best freezing rates (−0.5, −1.0, −5.0, −10 °C/min until −80 °C) that yield the highest number of viable type A spermatogonia cells. Results showed that all of these factors had significant impacts on post-thaw cell production and viability. DMSO was the most efficient permeating cryoprotectant at a concentration of 1.0 M. The optimal concentration of each cryoprotectant depended on the specific cryoprotectant due to interactions between the two factors. Of the non-permeating cryoprotectants, 0.2 M lactose with egg yolk consistently improved type A spermatogonia production and viability beyond that of the 1.0 M DMSO control. The overall best freezing rate was consistent at −1 °C/min, but similar results were obtained using −0.5 °C/min. Overall, we recommend cryopreserving blue catfish testicular tissues in 1.0 M DMSO with 0.2 M lactose and egg yolk at a rate of either -0.5 or −1 °C/min to achieve the best cryopreservation outcomes. Continued development of cryopreservation protocols for blue catfish and other species will make spermatogonia available for xenogenic applications and genetic improvement programs.

For externally fertilizing fishes, interactions between male and female gametes have been shown to have remarkable impacts on sperm performance. Ovarian fluid (OF) and its ability to alter the swimming behavior of fish sperm makes it a determining factor of fertility. With the expansion of channel catfish (Ictalurus punctatus) ♀ × blue catfish (Ictalurus furcatus) ♂ hybrid aquaculture, it is essential to understand the impacts during fertilization and the magnitude such gametic interactions have on sperm performance and subsequent male fertility potential. This study was conducted to address the following: 1) activate blue catfish sperm with/without channel catfish OF to determine impacts on sperm performance and 2) assess if sperm behave differently when activated in the OF from individual females. Sperm (n = 4 males) were activated without OF (control) and with diluted OF from unique females (n = 6), creating 24 experimental crosses. Sperm motility (%), velocity (VCL), and longevity were analyzed using computer assisted sperm analyses software. With OF incorporated in the activation media, sperm velocity was significantly higher than the control at 10, 20, and 30 s post-activation. OF did not have an impact on motility for any females at 10 s and 20 s post-activation but became significantly higher than the control at 30 s. In all cases, OF treatments greatly increased longevity. Male × female interactions were highly significant, such that motility, velocity, and longevity were dependent on specific male-female pairs. This information shows that OF should be incorporated in aquatic media to simulate natural spawning conditions and accurately assess the fluid mechanics of sperm propulsion for each male. Additionally, there are mechanisms that drive gamete interactions that need to be explored further, which may improve selection of male-female pairs for in-vitro fertilization. On a broad scale, our results also help to shed light on the complexities of fertilization and fish reproduction overall, which may have implications for recruitment variability and recovery strategies of threatened and/or endangered freshwater species.

Long-term storage of spermatogonial stem cells (SSCs) represents the next step for advancing aquaculture research by providing valuable genetic resources for genetic enhancement programs and xenogenesis technologies. Hybrid catfish, the cross between channel catfish, Ictalurus punctatus ♀ and blue catfish, I. furcatus ♂, are in high demand by the US aquaculture industry, but production can be limited by the availability of blue catfish gametes. Therefore, xenogeneic stem cell transplantation is a powerful method for generating blue catfish gametes within faster growing and maturing channel catfish hosts. These technologies could be facilitated by having cryopreserved SSCs from testicular tissue on-hand in gene banks. Cryopreservation protocols for blue catfish testicular tissues and for other fish species are still being optimized, and currently, data is lacking on the effects antioxidants and antifreeze proteins (AFPs) have on cellular post-thaw viability of SSCs. The objective of this study was to analyze the individual and combined effects of antioxidants (catalase, hypotaurine, and ascorbic acid) and AFPs (I and III) on post-thaw type A spermatogonia cell production and viability from testicular tissue. In this study, there were no improvements with individual antioxidant or AFP treatments. However, when the antioxidants hypotaurine and catalase were tested in combination with different AFPs, three treatments had higher type A spermatogonia cell production than the control, including hypotaurine 7 mM + AFPI 1 μg/mL, hypotaurine 3.5 mM + AFPI 0.1 μg/mL, and hypotaurine 7 mM + AFPIII 0.1 μg/mL. These treatments along with hypotaurine 7 mM + AFPI 0.1 μg/mL and catalase 100 IU + AFPI 0.1 μg/mL had higher viability than the control. From these results, we recommend adding these antioxidant-AFP combinations to future cryopreservation protocols. Overall, this is the first study showing that combining specific antioxidants and AFPs can improve spermatogonia cryopreservation for a fish species.

