Leptin is a pleiotropic hormone known for regulating appetite and metabolism. To characterize the role of leptin signaling in rainbow trout, we used CRISPR/Cas9 genome editing to disrupt the leptin receptor (LepR) genes, lepra1 and lepra2. We compared wildtype (WT) and mutant fish that were either fed to satiation or feed deprived for six weeks. The LepR mutants exhibited a hyperphagic phenotype, which led to heavier body weight, faster specific growth rate, increased viscero- and hepatosomatic indices, and greater condition factor. Muscle glycogen, plasma leptin, and leptin transcripts (lepa1) were also elevated in fed LepR mutant fish. Expression levels of several hypothalamic genes involved in feed regulation were analyzed (agrp, npy, orexin, cart-1, cart-2, pomc-a1, pomc-b). No differences were detected between fed WT and mutants except for pomc-b (proopiomelanocortin-b), where levels were 7.5-fold higher in LepR fed mutants, suggesting that pomc-b expression is regulated by leptin signaling. Fatty acid (FA) content did not statistically differ in muscle of fed mutant fish compared to WT. However, fasted mutants exhibited significantly lower muscle FA concentrations, suggesting that LepR mutants exhibit increased FA mobilization during fasting. These data demonstrate a key role for leptin signaling in lipid and energy mobilization in a teleost fish.

Leptin is a cytokine that regulates appetite and energy expenditure, where in fishes it is primarily produced in the liver and acts to mobilize carbohydrates. Most fishes have only one leptin receptor (LepR/LepRA1), however, paralogs have recently been documented in a few species. Here we reveal a second leptin receptor (LepRA2) in rainbow trout that is 77% similar to trout LepRA1. Phylogenetic analyses show a salmonid specific genome duplication event as the probable origin of the second LepR in trout. Tissues distributions showed tissue specific expression of these receptors, with lepra1 highest in the ovaries, nearly 50-fold higher than lepra2. Interestingly, lepra2 was most highly expressed in the liver while hepatic lepra1 levels were low. Feed deprivation elicited a decline in plasma leptin, an increase in hepatic lepra2 by one week and remained elevated at two weeks, while liver expression of lepra1 remained low. By contrast, muscle lepra1 mRNA increased at one and two weeks of fasting, while adipose lepra1 was concordantly lower in fasted fish. lepra2 transcript levels were not affected in muscle and fat. These data show lepra1 and lepra2 are differentially expressed across tissues and during feed deprivation, suggesting paralog- and tissue-specific functions for these leptin receptors.

Objective: The Striped Bass Morone saxatilis is an emerging cultivar in the United States due in large part to its rapid growth rate and the ability to be reared in salinities ranging from freshwater to full-strength seawater. The successful culture of this species has been advanced by successive generations of selective breeding and domestication that have improved growth characteristics by around 8% per generation. Previous studies have shown that implementing reduced feeding protocols in aquaculture can have minimal impact on growth performance while potentially improving feed efficiency. Therefore, we evaluated the growth of fifth-generation, selectively bred pure-strain Striped Bass and determined whether reduced-frequency feeding might improve production efficiency of fish in recirculating aquaculture systems (RASs). Methods: We evaluated the production characteristics of F5-generation domesticated Striped Bass that were cyclically fed either 3 d/week or 5 d/week in an RAS. Striped Bass fingerlings were stocked (n = 30 fish/tank) at an average body weight (BW) of 60 g and were fed a commercial diet to satiation once per feeding day. Sampling occurred at the beginning of the study (0 d poststocking [dps]) and then every 2–3 months (i.e., at 58, 113, 179, 262, and 351 dps). Results: We found that F5-generation domesticated Striped Bass grew to market size (1.3 kg) in 11–12 months (18 months posthatch) when fed 5 d/week in an RAS. Within 179 dps, the mean BW of fish fed 5 d/week was significantly greater than that of fish fed 3 d/week, and this divergence in size persisted throughout the course of the study. Fish that were fed 3 d/week achieved a final BW (1.1 kg) that was 16% lower than the BW of fish fed 5 d/week, and they consumed 18% less feed overall. Although the specific growth rates of fish on the reduced-frequency feeding regime were lower initially, by 179 dps their growth had reached rates similar to those of fish fed 5 d/week. The final feed conversion ratio (FCR) was not significantly different between feeding regimes. However, the FCR changed with age: It was initially higher in smaller fish fed 3 d/week, whereas by the end of the study, the FCR had subsequently declined to a level at or below the FCR of fish fed 5 d/week. The shift in FCR occurred at a BW of around 785 g. Conclusions: Collectively, the results suggest that a shift to reduced-frequency feeding in larger fish (∼785 g) may have the potential to reduce feed and labor costs while minimizing impacts on growth and improving overall production efficiency of domesticated Striped Bass grown in RASs. This study demonstrates that F5-generation, selectively bred Striped Bass can be grown rapidly (<12 months) in an RAS from 60 g to 1.3 kg, a size exceeding that of most domestically produced fish in the United States, which could allow for expansion of aquaculture to the high-value fillet market.

Flounder in the genus Paralichthys exhibit a unique sex determination system that combines genotypic and temperature-dependent mechanisms. In southern flounder ( Paralichthys lethostigma ), XY individuals develop as males, while XX individuals’ sex is influenced by juvenile developmental temperatures. Little is known about the genes involved in this process. This study uses pooled RNA sequencing to create a de novo gonadal transcriptome and identify genes related to sex determination and differentiation in wild-caught juvenile southern flounder from two North Carolina locations with differing sex ratios. The transcriptome assembly revealed 68,331 uniquely annotated genes with high functional specificity. Nine genes were linked to sex determination, including five novel to southern flounder and associated with epigenetic mechanisms like m6a methylation and alternative splicing, suggesting epigenetic influence in male development. Additionally, genes in the TGF-β signaling pathway were differentially expressed, supporting this pathway’s role in sex differentiation. This study provides a valuable transcriptome resource for future research on southern flounder.