Hybrid catfish, the progeny of channel catfish (Ictalurus punctatus) females × blue catfish (I. furcatus) males, are in high demand by the United States aquaculture industry because they outcompete each of the parent species for pond and raceway culture. Unfortunately, fry production can be a limiting factor due to a lack of natural hybridization between the two species and the necessity to sacrifice males for artificial fertilization. Males are costly to maintain in hatcheries and take longer to mature than channel catfish males. Therefore, it is essential to use the minimum quantity of sperm that allows for the highest fry production. In this study, channel catfish eggs from 9 females were collected from two different locations and fertilized with six different sperm to egg ratios ranging from 1.0 × 103:1 to 1.0 × 105:1. Embryos were then incubated under common environmental conditions until hatch. Overall average hatch success ± SE using sperm to egg ratios between 1 × 103 and 1.0 × 105 ranged from 18.3 ± 4.3 to 48.8 ± 8.3%. Overall, there was a significant effect of sperm to egg ratio (P 1 × 104:1 sperm per egg had no significant improvement on hatch success. Both study locations indicated significant impacts of sperm to egg ratio (P

Despite best efforts to optimize reproduction, egg incubation, and larval performance in captivity, inconsistencies in hatchery fish production are still created by high variations in egg quality from individual females. In some hatchery species, egg quality and generation of viable embryos are correlated to abundances of specific mRNAs. Channel catfish females show considerable extremes in egg quality, causing inconsistencies in channel catfish, Ictalurus punctatus, female × blue catfish, Ictalurus furcatus, male hybrid fry production. The objectives of this study were to examine relative transcripts linked to egg and embryo quality and determine expression between low-hatch and high-hatch egg batches through early development (0, 24, 48, and 96 h post-fertilization; HPF). RNA was extracted from eggs/embryos of nine females (n = 4 high-quality, n = 5 low-quality) and Real-Time PCR was used to quantify relative gene expression. The transcripts assessed in this study perform critical cellular functions, including tubulin β (tubb), cathepsin D (ctsd), cathepsin Z (ctsz), cathepsin B (ctsb), cyclin B (ccnb1), exportin-1 (xpo1), ring finger protein 213 (rnf213), glucocorticoid receptor-1 (GR-1), and heat shock protein 70 (hsp70). Relative gene expression of all transcripts except GR-1 and hsp70 were up-regulated in the high-hatch group and peaked at 48 HPF (neurulation stage), indicating the importance of these gene products at this threshold to normally progress until hatch. Due to lack of expression during earlier stages, maternally derived mRNAs for these genes do not seem to impact early embryonic development. Using mRNA markers as a selection mechanism for hatchery broodstock may lead to more high-hatch egg batches by reducing problems associated with poor egg quality.

Hybrid catfish, the progeny of channel catfish, Ictalurus punctatus, females × blue catfish, I. furcatus, males are in high demand by the aquaculture industry due to their superiority for pond culture. Unfortunately, fry production is a limiting factor due to lack of natural hybridization between the species and the necessity to sacrifice males for artificial fertilization. In this study, we used a quantitative genetic breeding design to assess genetic, environmental, and genotype by environment interactions to detail the genetic architecture of fitness during the “critical” early life history (ELH) stages. Males and females were crossed using a full-factorial design, creating 20 unique families. Offspring from each family were split into 2 temperature-controlled environments based on conditions that mimic early (26.6 °C) and late (32.2 °C) seasonal temperatures. Embryonic survival, hatch success, larval morphology, and deformities were quantified at hatch, mid-yolk sac transition, and swim-up stages of early development. Variation in early performance traits (calculated as variance components, VC) were partitioned to maternal/paternal effects as well as parental × environmental interactions, analyzed across and within temperatures. Embryonic survival ranged from 45 to 93% by 120°-hours post-fertilization and was not impacted by temperature. Maternal effects were responsible for large amounts of variation (VC = 51%), and paternal effects also became apparent during later stages of embryonic development but in smaller quantities (VC = ~7%). Temperature significantly impacted hatch success, in which hatch decreased at 32.2 °C (from 40% to 32%). Therefore, we conclude that temperatures at the start of the spawning season yield higher hatch success. Maternal effects were highly significant (VC = 65%), and there were also significant paternal effects (VC = 12%) with wide family variation (ranging from 14 to 71%). The deformity rate increased from 3.6% at 26.6 °C to 6.0% at 32.2 °C, but variation was driven more by maternal effects rather than temperature. For morphology, fry reared at 32.2 °C had smaller body sizes at each developmental stage. Maternal variation across morphology traits ranged widely from 9 to 80% and was highest for maternal yolk. Paternal effects/interactions ranged from to 0–29%. Genetic × environmental interactions were also observed for morphology traits since values for VCs differed within each temperature. This information showed the importance of environmental effects, parentage, and their associated interactions, which by isolating indicators of male/female quality, can be used to develop parameters for broodstock selection. Results can also be applied to improve incubation conditions for hybrid catfish during the crucial ELH stages.